Good morning, Gentlemen! I have a 1987 XJS V12 with an interesting speedometer issue... It works fine when I first drive the car, varying distances/times. When I stop and then return to the car to drive home, (after driving for 20-45 minutes), the speedometer does not work at all. This is a fairly new issue for the past few weeks. I know that the speedometer transducer was replaced last year prior to my purchasing the car, and I also know that they still encountered problems afterwards (unclear what those problems were...). When the speedometer is non-functional, I also lose the trip computer info, which I believe indicates that the speedo itself is not the culprit. When the speedo is operating, all trip computer functions return as well. I have not had the opportunity to see how long it takes for the speedo to return to operating status, so far I have only driven the car the following day or more in regard to delay, no tries after say a few hours.... When I do drive it after that much time lapse, the speedo seems to work fine. Any suggestions to what might be going on? Thanks in advance to all my Gurus!
John

 

Hi JJS

Inside the Boot/Trunk over the Top of the Wheel Arch (inside the Car) having first removed the Trim, if you look into that space with a Torch you should see a little Black Box next to a Blue Connector and this is called the Speedometer Interface

The Interface Splits the Signal into Analog and Digital and one of those Signals (can't remember which one) does the Speedo while the other one does the Trip Computer

It's Passenger Side on a (UK) Car but maybe on the Drivers Side on yours

So it could be the Interface or maybe a Bad Earth in the Instrument Binnacle but going by what you were saying about The Trip Computer

I wouldn't rule out the Interface as being the cause of the problem, as was suggested to me by either Mac or Warrjon, can't remember exactly as it was such a long time ago!

About 3/4 down the page of the Following Link is where you will see a Photo of the Black Box

The Interface sends a Signal to the Speedo on a V12 XJS

 

Is the Speedo operated by a gizmo on the side of the gearbox? If so no box in the boot.
If it is from the G box, find the plug in the loom and check it. It is about 5 inches from the gizmo

 

Quote:

I know that the speedometer transducer was replaced last year prior to my purchasing the car, and I also know that they still encountered problems afterwards (unclear what those problems were...).

Make sure the transducer wiring has flopped against the exhaust pipe.....which sometimes happens after the transducer is replaced

Cheers
DD

 

Right!

If you have the differential mounted speed sensor you'll have the box in the boot. Otherwise, no.

The change to the diff mounted speed sensor was at VIN 142xx or something like that. About mid-1987 calendar year....which would be 1988 model year for a USA car.

Cheers
DD

 

Doug,

Change seems to be at 144262

Cheers

Paul

 

Thanks for all the tips, guys! I will take a look... My serial number is 135xxx, so I am assuming from your info that I still have thye "black box" in the boot. With the symptoms I am getting, it seems likely to me that the box itself is no good (even though it was replaced recently), but wiring touching the exhaust is certainly worth looking at! I will research the cost of replacement for the box...
Thanks again!
John

 

No. You should have the transmission mounted transducer…..which does not use the black box.

Since your transducer is new-ish my money is on a poor connection.

Cheers
DD

 

Hi JJS

Before you go Spending any money on her, it may be worth taking the Speedo Binnacle out to check the Printed Circuit on the back of this for any signs of Corrosion

This is only a 10 Minute Job, even if you have never done it before, where the only thing that you really have to watch out for, is being Careful not to damage the Printed Circuit, that is on a big plastic Ribbon, on the back of it

Then you can use Cotton Buds dipped in Lemon Juice to Clean it or the Rubber on the end of a Pencil, then once having done so you could also fit a Secondary Earth

There are quite a few pages of what I did, when the Needle on my Speedo dropped to Zero

With a little help from the GF!

How I fixed the Speedo on my 1990 XJS V12 with lots of Photos

 

OK, Doug, I understand now. Poor connection seems like a good bet to me also....
Thanks!