Oil Pressure Gauge

This is the final result on my mission to get an operating oil gauge.

APT Instruments delivered a repaired guage which had been calibrated to my sending unit. I installed the guage and had no response from it whatsoever, just as it had been behaving before being fixed. I called APT and they graciously said send it back. I did. They returned it to me about 10 days later for no extra charge.

On the second try at installation I completely disconnected all the power by removing the battery cables. I then installed the newly repaired guage and the sending unit. I am happy to say that it now performs perfectly.

Apparently, on the first attempt at install, an electrical surge of some kind burned up the guage element. This seems to be the only answer and led to my extra caution the second time around. The moral of the story is to disconnect power before installing these sensitive parts.

Kudos to APT for their patience and willingness to work with me.

Now I am going to another forum to try to figure out how to stop the new leak from my oil filter.

Terry,

Very glad to hear that you got such great service from APT Instruments - another one to remember (though I think I have one of their cards somewhere....).

Thanks for letting us know.

Thanks for those ideas guys. What I ended up doing was sendng the gauge to APT Instruments International in Minneapolis. No good reason except that I have a son who lives there and I thought if worse comes to worse, I can always drive there and try to get it back, as well as visit part of the family.

Anyway, I sent the gauge on Tuesday, it arrived on Thursday, APT called on Friday with the estimate ($145 including shipping). They have now put it in the mail and I should have it Tuesday. The wire that heats the expanding strip to move the guage was shorted out of the circuit, so the guage never moved. I also sent my sending unit which they calibrated to the guage.

If this all works out I am pretty happy with the one week turnaround and the reasonable price. I will post to let you know the final result.

Terry Veldhouse

For us on the warm side, there is West valley Instruments in Los Angeles 818 758 9500.

To all:

PA Speedo is a popular vendor, but by no means the only. I only used them once (read into this what you will.....).

Nisonger Instruments in NY (914.381.3600, www.nisonger.com, infonisonger.com) is probably *the* recognized experts in Smiths gauges. Great inventory of parts and able to fix any Smith's gauge to a very high caliber.

Additionally, I have used, with great satisfaction, Bob's Speedometer in MI (1-800-592-9673, www.bobsspeedometer.com) for a Ferrari's Veglia gauge. They did excellent work at a fair price.

Good luck.

Terry when I bought my car in 2000 many of the instruments did not work. I sent them to Palo Alto Speedometer 650-323-0243. They did a nice job on mine so they may be a repair source if yours turns out to be bad.

David Barnes
68 E-Type FHC

Terry,

Don't worry about the internal resistance of your meter. I like the test you did with the ammeter. It does appear the sensor is working as it should. Could you do the same test again attaching the ammeter to the wire going to the sensor (normal configuration, gauge to ammeter to sensor) and see if you get the same results. Just to make sure you don't have a wiring problem between the gauge and sensor. You can also check that by just measuring 12V at the sensor with the ignition on. It does appear you have a bad gauge. Got to be lots around, but I am not sure of the best source. Maybe someone else may jump in and tell you were to buy one. Let me know if this is the fix.

John, I did the test to ground on both posts and the light lights up on both, so there is current through the meter. I took off the wires and tested the ohms accross the posts and the meter shows 23 ohms. Believe this is supposed to be in the 60's, so it looks to me like my gauge is shorted somewhere and not sending current to the heating element.

Secondly, I took the guage out of the circuit. I attached an ammeter between the +12V line which goes to the positive post and the sensor wire, leaving the gauge out completely. I started the car and after a few seconds, the ammeter showed a variable current which changed about every second between 0 and 175 milliamps. I believe this is about the right range to run the sensor, so I believe that the sensor is at least operating when I do this test. As I sit here writing this, I think you are probably wondering what's the internal resistance of my meter. I don't know, but maybe it's not important at this point.

Your thoughts?

If this gauge is defective, do you have any idea where to get one? My internet seaches are not productive to this point. Or, is there some way to refurbish mine? I have not taken it out of the dash yet.

Terry Veldhouse

Right about the gauge operation. If you tested across the gauge, it will not give you a good indication. You would need to go from each post to ground. The light should light on each post. That will confirm you have some continuity through the gauge. If the light doesn't light on one of the posts, you are correct, the gauge is toasted.
NOTE: I never used one of these lights but assume it is what that call a tracer light.

