Oil Pressure Gauge

Your initial problem was that the meter pegged. Now it doesn't move. Since I can't be there I need to ask questions. Did you reconnect the wires after you took the resistance measurements? Now take six measurements. Measure the voltage to ground from the two meter terminals and the sender terminal with the engine stopped and ignition on, and with the engine running. Let me know what you find.

Hello Michael;

Bob Grisar writing.

Still chasing the solution to my desire to obtain a mid-range reading on the oil gauge.

Here is some data.

With wires to gauge disconnected

Resistance of sender to ground (engine off) - infinite

Resistance of sender to ground (engine running) - starts at 62 ohms, then slowly drops to 29 ohms (slight wobble, likely due to engine rpm wobble).

So, needle should move? No. Not it is stuck at 0 (zero).

I am confident that sender is working, especially based on resistance change with engine running.

Meter reads 68 ohms without any connections.

So, total circuit is 97 ohms (68+29). Infinite when no pressure (engine off).

When running, gauge should read 60 psi (verified by mechanics equipment) when oil pressure is maximum at sender. So, drop across meter is 68/(68+29) X 12V = 8.4V.

Meter stays at 0 psi (I am scratching my head).

Wish you were here.

Best regards G.

Hello Michael.

I am very pleased to learn that your oil pressure issue is the sender and not the engine.

Please accept my thanks (again) for your help and continued thoughts of my oil pressure gauge issue. As you have before, your review is spot on and very much appreciated.

Very respectfully, Bob G.

Well I managed to get a wrench on my sender and moved it 1/12 of a turn. Luckily that was loose enough that I could unscrew it the rest of the way by hand. I cobbled together an adjustable pressure regulator, a pressure gauge and the sender and connected a spare meter to the sender. When I cranked the pressure to 50# the meter needle crept up and stopped at 10. Shorting the sender sends the needle to the top of the scale. I put a 'scope on the sensor and see that although it is indeed turning on and off it is very erratic and spends most of its time off, which explains the low reading. So it looks like I'm going to need a new sender too. Oh well, it's better than needing a new engine.

BTW, my money is also on a new-out-of-the-box defective sender. :-)

Yes, I saw that it was confirmed by "the auto mechanic testing equipment" but that left me with a question in my mind as to what this equipment consisted of.

Hi Tom,

He did that and told about it in the first line of the first post.

I haven't seen my method, of confirming the oil pressure, addressed yet on this thread. I had a sender unit fail (not new) long ago and I then purchased a mechanical oil pressure guage equipped it with the necessary fittings, and attached it to the engine. I found that the sender was faulty. I ruined another one myself by not protecting it while cleaning the engine.

The pressure sender and switch are mounted on an aluminum casting that bolts to the side of the engine, like yours but without the oil filter. By removing the exhaust and a panel I can see it but can't swing a wrench to remove either one. I then decided to undo the 4 screws and remove the casting from the block so I could work on it on the bench. The four screws are out but the subassembly is thoroughly stuck to the block. Using a long rod and a hammer I whacked it a few times to no effect. I quit last night before I could do any damage. I wonder if oil pressure would blow it off if I started the engine? Anyway, today I am pursuing another tack, installing a mechanical gauge by drilling out one of the bolts that plugs the holes in the oil gallery that runs along the side of the block. I got one out but will need to use copper line since the exhaust manifolds are right there. I'm off to the plumbing supply store now.
Considering the cost of those senders no one should ever have to buy 2!

OK. That is unfortunate. New oil pressure sensor purchased from XKS.com 4 months ago. Oh well.

Any progress diagnosing the oil pressure issue on your car?

Sounds like you need a new sender.

Hello Michael.

Per an earlier suggestion, I made a simple wire harness. One wire from +12V to my spare 60 psi meter, a second wire from my spare meter to the connector on the oil sensor. Four pictures are included. (1) the wires attached to the spare meter, (2) the meter face, key on, car not started (reads zero), (3) car started, picture of in-process movement of gauge, about 1/2 way plus (as quickly as I could take the picture, (4) and last, picture of gauge, motor running about 10 seconds - pegged. The dash board gauge and its wire to the oil sensor are not connected in this test. The gauge reads high without regard to engine rpm.

All I know is what I read and it's all in the owners manual, which is the first place I look when I have a problem with one of these cars.

OK Michael you make me look bad again. I assumed again the meter motion was like an old analog voltmeter. I am going to have to google the meter.

I'm sorry but I can't let that one pass. As described in the manual, the meter is simply a heating element wrapped around a bi-metallic strip. Current passing through the wire heats the strip, making it bend and moving the needle. The polarity of the current doesn't make any difference since it makes heat no matter which way it flows. The needle can not possibly move backwards. However, John's troubleshooting suggestion is a good one, just don't short anything while doing it. You might also measure the resistance of the sensor with the engine off. It should be an open circuit.

