My speedo was acting goofy (bouncing erratically) before it stopped working completely. It was bouncing around and wouldn't hold a constant speed before it quit. All the other gauges work fine

I now have the instrument cluster out of the car and have cleaned the contacts of both the plug and the printed circuit board. Is there anything else I can do before putting it back in in the car?

Getting it out was a pain. I don't want to do that any more times than absolutely necessary.

 

I would be looking at the transducer on the transmission before the speedometer itself. When the speedo was bouncing, try the cruise as both the speedo and cruise get their speed signal from the transducer.

 

Thanks, after doing some reading I think you're right about needing a new transducer.

I didn't understand what you were saying about the cruise. Mine doesn't work. Could the transducer be the culprit? I have a new solenoid unit on the car and have sealed the bellows with silicone.

There are a few jags in a local junk yard. Just have to figure out how to get to the transducer now. Thanks.

 

For the cruise, the transducer could indeed be the culprit. The speed signal into the cruise ECU comes from the transducer, so it won't work if it doesn't get a speed signal. There are other possible things too, the brake pedal switch was a culprit on mine - note there are 2 switches, the lever arm one is for the brake lights, the plunger is for the cruise. Mine shorted internally against the metal case, so the cruise ECU was seeing that as the brakes on, so wouldn't engage.

The transducer is accessed from underneath the car, left rear of the transmission. It's a big nut - maybe 1 1/4" from memory? Undo that and unplug the connector and it comes out. The V12 and 6 cylinder items are not interchangable.

There was a change, one type up to VIN 144262 and another after that. I think about 1988 there was a change to a diff mounted speed sensor? I'm not sure what year the change was, and if you have a transducer or diff mounted pickup. I'm more familiar with the V12 Series III sedans than the XJS in that regard.

 

Another clue would be the trip computer giving wacky readings, as it gets a signal from the transducer as well.

The 144262 VIN break signifies the change from trans-mounted transducer to diff-mounted speed sensor. This would've been mid-1987 calendar year, 1988 model year for the USA market cars.

Cheers
DD

 

According to the ROM HE Supplement page 86-13 the cruise control has a separate pickup on the rear suspension cage with magnets on the driveshaft, the TC and speedo come from the speed sensor. VR sensors are usually pretty reliable I would check the connections before you replace it.

There is a PDF here that may help with checking the cluster.

https://www.jaguarforums.com/forum/x...2/#post1798518

the only other circuit I can find for the CC is the 1989 MY supplement with the change to the diff mounted speed sensor.

 

That for the earlier cars with the cable-driven speedometers. When the change was made to electric speedometers (1983-84?) the cruise control operated off the signal from the speedometer transducer

Cheers
DD

 

Thanks,


My car is an early '87 model year built in Oct '86. VIN 138xxx.


The speedometer transducer seems like the most likely suspect at this point and the most logical place to start. Potentially replacing that could fix two birds with one stone.


Now back to the no-start issue (hopefully this weekend it is finally solved) and then trying to run down some help in England before the holidays with obtaining the bracket, pulley and manifold pipe hole plugs from and English junk yard for the anti-smog air injection removal project.


I really believe I am very, very close to having everything back and running correctly. Of course my belief could just be delusion masked as optimism.

 

Hi Brad

If you thought that getting the Binnacle out was a problem, that would be a 'Walk in the Park' compared to removing the VR Sensor

'if' you have the type that's in the Back Axle.

Where you 'Might' have to drop the Cage to get it out, unless you're doing nothing for the rest of the Week, as its a PIA to get to

And as Warrjon has already said, its unlikely to go wrong (or be the problem)

Much more likely to be a dirty connection where the Two Plugs go into the Sockets in the Printed Circuit, in the Back of the Binnacle

Or it could equally be a Bad Earth in the Back of the Binnacle, so if the Binnacle is still out you could add a Supplementary Earth Wire

Since your Cruise Control sounds a bit 'iffy' with punctured bellows etc, under those Circumstances the Cruise Control could 'possibly' have an

Unrelated Problem to the Speedo

So if you can get the Binnacle back (just pushed back in the hole enough to test it)

Take the Car out for a Drive and Check the Reading on the Milometer on the Speedometer against the Reading on the Digital Readout on the Computer 'Thingy'

Which updates in real time, as you drive along.