Terry,

Right about gauge operation. IF you put the test light across the two posts on the gauge, it will not give you a good indication. You need to use the light from one post to ground and then the other post to ground. If the light lights on both posts then you have continuity through the gauge. NOTE: I have never used the continuity light before but I think it is the same as they cause a tracer light. if so I am sure it won't work across the gauge.

Recommend a cheap analog Volt Ohm Meter if you don't already have one. Radio Shack has them.

You are right about the operation of the gauge. I am not sure about the continuity light, since I haven't used one, I always have used a VOM meter. But I don't think you can use it across the two posts on the gauge. I think you would have to connect the light to one post and ground and then the other post and ground. If the light lights on one post and not the other the gauge is open and as you said, "toast". If the light is on with both posts then you have continuity through the gauge.
If you don't have a Volt Ohm Meter, I would suggest you go to radio shack and buy a cheap analog one.

John, thanks for responding. I am going to try this tomorrow. Today, I attached a simple continuity light accross the two posts of the gauge and it did not show that current could flow through the gauge. From what I understand from the forum posts, there should be current going through which will heat up the bi-metal spade and cause the needle to move.

So, I am pretty sure the guage is cooked. Am I right?

Terry Veldhouse

Terry,

One thing that can cause you to read O PSI oil pressure is the oil pressure relief valve being open all the time. I had this problem with my car but it did show oil pressure at start up and would slowly drop to zero as it heated up. That is probably not you problem. Below is how to check out the system.

Actually it is simple. It was make complicated by Robert, Michael and I. Seems like a case of inflated egos. Anyway, is your oil pressure gauge stuck on zero or max pressure? I am assuming it is stuck on zero.

The first thing to do is turn the key on, check the voltage at the senor to ground for 12 volts. If you have 12 volts, with someone in the car tap the wire to ground and have the person in the car watch the gauge, it should move. When you tap the wire to ground and the gauge does not move your gauge is bad. If you want to do a better check of the gauge, from the work Michael and Robert did, you can check the gauge by adding a variable resistor. Remove the wire to the sensor, attach the variable resistor to the wire and the other side of the resistor to ground. If you vary the resistance between 40-60 ohms you should be able to show an oil pressure of around 50 PSI. (Resistor should be 1 watt and variable from around 0-100 ohms) This is not exact but will tell you if you gauge is operating and somewhat accurate.

So if all of this works as it is supposed to and you still show no oil pressure then, then the sensor is bad. Replace it. Do NOT leave external resistors in the circuit.

To All,

I just read this post/thread,(amazing), from top to bottom and although I can add nothing to the discussion that would be of any help, it brought to mind the comment many people make about how complex new cars are now with everything computerized/digitized etc.

Seems like this little old pressure gauge can give anything we identify as complex in todays world, a real run for it's money...and it's completely analog. I'm correct in saying this, right?

Just read my post. Hope you guys can figure out a little dyslexia. Lets go with gauge and "is there an easy test to determine if my gauge is wrong?" Told you i'm new at this.

Guys, I am a complete novice at forums, but I am learning. Really enjoyed and understood most of what was just posted about the oil pressure guage. My '69 2+2 has an oil guage that does not move, ever. I think it is supposed to peg right, so is there an easy test to determine is my guage is wrong? My sensor is a couple years old. The guage has never moved as long as I have owned the car, which is since 2009. Careless of me, no doubt, but I am just now getting the car going. Long story, not worth telling. Any way to check that guage easily?

Michael, you said it took 130 mA for the sensor to activate. Using the lowest resistance values measured by Robert and you, I have sensor 28 ohms; meter 60 ohms; added resistance 17 ohms (two 34 ohms in parallel). So I come with the resistance of 105 ohms. Now if I take 12.5V and divide by 105 ohms I come up with 119mA which is below what it takes to activate the sensor. Does the meter resistance reduce with current? I would think if anything it would go up but I am not sure.
Figuring these number, can you figure out how the sensor will work since the current is not high enough according to your number? Possible the meter is lower than 60 Ohms? Or will it not work with the amount of added resistance?

One snide remark too many.