Hello John. I like your suggestion. I will give it a try this afternoon or tomorrow. Thanks for the description of what would happen if I reversed the wires on the gauge. Yes, that would be bad.

Reversing the wire "should" make the needle deflect the other direction so it would be pegged hard in the "0" direction. I have been following this and it is a simple circuit, but there is something strange going on. When you find the problem it will be so obvious you will wonder how you ever missed it. I have worked in electronics for over 30 years and it is amazing how hard some simple problem are to fix. Normally because it is something you have discounted as not possibly being the problem and it is the problem. Try wiring in your extra gauge as I suggested earlier. That will totally isolate the oil pressure system from anything else in the car.

This is an incredibly simple circuit. +12V, two resistive elements in series, and ground. Is it possible the wires on the gauge are reversed? What would happen if I reverse connections of the +12V (green wire) and the wire that runs to the sensor (white-brown)?

What may be a good way to test it since you have already have an extra gauge.
Remove the wire from your sensor.
Run a wire from you sensor to your extra gauge.
Run a wire from hot side of your battery to gauge.
Now you have bypassed everything in car but the sensor. Start the car and see what you have.
NOTE: Make sure you have good connections and don't short it out. I will buy a spade connector for the sensor and terminal lugs for the gauge and the battery to ensure.

I am not sure about measuring resistance. If you are measuring resistance it is best to isolate whatever you are measuring. ie discount both wires to the gauge and measure across the terminals. Discount the wire on the sensor and measure to ground. But after Michael corrected me on the sensor being a variable resistor, I did some reading and I am not totally sure how it works as far as controlling current flow through the gauge.

Something is wrong somewhere. If your sensor has about 60 Ohms and the meter has about 60 ohms and they are in series across 12V then there will be 6V across each of them and the meter will read very high just sitting there with the engine off. But you say that is not the case. It's a very simple circuit with only two parts. If I were there I could pinpoint the problem in maybe 2 minutes. Since I can't be there you need to be my eyes and hands. So with the engine off, wire disconnected, what does the sensor read to ground?

Key on, don't start, reads zero.

What does the oil pressure gauge read when you turn on the key but don't start the car?

Hello Michael.

I made three measurements.

(1) Oil sender to ground (no wire connected): 66 ohms.

(2) Across oil gauge, removed from car, no wires connected: 62 ohms.

(3) Wire that attaches to oil sender to wire that clips onto oil gauge: less than 1 ohm.

Reconnected wires, installed gauge. Started car. Oil gauge climbs slowly from 0 psi to 60 psi, then pegs at oil gauge case housing. About 5 seconds.

Any luck checking the oil pressure on your car?

Hi Robert,

It was Robert who gave you the 28 Ohm figure and I who told him he was wrong about that.

Disconnect the wire to the sender and measure the resistance from the sender terminal to ground. It should be an open circuit between the terminal and ground without the wire disconnected. If it is anything else you have a bad sender. Leave the wire disconnected and isolated, and turn on the key. The meter should not move. Let us know what the results of these tests are.

Hello Michael:

With engine off, I measured +12V at oil sender (using trouble light). Then measured 66 ohms from oil sender's terminal to ground. Measured 62 ohms across oil gauge. These were different values that the 28 and 60 ohms you earlier offered. With engine just starting, dashboard gauge moved slowly from 0 to maximum deflection (about 4 seconds). Used formula 12V * 60/(28+60) ~ 8V. Substituted values for about 5V. This should be simple, so obviously it is something I am doing or not doing correctly.

Sorry to learn about your car Michael. I hope your oil pressure issue is only a gauge, sender, or wire. Thanks for explanation (and photo) of sender. Plan to check wiring, resistance, etc. this weekend.

No. If you look carefully you will see that the two tabs are really formed by a single U-shaped piece of metal with a rivet in the middle, so there is really only one connection. The whole point of having an idiot light as a backup oil pressure indicator is to have a completely separate system so if one fails the other will still work. Wouldn't you know it, on my car either they both failed or I lost oil pressure. That's why I'm not driving the car right now.

Hello Bob F and Michael Eck:

When I bought and installed my replacement oil sending unit from XKS, it came with two electrical connections. I am guessing one is the variable output, and the other is for an idiot light. Is this possible?

Bob F. I appreciated your posts. I printed off this whole thread and put it in my shop manual along with the other good tips I have seen on this forum over the years. Thanks

David Barnes
68 E-Type FHC

In order not to mislead anyone else with bogus information I will just keep my mouth shut from now on, I was just trying to help

Hi Bob,

If the sender measures 28 ohms with no oil pressure and the meter measures 60 ohms and they are in series with 12 Volts across them then there should be about 8 volts across the meter, which would measure full deflection with no oil pressure. Since that's obviously not true there must be a flaw in your explanation. If you look at pages P.51 and P.52 you will find a detailed explanation of the gauge operation. The fuel level sender is a variable resistor but the oil pressure sender is not.

