hi all,
well finally after 2 months I have got the XJS off of the axle stands (lots to tell, will post here ASAP!)
Anyway, took it for a brief spin yesterday and the speedo wasnt working!
I have basically done a head rebuild, head gasket, new waterpump, new o-ring in oil return/out hoses and undersealed the car. Non of which surely would affect the speedo? On my XJ40 the speedo sensor was in the back of the diff, but even in the dark shadows under the car I am pretty sure I never undersealed near the diff..? So surely it cant be that thats stopped the speedo from working.
Anyone any other ideas? Thanks

 

What year XJS?


Cheers
DD

 

Oops sorry its a 1991, about the 100th-ish Facelift made *grins*

 

Besides the diff mounted speed sensor there's the trunk mounted interface unit. I'm not sure how to test either although a volt/ohm meter could be used, prehaps, to test the sensor.....just to see if it's flat-out dead.

Poor connections at printed circuit on the back of the instrument cluster are rather common. You could remove the cluster and gently clean the contact points. Add a redundant ground while you're in there.

Cheers
DD

 

Hia,

Thanks Doug will have a look around. Had no idea about the trunk device ... where is that? I know the speedo sensor in the XJ40 can be mounted into the diff thro the back of the boot.

Ugh, I dont know, this is the 3rd car in a row I have owned (2 of which were Jaguars haha!!) that have had speedos stop working! This is probably the most peculiar tho as it has just stopped out of the blue, the XJ40 just stopped working intermittently, but this has given no clues. Maybe it is just a matter of seizure as it hasnt been used for 2 months, I will give it a blast up the road properly and if to no avail then its dash out and sensor clean up lol!!

 

I'm just curious, does your cruise control and/or odometer still work? My speedometer, cruise and odo don't work ('82 XJS), and I've learned that this is an indication that the speedometer transducer could be faulty. They fail pretty commonly on our model I've been told.
Mat

 

1992 XJS problems. Speedometer now is pegged past 150. Disconnecting battery makes no change. Anyone have ANY idea what might have happened. Happened while changing out radio of all things??????????

 

Doug,
1992 XJS. If you will help me out here it would be appreciated. Installed a radio last night and removed and old Jaguar installed Radar detector. May be completely unrelated, but Speedo now reads 160, off, battery disconnected, battery connected, doesn't matter. Any idea what I might have screwed up.

 

Cleaning those contacts on the insrtrument panel may very well be it . On my 91 xjs conv. I took it out to get my blinker indicator lights to work and found the bulbs to be fine. Cleaned the harness contacts, put it back and only one side started working. I then took it out again and wiggled the harnesses while plugged in and got the other one to work. My real point is that after doing this, I put the dash back and found that my irratic fuel guage now worked fine, so, after this I've concluded that the best advice for anyone with an xjs new or old is to CLEAN as many of the contact points that you can. Disconnect and inspect EVERY plug check them plug em back in, some, may be corroded or dirty but, mostly the ground wires need to be checked. Cleaning the ones to the right of the battery fixed my odometer and temp gauges. Hope this helped Bud

 

The speedo sensor outputs a pulse and the unit in the boot turns that pulse into a voltage. I have not put a scope on the sensor (my car is still not operational) but if you have a DMM that has frequency you could put the car on stands and connect the meter to the input of the unit in the boot and have someone put the car in drive. As the speed rises the frequency should rise also. Judging from what the diff emitter looks like the wave should be a square wave.

The boot mounted unit is most likely a frequency to voltage converter so as the frequency rises the DC voltage on the output of the unit should rise in proportion.

 

Gee, that's pretty strange!

Wish I had a snappy answer but I don't. Definitely re-trace all your steps, do some gentle wire jiggling, and clean all connections relative to the instrument cluster as mentioned.

How/where was the radar detector wired in? Was it just a simple "power and ground" type installation? Maybe something was disturbed realtive to the detector but a speedo peggged and unresponsive to key on/key off is weird.

But somehow I have a feeling that this failure is unrelated to your other work and is probably a problem within the speedometer itself.

Sorry

Cheers
DD

 

While adjusting bracket on pipe (runs real close to tailshaft), had "bumped" the sender and shorted it to ground. Even with battery disconnected though, the Speedo would not return to -0- default. Hmmm, turns out it had pegged hard enough to wedge on the top of the face bezel. Pulling the instrument panel out, taking off the back cover and using a tie strap to slide in and move it back off cured the problem.

 

Good sleuthing :-)


Cheers
DD