1. Fluctuation you saw was probably your alternator kicking in and a voltage variation. When you see a 10 to 20 PSI change then you may have some results.
2. FACT: when you added the resistor in series you dropped the control voltage and current for the contact control in the sending unit. You will not be able to tell what your oil pressure is because the sensor is calibrated for a higher current/voltage. In your case you will still just read 60PSI or 0PSI.
3. FACT: Meter movement will be extremely sluggish because you have significantly reduced the current through the meter.
4. FACT: if you have to add a resistor in series to make the sending unit work, the sending unit is faulty. Replace it. They are not that expensive.

If anyone would like to tell me where I am wrong on any one of these, please let me know why I am wrong. Always trying to learn. Please limit to these 4 items so we stay on track.

Hello Michael.

I live very close to sunset point beach in Florida. Tonight we plan to take a drive, top down of course, and enjoy the sunset and perhaps a dinner with cocktails. I'll keep an eye on the oil gauge. I already can see it fluctuate when the car is in the drive way and I rev the engine, so this will be a preamble to the measurements you suggest on Sunday morning.

I cannot begin to tell you how much I love this car. When I first got it (only 18000 miles) it was cherry with just a few minor issues. You fixed one of those issues with your marvelous Jaguar clock conversions. The clock has run flawlessly ever since. I only touch it for daylight saving time changes.

The oil gauge is another extremely minor issue. Great engine. Burns no oil. Nothing leaks on the garage floor. I just wanted to see the oil pressure gauge show something (anything) other than pegged. Changing the gauge from 60 psi to 100 psi really only changed the face, not the internals. Your suggestions helped me understand how the oil sender works, make the appropriate, meaningful measurements, and select a high wattage, low value resistor to gently reduce the voltage drop across the gauge.

Your help is a pleasure. You obviously read and try to understand what people post before you offer intelligent advice.

Best regards, Bob G.

Hi Bob,
Congratulations on getting the meter to read in the linear region, but you are not out of the woods yet. I'd like you to run another test. With the engine running and everything connected, measure the voltage between the sender terminal and ground. If you get a nice stable reading then your sender is bad and by adding a resistor you have simply converted your gauge into an expensive idiot light. If the reading fluctuates all over the place then the sensor and your system are probably good. You can verify that quite enjoyably by taking the car on a long high-speed run and then stopping and idling. That will have the effect of thinning the oil and lowering the oil pressure. If the gauge still reads 60 PSI then it is probably not reflecting reality. If it indicates something like 20, the way mine would, then your solution is good and you have solved the problem.

Dear John R.

Please accept that I understand your continuous need to communicate. Unfortunately, the solution you continue to offer is not part of my initial requirement.

Please take a moment to re-read my initial post dated 12 June.

The good news: my car has 60 psi oil pressure, verified by auto mechanic testing equipment. The challenge: The 60 psi oil gauge in the dashboard is always pegged at 60 psi. So I replaced it with a similar Smiths era oil gauge that reads 0-100 psi. Now that gauge is pegged. I would like to add a resistor in series with the oil gauge to reduce the reading to the 60 psi range. I do not need a perfect reading, just want to see the gauge read mid-range. Any ideas about the resistance, or is there another way?

Every part of my requirement has been satisfied. Michael helped me. He provided me with the method to test and understand the data, which lead directly to the solution I needed.

I wish you a nice day.

Hello Michael.

Here is a photo of the simple cable I fabricated. Male clip on one end attaches to oil gauge. Female clip on opposite end attaches to White/Brown wire (inside dash board) that runs to oil sensor. Cable is cut in center, allowing insertion of different combinations of 2W resistors. (I used 2W because of the high current.) I had some 68 ohm resistors (too high), so I put two in parallel (34 ohms) to achieve the desired result.

When I start the car, gauge slowly climbs from 0 psi. Pulses up about once per second. On my 100 psi substitute gauge, settles in about 8-10 seconds at about a 70 psi reading. Just a little higher than I wanted, but exactly works as I had hoped. Probably needs a slight bit more resistance to read at 60 psi, but good enough for now.

Thanks again for all your help Michael, and good news on the oil pressure on your car. Clearly a victory for all.

Very respectfully, Bob G.

