"My XJ-S is a ‘88 5.3L with the differential mounted speed sensor. [mine has the gearbox transducer] My traveler [he means trip computer] 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 [a yellow wire is the OEM diagram colour sending the signal to the trip and the speedo] . 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 [i.e. the diff sensor that feeds the signal into the interface unit in the boot] - there was an approximate sine wave of 67hz at 2.5v peak to peak [i.e. the diff sensor produces a sine wave]. 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 [i.e. the speedo recognised the square wave output from the interface unit] .
So in summary the [differential] 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 "