I caught my trip computer registering no distance traveled today. Speedo is reliable. Where does the TC get its distance input from?

 

The odometer.

 

To elaborate a bit on Mikey's answer....

The speedometer & odometer, trip computer, cruise control, and (when fitted) service interval counter all share a signal from the speedometer transducer which is located on the LH side of the transmission. The transducer is also described as a 'pulse generator'.

Cheers
DD

 

If the speedo is reading Ok then the fault is likely in the TC itself

Cheers
DD

 

What year is your XJ-S?

They switched to a differential mounted speed transducer (not sure which year) and the signal splits at a speed interface unit mounted in the trunk/boot, one goes to the trip computer and one goes to the speedometer. If one or the other works, but not both, the speed interface unit is the problem.

 

1986 according to the signature line.

The diff mounted sensor came along at VIN 142xxx or 144xxx or something like that. That is early/mid 1988 model year (in USA terms)

Cheers
DD

 

Thanks for the info guys. I'll pull the TC and check its connections. Speedo has never faltered so I assume the signal is being lost behind the dash.

 

Does the odometer register?

 

As often as not just unplugging and re-plugging the TC clears erratic quirks. At least temporarily. I think these old TCs suffer degraded solder joints like so many other decades-old devices. The unplugging-replugging jostles the board enough restore the connection. It's the fiddle factor.

Worth a try before getting elbow-deep in dashboard wiring, perhaps.

Cheers
DD

 

It's a good question. I'll check at lunch.

 

Ill try checking and cleaning, dielectric greasing the plug and reconnecting. It's intermittent, of course, so I won't know if it's fixed for a while. I know my routes length so I'll compare daily.

How does one remove the Trip computer?

 

Interesting, I didn't realize you can only see someone's signature line if you are logged in. I'll have to remember that.

 

It's on spring clips, it should just pull out. The later facelift versions are bolted in.

 

Everything you ever wanted to know about the trip computer: Jaguar XJ6 SIII trip computer details