Let's see if anyone knows a bit more about this. I saw on an old thread that the Series 3 cars up to '87 (a different instrument cluster?) has a pulse generator for the speedometer, cruise and trip computer that clicks at 8000/mile. So that means 8000 pulse per minute (ppm) at 60 mph (one mile per minute), 4000 ppm at 30 mph, etc. That is pretty straight forward. I also saw that the pulse is an AC square wave. This is probably incorrect. AC, alternating current, is generally a sine wave that transitions from +V to -V and back to +V, doing this at a frequency, X per second. A square wave (I think the usual pulse generator signal) would go from 0 volts to + volts, basically on - off at a frequency. In this case 133.3 cycles per second (8000 per minute divided by 60 seconds per minute) at 60mph. But if one wants to bench check a speedometer, or trip computer I think the voltage is pretty important and I can't find anything that reports the "on" voltage. A guess would be 12v because 12v is the input to the pulse generator on the schematic. But if the generator drops the voltage for the electronics in the speedometer or trip computer, 12v could fry the circuits.
I could simply put the thing on a lift, put an oscilloscope on the generator output and measure it, but if anyone knows more about the pulse generator signal I can bypass that step. Anybody have any information on this system?

 

This article in the Trip Computer generally, looks like it contains some clues:

Jaguar XJ6 SIII trip computer details

 

Hints indeed. Very good article. Do you have a notion of who the writer may be? I have 5 trip computers beyond what I need but I'd like to be able to test them before I make them available. I appears that the writer has researched the data to be able to test.

 

Sorry, I've no idea who wrote the article. I first found it through this forum, while searching for "trip computer".

Good luck with your investigations.

 

His name was Duncan and if you search Jag-lovers archives from 10 years ago or so you might be able to pick up a trail

Cheers
DD

 

Doesn't look like there is much interest, but I put an oscilloscope on my car and found this:
Speed pulse: +/- 3.2V DC
Fuel Pulse: +/- 6.0V DC
Both are square waves
I also confirmed that the speed pulse is rate 8000/mile. That converts to 133Hz at 60 mph, 67Hz at 30mph. I read about 69Hz when the car indicated 30mph. I'm going to call that good.

 

Thanks of advance of any help.

 

The pulse generator on an XJS V12 of this era does indeed generate a square wave. 12v is the input voltage. The cruise control and the speedo need that square signal and they run on 12V.
I know this for a fact as i have installed a replacement Dakota Digital cruise control, for my non-functioning OEM one, relatively recently. FYI the solid green wire into the pulse generator on the GM 400 carries the 12v input.