The speedometer in my 90 XJ-S has been working erratically ever since I woke the car up from a two year slumber in my garage several weeks ago. It either works fine or not at all. Either condition can last a couple of miles or fifty miles or more.
I pulled the instrument cluster today to check for any damage or corrosion to the circuit board. Everything was bright and shiny with no hint of damage or corrosion.
The cruise control still works, even when the speedometer does not.
Where do I go from here?
Thank you!
Michael
A 1990 should have the Diff mounted transducer module (like a VR ABS sensor, or VR crank position sensor etc) and the trunk translator/interface/signal-conditioner module).
These typically don’t fail as often as the older style transmission mounted gizmo, but it’s not unheard of for the Speed Interface to go wonkey.
I wish I could tell you more about the Interface module, other than it looks like it has come caps on it which could go bad and would be easy to replace. Not sure if the ICs ever fail, and I’ve only found the datasheet on one of them: TC4520BP.
Sorry… that’s all I have.
~Paul K.
That means it’s gotta be the speedo itself, doesn’t it?
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Every thread I can find on flakey speedos and tachos, or one’s signal crossing with the other seem to indicate to clean the grounds, and add a ground in the cluster. I keep wondering if all these problems are related (when it’s not the TH400 side mounted transducer) and if we really know what’s wrong. Variants of this seem to be a common problem (beyond that older unreliable transducer).
~Paul K.
I’m pretty sure the gearbox transducer is also used on the same period XJ6. Mine went bad and got replaced. When I looked around for another XJ6/XJ12 I came across one for sale that had a remarkably low mileage for the year. The MOT records showed that it had only done a couple of hundred miles in the last 4 years. The owner said that he “only ever used it to drive to the golf club around the corner once a week”.
Somewhat predictably, 4 minutes in to the test drive, the speedo dived to zero and the odometer stopped turning.
Big Michael,
If the cruise control (and trip computer) in your 1990 XJ-S work properly but the speedometer does not, then you have a failing speedometer. Since you checked the instrument panel flexiboard and wiring and didn’t find any issues then it is time to remove and replace the speedometer. Used and New Old Stock (NOS) speedometers show up on eBay from time to time. There are also shops that repair speedometers. I have never had a speedometer repaired but I see shops mentioned occasionally on Jag-Lovers. If you search the archives for “speedometer repair” you are bound to find some mentioned.
Paul
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Assuming it’s your speedometer…give it a bit of time before spending a lot of $$ and work…i had a similar issue with my tachometer. I purchased a used instrument cluster (probably sat around for years), and swapped in my speedometer but left the tachometer that the cluster came with. I sprayed electrical cleaner on everything, EXCEPT the tach.
For the first few weeks, I had to actually tap on the tachometer to get it to work.
Then the next few weeks, it would work by itself after about 5 seconds of engine running.
Now it works right away as it should.
Hi Paul!
Thank you for your input. “Grounds” always seem to pop up as a
possibility when discussing Jaguar electrical issues, don’t they? 
Michael
Quoting Paul Kobres via Jag-lovers Forums noreply@jag-lovers.com:
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Hi there!
I’m going to take your question as a statement, since I don’t have
sufficient expertise in how the speedometer works in relation to the
cruise control to warrant an opinion of my own.
Thank you!
Michael
Quoting “Author of the Book former owner of an '83 XJ-S H.E. via
Jag-lovers Forums” noreply@jag-lovers.com:
Hi Paul!
Thank you very much for your insight. I will start the search for a
“new” speedometer or a repair shop immediately.
Michael
Quoting “Paul M. Novak via Jag-lovers Forums” noreply@jag-lovers.com:
AFAIK they didn’t change the speedo on the pre-facelift when they changed the transducer, so presumably it still gets the same type of 8000PPM signal the transmission transducer created, but from that interface unit.
Here’s what the guts look like:
The tacho is built almost the same way with an almost identical interface board, but the Speedo has the extra mechanicals for the counter.
~Paul K.
It is my impression, based on reports surfacing on these forums over the past coupla decades, that the problem more often is that interface module than the diff transducer itself. And I’ve only recently learned about capacitors and how often you can bring stuff back to life with a soldering iron and a few cents’ worth of new caps. But that diagram above shows the same lead from the Speed Interface going to both the speedo and the cruise control, so if the cruise control works it must be the speedo itself – or the wiring in its immediate vicinity, the flexible circuit board or its ground.
nods Yes, it does suggest something instrument/dash area related.
It’s just that there so often seems (I’ve encountered folks w/ these issues nearly a dozen times now) to be dash area related issues where the Tacho or Speedo fails, or flakes out, or… very often, flakes out in unison with the tacho… and/or acts as if there is cross talk between them.
~Paul K.
I strongly suggest going through the wiring behind rear sofa (upper corner, UK passenger’s side) usually you will find cuts/fray in there (rubber boot decomposed).
Usual dents/cuts on screened cable are found in the area under the fuel tank.
The main issue is usually related to speedo flexi circuit board - it may look shiny but the copper terminals on the multi-pin plug are usually oxidised. Had that with mine. Checked everything, then returned to the instrument cluster in suffering to discover easy fix.
Hi Janusz!
Thank you for your input. When the terminals are oxidized do they look
any different from ones that are not? Or do you just clean them all
up, whether they look like they need it or not?
Thank you!
Michael
Quoting Janusz via Jag-lovers Forums noreply@jag-lovers.com:
On an XJS I’d always suggest giving the terminals a clean when you have the opportunity. Sometimes oxidation & or other interfering materials aren’t obvious. And eraser and maybe a coat of WD40 or other electrical terminal protectant is good for the metal you can reach, and a good terminal cleaner fluid when you can’t mechanically get to the surfaces.
~Paul K.
Clean them all where you can. When you’re in the middle of work on anything requiring disassembly - if something is visible, every ground cable, every relay - just do it, without any form of hesitation. Same with the bolts and screws. Replace with stainless, add copper anti-seize. That will bring benefits in the fututre. Regardless if you’re fixing XJS yourself or paying someone per-hour labour.
There is always good to apply silicone on the tight-fit connectors. I found best results with 3000cps silicone oil inside and silicone grease outside.
With your cluster - clean it even if nothing seems to be there - use designsted contact cleaner (f.eg WD40 contact cleaner - not standard WD40 in blue can)
While your cluster is out - remove small bolts and nuts and clean/remove corrosion from those as well…
OK. You guys shamed me into taking the cluster out a second time and cleaning all the terminals, etc., even though they looked clean to me the first time I inspected them. Guess what? The speedometer works fine now! A lesson learned.
Thanks to all of you who took the time to respond to my query!
Michael
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