Trip Computer & Interface Repair

XJ-S
1

Information for anyone interested-

My XJ-S is a ‘88 5.3L with the differential mounted speed sensor. My traveler did not work when I purchased the car 3 years ago with the exception of the clock; neither fuel used or miles driven were recognized. The speedo and cruise worked fine (well after fixing the bellows).

After verification of power and ground at the fuel interface (located near ECU), I opened the unit; only component likely to fail was an IC. I replaced the IC and the traveler recorded fuel used quite accurately. Still no distance recognized…

I verified power and ground at the speed interface module (left side of trunk/boot) and continuity of the red turns to yellow wire to the traveler. I then fed a 5v 133hz square wave to the yellow wire and the traveler recognized distance- the 133hz represents 8000 pulse per mile and the traveler showed 1 mile in a minute.

I then ran the car on jack stands at 30 mph and monitored both frequency and waveform from the speed sensor- there was an approximate sine wave of 67hz at 2.5v peak to peak. This corresponds to 8000 pulses per mile. When I fed a 1v RMS 133hz sine wave into the speed interface the speedo registered 60 mph.

So in summary the speed sensor produces a low voltage sine wave at 8000 pulses per mile; the speed interface amplifies the signal and shapes the waveform to a square wave at the same pulse rate. The traveler output is at 5v and the speedo is at about 2.5v

I then disassembled the speed interface (DAC4864 on mine); there are 2 ICs, one electrolytic capacitors, 2 transistors, and a bunch of resistors, film caps, and ceramic caps. I replaced the 2 ICs and the electrolytic cap and all is working.

One IC is a LM2903N, which is equivalent to a LM393N. The other is a TC4520BP. The electrolytic is a 47UF/25v (I used a 100UF because that is what I had).

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I have fixed a number of 90s era electronic devices by replacing electrolytic caps. They dry up, leak or otherwise fail. Replace them with 105C rated caps for best replacement life.

It’s certainly possible for the ICs to fail, but absent conclusive testing, I would put my money on the caps.

The travel computer never worked on my '90 XJS. I’ll have to remember this when I start the project of getting it back on the road next year. Thanks for documenting this!

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I lack the expertise to diagnose the circuit any further; but as only the one output on mine was affected, and the electrolytic was in a filter, I went with the processor. The 47UF value cap wasn’t part of anything critical, as they can vary +/- 20%

I agree that most often it is electrolytic caps that fail, supported by my experience with vintage audio. Followed by cold solder joints and corrosion.

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Excellent repair and a very informative post Robert !

I took the liberty to rename the tittle of the post for future reference to anyone searching, as it’s normally referred as the “Trip Computer”.

Best regards.

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Thanks for that, I had a difficult time finding much information about pulse rates and signal forms/levels. Hopefully someone can use the information.

I did measure the electrolytic capacitor that I removed, 41 UF with 10M leakage- was probably ok.

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