Robert,

Don't add the external resistor for the following reason:

a. you will only be hardwiring you gauge to read 60PSI all the time. Obviously the sending unit is not controlling it at all. If you don't believe than b should convince you.
b. The sending unit has a static resistance of about 30 ohms. When you add 34 ohms in series with it, you create a voltage divider. You voltage at the sending unit will be less than half of what it was designed to operate. Since the contact are open and closed with a combination of a heated element of some type (who cares) and pressure. We can safely assume the contacts are at a minimum not opening as much as they should. So by adding a voltage divider you have reduced the power to the heating element to 1/4 of the design requirement. Don't you think that will do a little number on the calibration?

Any way it is a bad idea.
And when Michael said "When my car has 60 PSI and the meter reads 60 PSI then that means the sensor is GOOD" that may be true if he didn't add a resistor in series to fool it into 60PSI. But reading 60 at 60PSI does not mean the sending unit would be good anyway. If it read correctly at 10/20/30.......... to full scale then he could say that.
Anyway, go buy yourself another sending unit.

Michael,

What if:

The contacts will not break in the sending unit. Note they are mated by pressure and temperature breaks the contacts (you told me that). So if it doesn't expand and break the contacts you will have a direct path to ground through the sensor resistor. What will the Oil Pressure gauge read: MAX PSI.

Assuming this is the case what would happen if you add an external resistor to make the gauge read 60PSI?
a. Does that mean your sending unit is good because 60 PSI is what you have at idle or do you need to run the car so it will have different PSI and see if the gauge actually moves? If the gauge stays at 60psi does that mean the sending unit is good or bad?

I think what has been done is create a constant resistance to ground through the gauge and it will read 60PSI no matter what oil pressure you have. Hopefully this explains what I was saying.

Not sure it is fixed. This the third time I have tried to submit this hope it goes.
Michael, I said Zero to 10 K because I was sure it would cover all possibilities and may save a trip to radio shack.
The reason I don't think it is fixed is this. You added a resistor to make the gauge read 60PSI. If the contacts in the sending unit are stuck you have a constant resistance value in circuit and you will read 60PSI all the time. NOTE if they are stuck you will read max psi without the external resistor also. Most cars do not maintain a constant oil pressure at all temperatures and RPMs, So drive and check. If pressure is constant your sending unit is bad. You just don't know how much I appreciate Michaels belittling and sarcastic remarks. I was trying to help and I appreciate Michael comments and his knowledge, but I don't agree with him on all area, but I learned a lot.

Not sure it is fixed. This the third time I have tried to submit this hope it goes.
Michael, I said Zero to 10 K because I was sure it would cover all possibilities and may save a trip to radio shack.
The reason I don't think it is fixed is this. You added a resistor to make the gauge read 60PSI. If the contacts in the sending unit are stuck you have a constant resistance value in circuit and you will read 60PSI all the time. NOTE is they are stuck you will read max psi without the external resistor also. Most cars do not maintain a constant oil pressure at all temperatures and RPMs, So drive and check. If pressure is constant your sending unit is bad. You just don't know how much I appreciate Michaels belittling and sarcastic remarks. I was trying to help and I appreciate Michael comments and his knowledge, but I don't agree with him on all area, but I learned a lot.

Not sure it is fixed. This the third time I have tried to submit this hope it goes.
Michael, I said Zero to 10 K because I was sure it would cover all possibilities and may save a trip to radio shack.
The reason I don't think it is fixed is this. You added a resistor to make the gauge read 60PSI. If the contacts in the sending unit are stuck you have a constant resistance value in circuit and you will read 60PSI all the time. NOTE if they are stuck you will read max psi also. Most cars do not maintain a constant oil pressure at all temperatures and RPMs, So drive and check. If pressure is constant your sending unit is bad. You just don't know how much I appreciate Michaels belittling and sarcastic remarks. I was trying to help and I appreciate Michael comments and his knowledge, but I don't agree with him on all area, but I learned a lot.

