Trip Computer XJ-S

Does anyone know what the amplitude (voltage) of the square wave signal from the fuel and speed interface modules to the trip computer are on a ‘88
XJ-S? This would be what the trip computer is seeing on the orange and yellow wires.
I know the speed signal is 800 pulses per mile, and fuel is 8000 per gallon US. I have read 10v and 4v…
My trip computer records gallons used quite accurately, but mileage registers only occasionally. Speedo and cruise both work fine, so I am assuming that the differential mounted sensor and the speed interface module are ok.

Spent a few minutes trying to diagnose the issue logically- checked for battery voltage at speed interface connector green wire with key on (13.7v running). Checked for continuity to ground on black (.3 ohms). Checked continuity from yellow wire at travel computer and the red wire at the interface connector (.6 ohms). All other functions of traveller are ok so didn’t bother with powers or grounds there.
I then calculated the frequency of 800 pulses per mile (13.3 hz) to simulate a 60 mph signal; this is lower than my signal generator would provide- so I doubled it (27hz) to simulate 120mph and introduced a 5V square wave of that frequency on the yellow of the travel computer. The travel computer then counter tenths of a mile every 30 seconds reliably.
This indicates to me that the traveler is looking for a square wave of at least 5V with 8000 pulses per mile. 8000 pulses per mile would be133hz at 60 mph, which is 10x what it would be at 800 pulses and matches the pulses from the fuel interface per gallon.
Incidentally there are only 4 wires at the speed interface- power, ground, traveler, and speedo/cruise (which both work in my case).
From the above, looks like the speed interface is faulty.

That’s good deductive work!

Mine does the same thing. I fixed it by never checking it. I just remember mileage at fill-ups and do the math. I think it will be replaced by a pair of gauges.

I’ll watch this space though. I’m curious.

DAC4864 is interface, still available new in UK for 54GBP or in US for $175 (ouch). I just took mine out and removed the cover, but can’t get the board out. Looks to have 2 integrated circuits, one probably a voltage regulator for the speed sensor. Also a lot of discrete components. May order one from the UK as have had no luck finding a used one at a reasonable price.

Usually average is about 12-13 mpg as no long trips.