1992 XJS V12 Conv. US Spec.

Strange problem here.... My check engine light works, meaning it comes on at ignition then goes out. It comes on when I actually have a fault, ie when I've started the car with the air flow sensor disconnected, etc, however, I never get the FF code or the CHECK ENGINE warning displayed on the trip computer. (I know I am supposed to switch on the ignition then wait for 5~10 seconds and the FF code is supposed to appear on the trip computer screen but it doesn't)

The trip computer is working fine, and is very accurate.

Is there a seperate wiring harness to control the CHECK ENGINE functions on the back of the trip computer ? or a seperate fuse for the CHECK ENGINE functions ? (I see 2 fuses listed for the trip computer, one for memory and the other for the computer itself. Both of these are ok, and the trip does keep its memory)

I now have the Check Engine light warning light on and I can't start to diagnose it because I don't know the code :-(

 

I don't know the answer to your question, I have only had a few OBD 1 XJS cars through the years as most of my XJS customers have earlier cars.

The thought is that perhaps a timed "drive cycle(s)" must be completed before the data is stored: that is, if you pull off a sensor and drive the car, it might take some mileage or consecutive starts with the fault detected before the code is flagged, even though an immediate fault is recognized and the check engine MIL is illuminated.

You might also try cycling the ignition key on and off a few times and see if the code appears. That was sometimes necessary to obtain a code from the early OBD 1 XJ6's. Just speculation, hopefully someone will sign in with specifics. Poboyblues is a new guy here: he's on the ball, and seems to have some experience with older stuff.

 

Thanks JTO, amazingly I managed to fix it.

Pulled the trip computer out of the dash, had a look at the connector block...... noticed one of the wires running into the block had been pulled out of the block and was dangling about... connnected the wire back in and hey presto, I now get nifty text at the top of the trip computer display saying "CHECK ENGINE" which comes on and goes off in sync with the light on the instrument panel when I turn on the ignition.

I then disconnected the air temp sensor off the air intake and got FF16 proudly displayed on the trip computer. Fabulous.

I should really have worked this out sooner because when I got the car the check engine light in the instrument panel had been taken out, and of course when I put it back in it came on and stayed on. I didn't make the connection at the time because I didn't even know I was supposed to get codes displayed on the trip computer. If only the previous owner had spent more time trying to fix the car rather than try to disconnect all the warning systems !

Anyway, battery power cycled and I am now code free so hopefully (should I say that?) the "real" problem that I had earlier today will come back and I'll be able to get a code this time.

Too much excitement for one day, I'm off for a beer, cheers

 

Cheers to you for the lesson, thanks! That was obviously the "upgrade" of a used car salesman you had to resolve, well done.