Speedometer and odometer in my 1987 XJS-C does not work, no bouncing, just dead needle at 0. The trip computer does respond except for speed related functions. This car has the speed sensor on the transmission (and these are expensive I have read there are repair possibilities?) I have examined the connector - it was - well a 1987 connector/wiring under the car but no obvious damage.

I am wondering are there tests I can perform without getting under the car - reason I ask is I have access to a lift but its 30 mins away. I have a nice comfortable garage at home so if can test things at my leisure to narrow it down to speed sensor then why not!

 

Whether you can test it in situ depends what measuring tools you have to hand, you can't test it without removing it or turning it somehow. Does the sensor have power ? - should have a permanent 12V if the ignition is on, a bulb tester may work for this - this is also a good way to measure if the 12V is good enough to light a bulb - in which case it is probably OK. You can also bell test the cables on the sender but disconnect before you try this, if you ground each cable to the car you should be able to measure a circuit to ground at the speedo and fuse #4 in the main box (a 10A) - be good to make sure this fuse isn't bad or not presenting a good connection - if your reverse lights work this fuse is OK ...

Not 100% how it generates the pulses though never had one apart so could it be repaired I can't say, I would doubt it without a serious Macgyver type job - at a guess it is some sort of copper contact that sends 12 each rotation to the speedo - yellow cable goes to speedo.

 

Thank you! This is very helpful - especially the basic does it get power check bit (I have a multimeter, test light, convenient 12v source + wires). Reason I was hoping to test like this is I think I have a systemic fusebox or fusebox area wiring related problem. I have an intermittent outage on I think its fuse #7 (gauge backlights) that needed to temporarily bypass with positive feed at the dimmer to get guage backlights. I've also got intermittent HVAC control response (some days every setting works, some days it can only do one thing - turn on fans)
I assume that's unrelated - but is related in the sense of this a 1987 car of unknown to me history. This winter I plan to the full remove the dash overhaul the delanair unit and sort everything.

 

Electrics can be very fickle and the cause of a myriad of 'strange' issues that they can initiate. Any car that has stood for any length of time needs 'ideally' for every connector to be separated, cleaned and reconnected, and when I say cleaned I don't mean with sandpaper, the issues could be related.

Just because you have a circuit doesn't mean that circuit is capable of carrying current / load - hence the bulb suggestion, good luck with the plan as it is a good plan but it is going to be equal parts fun and frustration - probably need some cases of beer handy ...

 

Small update, there was 12v at the green wire - oddly I was disappointed by that
Connector wasn't in greatest of shape it was melted on one side. So to rule that out since I have nothing to lose at this point, I butt connected the wires, wasn't super hopeful and alas was proven correct. On to the next tests...

 

Connecting the wires will do nothing, I think. The signal should generate 8000 pulses per mile (PPM), square wave signal.
Remove the generator from the gearbox and see if spinning the square driveshaft in a drill, ignition on, wires connected, moves the speedo.
If it does, then the plastic gear the generator unit connects to is shot, or more likely the square speedo like piece is turning in the gear piece.
If so the gear just pulls out and they are freely available. From memory it has 39 teeth and is coloured brown. Easy to find them, just make sure the tooth count is the same as on the old one.
If it does not, then it is a cabin wiring/speedo cluster problem.

 

Hi, Thom!
Please take my reply with some grain of salt, as I am not an expert like Greg, Ben, Grant, and others.....
HOWEVER, I do have a fair amount of experience sorting a similar dead speedo on my 1987 V-12 XJS. I worked with Greg and Grant (please bow to the Great Prophet of the Southern Cross). After much testing, we determined that it was the sensor unit attached to the transmission. Interestingly, the original (bad) unit would show good mph on the speedo when the unit was turned by hand (electric drill) from underneath the car, but would drop dead shortly afTer driving the car a few miles when re-attached. Replacing that sending unit solved the problem completely. Fairly easy to replace, and I did it in my garage with just a set of small drive-on ramps. 20 minute job, have never looked back.The part number was DAC-4569, and I ended up getting it from Welsh for about $188. Greg's memory is very good on the 39 tooth driven gear (which I ended up replacing as part of the process, also very easy).
Hope this helps!
John

 

Did both gauges previously work?

if so, did they fail simultaneously?

if so, I would suspect a bad instr cluster ground, or power supply to the two gauges failure.

if they never worked since you got the car, it could be anything, but what i suggested still applies.

Doug