Not sure it is fixed. This the third time I have tried to submit this hope it goes.
Michael, I said Zero to 10 K because I was sure it would cover all possibilities and may save a trip to radio shack.
The reason I don't think it is fixed is this. You added a resistor to make the gauge read 60PSI. If the contacts in the sending unit are stuck you have a constant resistance value in circuit and you will read 60PSI all the. NOTE is they are stuck you will read max psi also. Most cars do not maintain a constant oil pressure at all temperatures and RPMs, So drive and check. If pressure is constant your sending unit is bad. You just don't know how much I appreciate Michaels belittling and sarcastic remarks. I was trying to help and I appreciate Michael comments and his knowledge, but I don't agree with him on all area, but I learned a lot.

Not sure it is fixed. This the third time I have tried to submit this hope it goes.
Michael, I said Zero to 10 K because I was sure it would cover all possibilities and may save a trip to radio shack.
The reason I don't think it is fixed is this. You added a resistor to make the gauge read 60PSI. If the contacts in the sending unit are stuck you have a constant resistance value in circuit and you will read 60PSI all the. NOTE is they are stuck you will read max psi also. Most cars do not maintain a constant oil pressure at all temperatures and RPMs, So drive and check. If pressure is constant your sending unit is bad. You just don't know how much I appreciate Michaels belittling and sarcastic remarks. I was trying to help and I appreciate Michael comments and his knowledge, but I don't agree with him on all area, but I learned a lot.

Not sure it is fixed. This the third time I have tried to submit this hope it goes.
Michael, I said Zero to 10 K because I was sure it would cover all possibilities and may save a trip to radio shack.
The reason I don't think it is fixed is this. You added a resistor to make the gauge read 60PSI. If the contacts in the sending unit are stuck you have a constant resistance value in circuit and you will read 60PSI all the. NOTE is they are stuck you will read max psi also. Most cars do not maintain a constant oil pressure at all temperatures and RPMs, So drive and check. If pressure is constant your sending unit is bad. You just don't know how much I appreciate Michaels belittling and sarcastic remarks. I was trying to help and I appreciate Michael comments and his knowledge, but I don't agree with him on all area, but I learned a lot.

Hopefully it will work. I am not sure, which I would explain below. I was wrong about the size of the resistor. I thought around 1K.
Michael, I said Zero to 10K because you might as well buy a resistor that would cover any situation. Might save you a trip to Radio Shack. Note 100mA is a low current circuit of course that is relative to what you have been working on.
Why I am not sure it is fixed. I said if you add resistance to make the gauge read 60PSI which is your know idle pressure and you drive the car and the gauge stays constant at 60PSI your sending unit is bad. I say that because I never owned a car that stays at 60PSI at all temperatures and RPMs. If it does stay at 60PSI it is probably because the contacts are stuck in the send unit and you just have a constant resistance in you circuit (gauge to external resistor and throw the resistor in the sensor to ground) and not measuring oil pressure at all. As for Michael sarcastic comment "When my car has 60PSI and the meter reads 60PSI then that means the sensor is GOOD". What is it reads 60PSI when oil pressure is 60PSI and reads 60PSI when the pressure is 20 PSI/40PSI/0PSI, is your sensor still GOOD? It is only good if it is correct or close to correct at different values. I was only trying to help and didn't deserve the belittling comments and sarcasm from Michael. Be sure and draw out the complete circuit and I think you will see what I mean.
I still believe your senor is bad so go drive and bring the car up to temperature and see if it stays at the constant 60PSI. Please let us all know. I am due to be right for once. Don't worry Michael I won't send any sarcastic or belittling comments. I will be content if the car is fixed and on the road.

I was wrong again. I was sure it would take more resistance and when I am sure I am normally wrong.
Note to Michael. I wasn't saying 10K but if you were buying a resistor why not go ahead and get a resistance range that would more than cover any situation so I said ZERO to 10 K and that could save you a trip back to radio shack.
The reason I said if it stays a constant 60PSI "while driving the car" the sending unit is bad, is because oil pressure normally does not stay constant as you drive the car. So if you add the external resistance and the gauge doesn't vary you are probably reading straight throw the external resistor and the resistor in the sending unit to ground. The contact is never breaking in the sending unit. So it would be like adding resistance between the gauge and straight to ground. And that may still be the case. Robert let us know if you see any variation in you oil pressure while driving and the car heats up, if not your contacts may not be breaking in the sending unit. Hopefully that is a good explanation and I think that is what the problem really is. If that is the case, I will be sure to post a Sarcastic remark like Michael does. Actually I won't because it is counter productive. My intent was only to help and my EGO is not so large that it gets hurt when someone has a different view. I think Michael knows his stuff and hopefully he will respond when I have a problem. I learned a lot from his comments but that doesn't mean I have to agree with all but I do owe it to him to respect his opinion. I have learned you learn more if you listen to everyone answers because you can always learn something for everybody.

