I have an 1987 XJ-S V12, s/n 134879.

For the past many months, the speedometer has been flaky. Most the time it just sits on zero, but when it shows signs of life it will show something similar to the actual vehicle speed. Then again, sometimes it just jumps to 120 and sits there.

Anyway, I've located the transducer (I think). So my questions...

1. See photo. Is this it?

2. How to test it? I've read what Palm has to say, but I'm having to do this operation by myself (no helper) available. Is there another way besides scratching a screwdriver across a file as Palm suggests?

Or, an alternate way to look at the question, anyway to test the speedometer to see if it is the problem?

3. Part number. Anyone have it?

4. I want to guess that the transducer is some sort of magnetic device, and as such that it is not open to the inside of the transmission...in other words, if I take it out will the tranny fluid stay where it belongs?

Thanks,

John

 

If that is on the LHS of the GM400 towards the rear, then it is. You can remove it with no fluid loss worries. New ones are not that expensive. It could be the transducer - which is actually a little dynamo that generates current in line with gearbox rotational speed, or the little gear cog thingy inside the box which turns the transducer spindle. These latter can fail. It looks like this and you need a new one with the same number of teeth:

 

I used to just 'electrically' connect a replacement transducer (with car raised) and spin it with my fingers while someone watched the speedo in the car.

If I was alone I would setup a video camera focused on the speedo and see if the needle jumped.

bob

 

Well, maybe this isn't it (see photo) since it looks nothing like Greg's, although it is on the LH side and towards the rear of the tranny.

Granted the gear part could be inside this unit, but I don't see any way of opening it without destroying it.

The issue with testing it with a replacement transducer is that once I've opened the box/unsealed the unit, most (if not all) places won't take it back.

I would think this unit would generate a voltage based on how quickly the shaft is turning...or possibly change resistance...but it measures a steady 450K ohms across the terminals, and no voltage at all when turned. But maybe it doesn't work that way at all.

Thanks,

John

 

Well, playing around with it a bit more I find that the transducer does generate an AC voltage. Found this out by putting the shaft in a power drill and monitoring the voltage as I adjusted the drill speed.

At this point the above is fairly useless information, since just because I am getting some AC voltage out of it I don't know if it's the correct AC voltage.

One way to find out would be to generate an AC voltage into the leads under the car and see what the speedo does, but generating a very low AC voltage isn't all that easy.

Thanks,

John

 

Figuring I had to start somewhere, I ordered a NOS transducer from Parts Geek. Installed it, and speedo is working fine now.

Before installing it, I put it on the drill and measured the AC "voltage". There wasn't any. It also had the same resistance across the terminals as the bad unit (450K ohms). So, these two things won't help much for troubleshooting.

Thanks,

John
1987 XJ-S V12

 

A speed transducer is nothing more then a signal generator. I'm not sure what the signal is, on the Jag. Most are an AC voltage, on, off, on, off, on, off,etc, that the computer turns into a signal, for the speedo. Best I can say is you fixed it. Congrats.

Jack

 

Greg's pic is the gear inside the transmission, not the transducer mounted on the outside like the pic you have.

bob

 

John, I have a1990 XJS and the speed sensor for the speedometer is on the rear differential. I am looking for one myself and it is no longer being made. Trying to find a good use transducer.
Testolas

 

Speed transducer can be removed without losing fluid, however to access the gear you need to pull out the aluminum housing which will make a bit of a splash on the ground(dont ask me how I know lol). Remove the bracket holding the housing, twist and pull. I have a gear if you need one, came off my 1986.