I took the car out for a spin tonight to help get it used to regular use. I noticed that the speedometer no longer reads accurately - it seemed to happen suddenly, a few minutes after I set out. It seems to be reasonably good up until about 30 kph, but above that it fluctuates wildly between say 60 kph and 20 kph, and when I was going fairly fast (80kph? More?) it stayed firm on 20 kph. Mostly it danced up and down. Sometimes it seemed to stay in one place as if it was registering the correct speed, though I don't know if that was actually the correct speed, as it was mainly a little under 60 kph and I am sure I was doing more than that. Maybe not. That's the thing about a bung speedo....

This is mainly annoying rather than worrying like smoke or anything, but has the potential to lead to traffic tickets, which I would prefer to avoid, and the wavering up and down is a little distracting.

Is this a simple fix, or am I going to have to spend wads again? I've never done anything electronic, so if resoldering etc. is involved, it might make sense to get it done professionally rather than screw it up. I don't think it's connected, but the speedo dial itself is cracked, as if someone tried to pull it out and pressure from the needle housing cracked it (a couple of cracks radiating from the needle housing, in other words).

Edit: The tach and other gauges seem normal. I didn't think to see if the odometer was turning, actually, which I shall do tomorrow. I might be able to use the trip meter to see if that sensor is working as well, although I'm not really used to using a trip meter (this is the first car I've owned that actually has one...).

 

I had a similar issue, so let me give you some background to help you isolate the problem.

On later XJSs, the vehicle speed is picked up from the differential by the speed sensor, and it goes from there to a Speed Interface Module mounted in the trunk. The Interface module interprets and splits the signal, with one going to the speedometer, and the other going to the trip computer.

Try going for drive, get up to a steady speed and put the trip computer in average speed mode, hold reset until it resets and registers a speed. If the speed is accurate, it's unlikely that the speed sensor is the issue. Given your description, I would suspect either the Speed Interface Module or a wiring fault.

I fixed mine by simply replacing the Speed Interface Module with a used one I found on eBay. The part numbers were not the same, but it still worked. For some reason the modules were much cheaper in the UK than in the US, so I picked one up when I was there. Check the wiring connections because they changed at some point, because I know the later '94 or so were different than on my '90. Not sure what you'll have on your '92.

 

Probably an earthing fault as these cars do tend to get a little age on them and the electrics weren't 'the best' back then...

 

Had the same problem recently on my '92. Dancing speedo, other gauges were fine. I would check your battery terminals first for clean, firm connections.

The positive battery terminal on my car was an aftermarket one and I had noticed it wasn't fitting snug on the terminal post, even after trying to tighten it. I replaced it with one that fit snugly and that fixed the speedo problem. Yours may not be this easy, but I'd give it a look.

 

Thanks for your replies. I'll check the battery terminals first, then do what Mac Allen suggests. I was poking about in the trunk yesterday, so it's possible I could have dislodged a battery terminal. That's an easy check, at least....

 

I had a similar issue and found out my battery was dying and not giving the system the voltage it needed

 

I took her for a run this morning, trying out Mac Allan's suggestion, and to my surprise the speedo was fine. Then I hit a tunnel (not literally, luckily), and turned on the lights.

And the speedo started dancing again. Lights off - normal operation. Lights on at first position (no headlights) - dancing. Lights full on - dancing. Clearly something electrical is interfering, but I have no idea what connection the lights would have with the speedo....

For what it's worth, the voltage dial in the car was reading normal.

 

I am having a similar problem with the speedo in my 1991 xjs V12, however instead of dancing it is sticking at around 40mph and drops to zero when I stop, but only when I start to drive away. I had the battery disconnected for a couple of months while I changed the starter and worked on some delayed repairs. Prior to any of this the speedo worked fine.

 

Classic case of a poor ground/earth.

On my XJS it was the tachometer that went wonky when the lights were turned on.

Remove the instrument cluster, clean all connections and contacts, add a redundant ground wire.



Cheers
DD

 

Thanks for that. Nice to know it's a pretty obvious problem.
Looking up my ROM, removing the cluster seems easy enough, and I'm sure I'd recognise a connection/contact when I see one (clean with cotton buds and pure alcohol, I assume), but how and where would the redundant ground wire be fitted?

 

Right, clean the harness connector contacts and all the contact points on the flexible printed circuit on the back of the cluster. I use ultra fine sandpaper ....but the PC is fragile so a light touch is required when dealing with the PC in any regard.



My experience is with the earlier clusters where a horizontal bar at the bottom of the cluster (or is it at the top? No matter, you'll see it) provides a good ground point. I assume the later cluster has the same, but perhaps not. In any case, just run a wire from the metal bar to any good ground point behind the dash.

If the later clusters are different in this regard someone will come along and tell us

Cheers
DD

 

This might help but it's the earlier style cluster

https://www.jaguarforums.com/forum/x...t-panel-76713/


Cheers
DD

 

It looks like the later cluster doesn't have that metal bar....

Instrument Pack - Parts For XJS from (V)179737 to (V)226645 | Jaguar Classic Parts UK

.....so you'll have to do a little sleuthing to determine how/where the PC grounds and run your redundant wire from that point

Cheers
DD

 

This is definitely a BAD ground, or dirty connector.

I'd be cleaning ALL the grounds first. Use sandpaper to clean the metal on the car and then STAR washers, coat the clean ground in seam sealer and your done for good.

 

Thanks for all your help everyone - very grateful. I'll look up more stuff on cleaning electrical contacts, and when I get a fine day (no garage, and lots of rain right now due to typhoon - she's under a five-layer protective cover for the nonce) I'll give it a go.
Actually I might see about getting hold of a new speedo dial and replacing the cracked one if that's not too hard a mission (not making things worse is my primary goal).