John, I truly receive no pleasure in correcting you but you keep showing that you really don't know how the system works. Please don't make this personal, I'm just trying to help Robert solve his problem. As I said before, this is NOT a low current system. The sensor needs over 100 mA just to start working, and the meter needs more than that to read full scale. If you put a 10K resistor in series with the system the current would be about a hundredth of what it needs in order to show any indication. In order to get any reading you would need to drop the resistance to the 33 Ohm range that Robert suggested, so a 10K pot would not be a good place to start. Perhaps a 100 Ohm pot would work, but be sure it could take the current. However, I don't understand your test. If he has 60 PSI and adjusts the meter to indicate 60 PSI and it keeps saying 60 PSI then how does that show the sensor is bad? When my car has 60 PSI and the meter reads 60 PSI then that means the sensor is GOOD.

PROJECT COMPLETE.

Thank you Michael for staying with me and patiently walking me through the meaningful tests.

My original desire: Oil Gauge (60 psi) was pegged. Substituted 100 psi oil gauge - same result. Mechanic verified 60 psi operation with independent5 measuring equipment.

If you ignore my clumsiness, solution was very simple. Added resistance in series between oil sender and oil gauge. Created a simple wire extension with male and female plugs on each end. Open gap in center allowed me to splice in resistors.

Tried 68 ohms. Too large. Reduced current in circuit too much. Added second 68 ohm resistor in parallel, effectively reducing additional in-line resistance to 34 ohms. Everything works, but still displays too high. Looks like the proper answer is greater than 34 ohms, but less than 68. That is next project. Easy to do when I get more resistor parts/values.

Thanks again to all who helped with their experience and guidance.

Very respectfully, Bob G.

Did not say you could, just said it might be possible with an Analog meter. If you cannot you cannot. Seem to be hung up on me being wrong. Sending unit is a resistance varying device.

Let me give you one more chance to prove me wrong. I know it gives you so much pleasure. Robert's question on adding the 33 ohms resistor, his math is correct but since the meter is hard pegged it is not accurate. I believe most gauges are low current devices. So I suggest he putts a 0-10K ohms resistor in line with his sending unit. Set it for 10 K and start the car. Then adjust it for 60PSI on his gauge. Then secure it and drive the car. If the pressure stays at a constant 60PSI then the contacts in his sending unit are hung up. Don't like adding a permanent resistor.

Don't worry, I will give you more chances to prove me wrong.

Have a good day.

John, you just repeated what I just said, describing the PWM system. However, you were wrong when you said that you can measure the average resistance with an ohmmeter, either analog or digital. If you measure the resistance you will simply read the 30 Ohm resistor, no matter what the oil pressure is. That's because it takes 130 mA to heat up the sensor and make it open and start the on-off cycle. An ohmmeter won't do it.

Robert,

You math is correct but you left out one thing. The needle is hard pegged. If it wasn't pegged you would be correct. Most meters work on a very low current so I am confident that with the values you have already measured you will need a resistor in K ohms range. I would suggest you buy a variable resistor (up to 10K) from radio shack and put in line with you sending unit. Have it set for 10K and start your car then adjust the resistor until you have 60 PSI. Then secure it and drive the car for a while. If the contacts are stuck in you sending unit you will show 60PSI all the time.

Michael,

You are looking at it physically and I am looking at it functionally. It does vary the resistance. It does it between Open and 30 Ohms. Now what I said was AVERAGE resistance. I understand instantaneous resistance is going to Open or 30 Ohms. Of course that mean the sending units varies resistance. Agree? Now to go on functionally. Think about figuring the AVERAGE resistance of the sending unit in 10 second intervals.
1. Let say the unit is pulling 0.01 amps when the contacts are closed and 0 amps when they are open. So if the contacts are closed for 8 second and open for 2 second you would have pulled .08 amps of current. Using Ohms law V=IR 12v=.08 x R then R = 150 ohms. So average resistance for the 10 second period is 150 Ohms.
2. Now let reverse it. Closed for 8 second and open for 2 second. Than the AVERAGE resistance for that time period would 12V= .02 X R R= 600 Ohms.
So by varying the opening and closing of the contacts of the sending unit as you described it is varying the Average resistance.

