Computer problems, part 2

[KeithC]

So as to not hijack the other thread.....

I'm having trouble with the fuel readouts from my trip computer. The displays show 0, 99, 0 for the Fuel, Inst and Ave readings respectively even after resetting. All other figures are correct. The Inst fuel reading resets from 99 to 0 when the vehicle is staionary, then back to 99 when in motion again. I like the idea of 99mpg, but fear the 0 figure far closer to the truth
Any ideas where I should start to look for the problem?

Thanks in advance

Keith

[Mark_B]

Hey that's a good one 99 - 0 - 99

I'm not 100% expert but there's 2 signals invloved that used calculating the mpg, the wheel revolution pulse rate from the diff and the fuel injector pulse width.

Sound to me like failure of the injector (pulse width) signal getting to the computing bit. Especially if the fuel used reading never leaves 0.

The computer pulls out and hangs by its wire harness, you could scope each wire till you find an injector pulse-looking signal. As it's so easy to remove it you could try out your suspect unit on another xjs....

[KeithC]

Thanks for the reply Mark,

I wish the computer would talk to the fuel gauge regarding the "fuel used =0"

I guess there must be a fuel sensor/signal problem, maybe a broken wire or something simple. Finding that wire is anything but simple though

Good suggestion about trying in another XJS..... if only I had another !!

Thanks again though

Keith

[Mark_B]

If it's what I think it is it's not that bad really. You know the signal is present at source and there's no sensor problem, otherwise the engine wouldn't run! Wires themselves don't break easily (unless they are part of a PCB). That leaves most likely the computer and that's where my money's on

[poprivet]

Trip computers can often be found on Ebay, but beware any adverts saying it will fit both 3.6 and V12 as they are different
Pop

[KeithC]

It's all sorted and working now. All it was was the connector on the .... "ECU interface unit".... (for want of a better description) hanging off. It's the thing with four connectors inside the right hand side boot next to the battery. Sorry to be so... technical.... in my description but I'm damned if I can find out what it is from either the parts list or repair manual.
At least I can now see why the fuel gauge doesn't like heights

thanks for your suggestions guys

Keith