To expand on William answer: If you turn the ignition switch on (not starting the car) and the oil pressure goes full on the gauge, remove the wire from the sending unit. If the gauge stays full then the wire between the sending unit and the gauge is shorted to ground. If the gauge goes to zero then the sending unit must be shorted internally.

Something to consider:
On Series 1, the battery + goes to the gauge first, then to the sender.
If you have a ground, you will read full oil pressure.
There is a contact in the sender that closes with some oil pressure; so with NO pressure (engine switch on, but engine off), you should read a open contact (figure 51 in the manual, page P.52) and zero on the gauge.

Glad to be of help.
here are a few more reference points. The sender measures about 28 ohms cold, engine off and disconnected from the wire on top. I did not start the car as I suggest you see if yours is close to this to begin with.
second, the smiths gauges are all basically the same design, that is the fuel, water temp and oil pressure. disconnected they should read somewhere in the range of 60 ohms across the terminals.

if you apply 4.8 volts you should get a reading about half way. or if you look close there are usually two small dots on the bottom of the gauge, this is the calibration point.
7.6 volts should put it at the point up near max deflection, or a full tank on the fuel gauge.
and for those keeping score, 2.0 volts is the dot down by empty.

you can hook your gauge directly to a variable DC source if you have one to see if you are close to the factory settings. the gauges can be calibrated, but its crude. and if you are anywhere close I would suggest you dont mess with it.

SO. since you know you have 60 psi from the other test. here is something else you can do to check the sender.

using the same DVM. measure the voltage on the meter with the car running. what I mean is the voltage between the input wire from the sender, attach the black lead from the meter to a ground, then with the red lead measure the voltage on both leads of the gauge one at a time. one will be around 13-14 volts, or battery voltage, the other should be around 7.6 (ish)since you say the mechanical test was 60 psi.
if you have less pressure it will read a lower voltage.

thats all for now, or I am going to start sounding like Cliff Claven. or James May from Top Gear for the younger crowd.

Bob F

you might consider that maybe the oil pressure is excessively high...I am looking into that issue on my car right now

two gauges reading the same thing raise suspect....

when the car warms up, does it drop at all? Does it move at all from ?

Hello Bob F. Your short summary is EXACTLY the information I needed. Thank you very much. I searched high and low over the Internet, and no one, no where, summarized the test(s) as succinctly as you have, in the sequence you presented. I will post the results in a few days.

Robert, this may get long, but the simple answer is that wont solve your problem, it will just give you a fixed reading and if the actual pressure drops you will never know.
in a properly working system the sender is a variable resistor and should do what you are asking for.

I will try to break down how the system works in 28 words or less...

the gauge is a simple voltage reading device, its a volt meter that senses the voltage across the terminals. the sender at the engine is a variable resistor, as the oil pressure increases the resistance decreases , this causes the gauge to read higher. this is because the gauge and the sender are across a 12 volt source, as the pressure goes up, resistance drops and more voltage seen at the gauge.

what you need to do is verify a few more things, you already ruled out the gauge not functioning. there are more tests you can do to the gauge if you have a volt meter and variable power supply. if you dont have a volt/ohm meter go to home depot, or harbor freight and buy an inexpensive DVM (digital volt meter)

first thing I would do is to remove the wire from the oil sending unit and connect one lead of the meter there, attach the other to ground, put the meter in OHMS x100 for a start.
you should have a high number ( I dont know what the factory resistance is off hand)
if it reads ZERO or very close I would suggest you replace the sending unit.
if you get a reading of any number, start the car, as the pressure goes up the number on the meter should go down. if not, replace the sender.

now, still more to check, the system runs off the high side, or battery voltage side of the regulator on the back of the gauge panel. use your new meter to make sure one side of the oil gauge has the same voltage as you would measure across the battery. they tap this off the unregulated side of the little regulator at the top left. you should see a wire connected to both

another test you can try, and maybe even easier than reaching into the engine to measure the sender is this.

set your meter to DC volts in a range that wont have +12 Volts go off the meter, so maybe a 50 or 100 volt range, depends on the meter. now put the red lead on the gauge terminal that gets the +12, put the black lead on the other terminal.

what voltage do you get? now rev the motor, does this change at all? if not, we need to find out why. could be the sender, could be a wiring problem..

when you tell me it pegs both meters I am going to guess you have +12v across the meter all the time. either the gauge wiring is not correct or the sender is bad, verify the sender works by measuring it as I show above, if the sender has a variable reading when you try the resistance measurement then the wiring to the gauge is wrong and somehow the gauge sees +12 v all the time.

If you get stuck, let me know, I can make a few measurements or take pictures for you.
but if you dont have a cheap volt meter go get one first.

Bob F