I think you agree that at 30 Ohms that baby is pegged full pressure and at an Open no meter deflection. So it not going to work in either one of the static conditions. And I would be a whole lot of money that the design of this was based on the Average resistance/current of the circuit. It had to a been. They have a meter that required X amps to get full scale deflection. To get X amps that had to figure the average resistance in sensor to give you X amps. I content the design and the function is built around the sending unit have definite average resistance values for particular oil pressure values. Since it goes between 30 Ohms and Open it has to be based on Average resistance.

Michael,

We are saying the same thing. You have a fix resistance of 30 Ohms which allows some the amount of current for a fraction of a second. Then you have an open circuit which has no current for a fraction of a second. The gauge is a slow reaction gauge it will see some current for a short period of time then no current for some time. For less oil pressure the open period of time will be longer. For high oil pressure the closed contact period will be longer. So if you took a ten second sample of the TOTAL current flow ( say .14Amps for 1 second and 0 amps for 9 seconds than your total current for that time period is .014 amps and that will give you an average resistance over that time period. Using 12V the average resistance will be 12/.014 = 857 ohms. If we reverse the closed and open time then your total current for ten second (open for 1 second and closed for 9 seconds) is 1.26 amps. 12V/1.26 = 9.56 ohms. So by open and closing the contact rate you are effectively varying the average resistance. They use the slow reacting bimetal meter so as to average out the current through the meter. In a simple electrical circuit the only way to vary current is by varying resistance or voltage. Since the voltage is fixed, with minor variation in gauge, the sending unit has to vary resistance. It does that by varying the resistance between an Open and 30 Ohms. By using the oil pressure to time the opening/closing of the contacts, it is able to vary the AVERAGE resistance of the sensor. As far as the slow reacting gauge is concerned it is seeing a varying resistance from the sensor. You are right it is not a wire wound resistor with a contact arm, but it controls the gauge by varying AVERAGE resistance over a time period.

Now to answer Robert question on the 33 ohms. You math is correct but it will not work that way. If your meter was not pegged, you would be correct. Since your meter is pegged you don't know how much resistance you need. Since most gauges work on a very low current I would guess you are going to need in the 5K ohms area and that is a guess.

You can go to Radio Shack and but a variable resistor and I would suggest up to 10k ohms. Put that in line with you sending unit at the full 10 K and start the engine. Then adjust the resistor until you have 60PSI. Then secure the resistor so it doesn't short out. Drive the car. If you oil pressure stays at 60PSI constantly then you sending unit has to be bad and also you are not really measuring oil pressure.

Sorry John, I don't mean to pick on you but since these exchanges are stored in the archives forever it is important that the information be correct. Pages P.51 and P.52 in the service manual describe the operation of the oil pressure measuring system on these cars. They say that the sender is NOT a variable resistor OR a device that varies resistance. It is a FIXED RESISTOR that measures around 30 ohms as long as there is any oil pressure on it at all. When pressure is applied to it a switch closes and connects the fixed resistor to the meter. Current flows through the meter and the sender resistor, heating them both. Heating the meter causes the needle to deflect. Heating the sender causes the switch to open. The switch opening causes the current to stop flowing and stop the heating. When the heat stops the gauge needle stops moving up and the sender starts cooling down. When the sender is sufficiently cool the switch closes again and the heating process resumes. This switch-on, switch-off cycle occurs about once per second. Oil pressure changes the amount of heat that is required to open the switch. The more pressure, the more heat is needed to open the switch. With more pressure the switch stays closed longer, heating the meter more, causing the needle to deflect more. This seems like an unnecessarily convoluted way of detecting oil pressure, but that's the way Smiths did it, without using a variable resistance.

Also measuring the voltage would be more accurate with an analog meter.

