Speedometer Problems - 1988 XJS

There is a sort of triangular bracket underneath the glove box. On top of it is a black box a little larger than a pack of cigarettes. It has two connectors, one a single and the other has about ten wires. Unplug them and try the speedo. If that doesn't gain you anything, you'll need to trace the wire between the pulse generator at the trans and the connector on the back of the instrument pack. The wire between the two should be yellow. Connect an ohmmeter and see it there is continuity, should be below 1 ohm. Also check the green wire at the instruments for 12 volts when the key is turned on.

Steve, how do you disconnect the cruise control? I tried taking the fuse out but that did not help. The speedometer still did not work. I also tried unplugging the switch. That also did not help. Thanks. John

Geez, I should have remembered that! Old age, I tell ya.

1988 model run began at 139052 and ended at 148781. Mid-year the switch was made to the differential mounted speed sensor. For early '88 cars the '87 wiring was continued.

So John, scratch the interface theory. You might try disconnecting the cruise control module that is located under the glove box. The wire from the pulse generator in the transmission that goes to the speedo also goes to the cruise control. I have seen the module fail and take the speed signal down. Beyond that, you are about down to doing some wire tracing to find out where the signal circuit might be interrupted.

Sorry for the wrong turn!

Good luck!

Geez, I should have remembered that! Old age, I tell ya.

1988 model run began at 139052 and ended at 148781. Mid-year the switch was made to the differential mounted speed sensor. For early '88 cars the '87 wiring was continued.

So John, scratch the interface theory. You might try disconnecting the cruise control module that is located under the glove box. The wire from the pulse generator in the transmission that goes to the speedo also goes to the cruise control. I have seen the module fail and take the speed signal down. Beyond that, you are about down to doing some wire tracing to find out where the signal circuit might be interrupted.

Sorry for the wrong turn!

Good luck!

Actually, the trunk mounted speedo interface is used only on the cars with the differential mounted speed sensor...which was used starting with VIN 144263

Cheers
DD

John,

I'm pretty sure that the AllData reference is probably to the "Speed Interface", which is located in the boot [trunk] on the LH dog leg. The dog leg is the black hockey stick shaped bolted in panel that is covered by the LH front carpeted trim. As I recall, it's a black box about the size of a pack of cigarettes. Should have seven wires to it, 2 Red, 2 Blue,1 Green and 2 shield wires which are usually black. Sometimes the shields might look like they have some tin foil on them.

The speed transducer [in the transmission] outputs a signal to the interface, then it is split and sent to the instrument pack, the fuel interface and to the trip computer in the dash. The instruments need the signal in a different form than the other electronics, hense the need for the "speed interface".

Good luck!

Gentlemen:
Thanks to all for your help. Here is the latest. I am still struggling with the speedometer. I have replaced the entire instrument cluster and the transducer and still no luck. I recently read Service and Repair instructions for the Speedometer from AllData. These instructions recommend replacing the "Speed Sensor" found in the "LH boot liner" I would appeciate it if you would help me with a couple questions. I do not know what is meant by the "boot'? Is that the trunk? Is the "Speed Sensor" the transducer or something different? Do you think that the Speed Sensor may be my problem? Does my car VIN 139393 even have a Speed Sensor?
Thanks again for all of your help.
Sincerely,
John

Gentlemen:
Thanks to all for your help. Here is the latest. I am still struggling with the speedometer. I have replaced the entire instrument cluster and the transducer and still no luck. I recently read Service and Repair instructions for the Speedometer from AllData. These instructions recommend replacing the "Speed Sensor" found in the "LH boot liner" I would appeciate it if you would help me with a couple questions. I do not know what is meant by the "boot'? Is that the trunk? Is the "Speed Sensor" the transducer or something different? Do you think that the Speed Sensor may be my problem? Does my car VIN 139393 even have a Speed Sensor?
Thanks again for all of your help.
Sincerely,
John

John call Coventry West (one of our sponsors--bet they have 100 of them!

To All:
I have replaced the transducer and the printed circuit board. The speedometer is still not working. When I had the cluster out, I took out the speedometer and tried to test it by running 9 volts through it. The speedometer did not move. I am now going to replace the speedometer only unless anyone else has any ideas. I have placed an ad for a used speedometer. Does anyone know where I could buy a new speedometer only?
Thanks again for all of your help.
John

Edited on 2010-08-27 11:14:22

Edited on 2010-08-26 19:08:04

To All:
Thanks much for all of your comments. I took out the instrument panel and took off the plastic cluster backing with the copper spider wiring running throughout it. I cleaned all of the dirty copper with alcohol. I also cleaned the plastic base of the instrument panel which was also dusty and dirty. I put two redundant grounds, one just for the speedmeter and one for the whole cluster. I thought for sure I had the problem solved. I put everything back together, and the speedometer is still not working. Could it be that I just have a bad speedometer, or a bad cluster? I might add that my trip computer was previously counting mileage travelled but it is no longer doing so. It was. The trip computer is noting gallons used. Did I miss something when I put everything back together? How would my work on the instrument panel affect the trip computer though? Also, since cleaning the cluster my oil pressure gauge is maxed out. This one is testing my patience. Help please???!!!
P.S. The tach is working. The cruise control is not working but hasn't been working for way long before the speedometer stopped working. I understand I can purchase a new cluster for about $75. Should I do that?