Both readings should be exactly the same to within about One Mile

If they are any different then its probably a bad connection in the back of the Binnacle

But since you've got a Scrapyard full of Spares where you are, then it might be a good idea to come back with another Binnacle

(unless the different mileage reading already on the Binnacle) would be a problem?

It should only take 10 Minutes to get that Binnacle out and I have done it on a Steering Column with a drop down Lever and on another one that's got a Collar

If your getting one out of a Scrap Yard, its easier to take the Steering Wheel off, which is about another 10 Minute Job

 

Other way around, the VR sensor has to come out before the rear IRS is dropped. It easy to get out, a single bolt. There is enough space. However, he doesn't have that type.

 

Hi Jagboi

If I could have got my Transducer out I would have but it was such a nightmare to get a Spanner on it I didn't bother

Just as well, as it was a Binnacle problem and not the Transducer, so that would have been a lot of work for nothing

 

I could not get mine out either there is not enough room without lowering the rear cage, until I found the inspection plate in the boot - OH well.

 

I looked everywhere for that diagram but could find no circuit that used the trans mounted VR. Do you have a link?

Also does the speedo/TC use the same pulse count as the CC, I know in the later cars with diff mounted speed sensor the CC uses a different pulse count than the speedo/TC.

 

This is the XJ6 diagram, but should be similar to XJS. I know in the middle 80's XJS and XJ12 used the same transducer.

Speedo, Cruise and Trip computer all use 8,000 pulses per mile. I added a trip computer to one car and it was accurate using the transmission mounted transducer

 

Now that you mention it, I recall getting the bolt out no problem and unplugging the lead, but I left the sensor of the diff cover.

 

All,

thanks for the valuable input. My car is an early '87 so it sounds like the Transducer will be on the left side of the transmission and not back on the differential.

The instrument cluster (Binnacle) is out so that I could clean the contacts on the printed circuit board and the plug. Should be no issue just connecting it back up and giving it a test (once I get the car started again) before I go blindly pursuing replacing the transducer. Its worth a shot anyway.

What is the VR?

My cruise control bellows is fine. It does not have a hole in it. I only put silicone around the edges to ensure a perfect airtight seal. I also replaced the front actuator plate (with the electrical connectors) so I'm relatively sure that is not the issue.

 

Variable Reluctance. It's the type of sensor that is used in the diff mounted sensor. Usually it will generate a sine wave based on close proximity to a toothed wheel going past it. These are used for ABS sensors in newer cars, and for crank position sensors.

 

You can test a VR sensor with a volt meter (DVM) it's not definative but will give you an idea.

A VR sensor generates a 5volt peak to peak sign wave as JB said. The one in my 4L60 which is similar to the TH400 unit generates 5v p-p which is 3.5volts RMS as you would read on a meter on AC volt range.

If you jack the back wheels up so they can rotate. Use the DVM on AC volts and set the car going at about 40km/h (25 mph) you should measure about 3.5 RMS volts on the DVM anything much less and the VR sensor is most likely bad.

 

The transducers have a history of being intermittent and a bit temperamental. I originally though it might have been related to heat, but I've experienced drops offs in all conditions. A friend of mine did describe a procedure that actually worked for him:

1. Buy a new transducer.
2. When it arrives, take it to the garage and present it to the Jaguar.
3. Put the new transducer in the glove box and leave it there.

After that he had no trouble with the old unit ( still in place!) until he sold the car.

 

HAHAHA........the sad thing is there is probably more than just a grain of truth in that approach.


The contacts and plug connector points are all cleaned and ready for re-install. If it doesn't work then I'll move onto getting a replacement transducer.


A transducer replacement will hopefully get the cruise control working again as well.