One more retraction. I was correct when I said the sensor is resistance varying device. It is actually an average resistance varying device. It changes the average resistance based on the amount of time the contacts are closed or opened. That is why it has to have the bimetal gauge because it has to work on average current. So if you are going to measure resistance of the sensor with the engine running you have to unplug the wire and measure the resistance with a cheap analog meter and NOT a digital meter. Of course it still depends on the frequency of the open and closing contacts for the analog meter to work but it will be more accurate than a digital meter. Not sure what you were using. But if you measure full scale and half scale on the gauge as I described, actually at 60psi, then you will know what the sensor average resistance must be at idle,60PSI. So idle the car and measure the resistance with an analog meter.
It would be nice to know what another gauge resistance is required for full scale and 60 PSI. If your two measure the same then that would give you confidence.

Just can't shut up. Note: I would not trick the system by adding a resistor.

I have a feeling your contacts in the sending unit are not opening up.
NOTE: You can alway get some good information out of bad information.

Hello Michael.

Of course you are correct. It is one microfarad. My age plus weak eyesight incorrectly read the micro symbol (small u with a tail) as a small p.

Back to data.

On my car (71 E-Type) the +12V wiring to the gauge connects through a relay.

Voltage on each side of gauge with key OFF is zero. Gauge reads zero.

Key on (engine not started) on +12V connection side of gauge 12.19V. On opposite connection (wire to sender) 12.18V. Bounces slightly, so basically identical. Both measurements made to ground.

Voltage on sender, key off (nothing). Key on (relay closed) 12.19V. With engine running: voltage immediately drops from +12V to about 3.42V.

Oh yes, the meter is pegged. Clearly I must have had a loose connection earlier.

Hopefully, I did this correctly, and haven't tried your patience.

Since we have 60 psi with engine idling (proven) and both the 100 PSI gauge is pegged (so is the 60 psi gauge when swapped), I would like to add resistance, in series with the wire from gauge to sender, to have the gauge display about a 2/3 reading (meaning 2/3 of 100 psi.). We know the 60 psi results in 3.42V to ground across the sender's. That means we are dropping the balance (+12.5V - 3.42V equals roughly 9V) across the gauge. We measured the gauge resistance to be approximately 62 ohms (you would know best based on the experience). So, I think a 33 ohm resistor would get the desired result (see rough picture below). What do you think of my solution?

Gauge Sender
+12.5V ---/////---o---/////---o---ground (when engine running)

+3.4V to ground

added
Gauge resister Sender
+12.5V ---/////---o---/////---o---/////-----ground (when engine running)
68 ohms 33 ohms
+3.4V to ground

A little correction the sending unit is a pulsating unit. So it lets current flow based on the amount of oil pressure. So the more pressure the longer the current flow which allows the gauge to heat up more and deflect more. So it is constantly pulsation. So it is not a device that varies resistance. But I am not sure how you can figure out what the actual resistance should be in the sensor. You can figure full scale, half scale with the variable resistance and the gauge but I don't know how you can figure fixed resistance for the sensor. Michael will probably know. I am signing out. Screwing up way too much.

One other way is to find the specification for the smith gauge. That would tell you amount of current for full scale operation then you could easily figure from there. I haven't been able to find the spec.

Robert, I know I have lost all credibility but here is a suggestion. You have the spare gauge. Using a variable resistor in line with the gauge and a 12 V source adjust the resistor for full range on the gauge. NOTE: start with high resistance on the variable resistor. Disconnect the power source and measure the resistance in the variable resistor. Then reconnect and adjust the resistor for mid scale on the gauge. Disconnect and measure the resistor. This will tell you what the oil pressure sending unit should be measuring for the reading you tested at. I have to be careful how I word this since Michael will jump me, but the oil pressure sending unit provides varying resistance based on the amount of oil pressure. NOTE: I did not call it a variable resistor, but a device that varies resistance.

More likely a 1 uF capacitor which is used for radio interference suppression and will not effect the operation of the meter.

Oops. should have read 1/2 (0.5 inch diameter).

Also, before I go out again, there is a large (maybe 1.2 inch diameter by 1.5 in long) , 1 pF (pico farad?) capacitor from the 12V gauge's terminal to ground. I tried testing both ways (with it connected and disconnected). No difference in operation.

I will do so. It is 97F here, and even hotter in the garage, so I can only work 10 minutes at a time.

Your explanation of meaningful measurements is straightforward. I will complete these today.