Almost as Doug said but the clip is at the top in the centre, remove this then the the other two are on the sides, there isn't one on the bottom, all phillips

It's been a few years since I've done this so I can't remember every detail, but....

In each upper corner of the cluster this is a small plastic cover which can be popped off to reveal the cluster mounting screws. Then there are three screws which hold the cluster trim in place...one at the top and two on the lowers sides. This is the rather flimsy trim that runs right against the cluster lens.

Remove the mounting screws and the flimsy trim...and I think you'll be ready to *gently* tilt the top of the cluster outward and wriggle it away from the dash. It'll only come out a couple inches so don't force it.

If I have left anything out someone will come along and correct me. What I can't remember for sure is if there are mounting screws at the bottom of the cluster.

The connector release tabs are pressed, not turned. You can just barely press them down then wriggle the connector a bit to free it from the cluster. There's a big connector and a small connector

Cheers
DD

Doug, thanks for your help. Where are the cluster hold down screws? I think I removed them and I think I located the tabs. Are the tabs black? Do you turn the tabs?

Thanks, John

If the trip computer is recording miles driven then the transducer is working...as both the speedo and the trip computer (and cruise control, for that matter) take their reading from the same transducer.

It is common for the contact points on the instrument printed circuit to become corroded. Remove the instrument cluster and gently clean all contact points as well as the tabs where the p/c connects to the main instrument harnesses (two--one big, one small). You'll have to carefully remove all the little nuts that hold the p/c to the back of the cluster. I do 'em on at a time. Be gentle ! Add a redundant ground as well. This is a good time to replace any burned out cluster bulbs.

The main cluster connectors are tricky to release as someone mentioned above. After the cluster hold down screws are removed wriggle your hand behind the cluster and you'll feel the release tab on each of the connectors. It gets easier after the first few times :-)

For the record, the change from transmission mounted to differential mounted speed sensor/transducer took place part way thru the 1988 MY at VIN 144xxx or thereabouts.

Cheers
DD

John, these transducers have been known to be defective right out of the box.......I would return it and try and find one known to be good..

The instrument panel has one or more plugs that plug into the main dash.
Now, depending on year, they are fixed or on pigtails.

You should be able to pull the transducer back out, connect your voltmeter, and spin the gear (both directions) and see if you have a reading.

There is always the possibility that the trans gear is bad or slipping, failing to turn the transducer gear.

Now, I can't recall if the speedo on an 88 is internally electronic or if there is a mating transducer plugged into the back of a standard speedometer.

Seems I remember that.
From somewhere.

Is this the model with the transducer mounted on the differential?

I purchased a new transducer. It had a gear. I am assuming that it worked. I did use a voltmeter and tried to test the output from the transducer by getting the pins from the voltmeter and touching the voltmeter pins to the wires from the transducer under different scenarious and even with just the positive voltmeter pin to each transducer wire and the negative voltmeter pin to a separate ground. No luck. Zero readings under all scenarios. Either I did the test wrong or there is no voltage. I only did it with the engine running - - no wheels turning. Considering that I my fuel gauge is not working, the odometer on the speedometer is very important. I rely on the odometer to determine when to fill my gas tank. Although, I am now relying on the trip computer's consumption of fuel information. I looked through the ROM re the speedometer repair and did not see anything about testing the voltage coming out of the transducer. The ROM does mentiona cable. See section 88.30.01. Any additional help/comments would be greatly appreciated. Where is the multiple pin plug you speak of? Thanks again.
John

If you replaced the TRANSDUCER, it means you have no CABLE.
Speedometer is electronic.
Not sure what you are missing to have the instrument panel come out, but generally, on other marques at least, once you get everything else loose, it's the multiple-pin plug, and they're just stiff sometimes.

When you replaced the transducer, was it with or without a gear?
If I recall, there were two types, but don't ask my anymore which one fit which car.
Do you know if the gear is good?
Or that it actually turns?

There should be a procedure in your book for placing a voltmeter at the ends of the plug from the transducer and measuring voltage as the driveline turns.

Could be a corroded ground.
Later cars the signal goes through the on-board computer, can be there or the gang plug into the computer.
But I think on yours it is direct.

Personally, before I broke into the dash, I'd want to know if I really did have an output from the transducer.

Dave