I pulled it out from the dash and disconnected both plugs. I waited about ten minutes and plugged back in. Set the time in the clock and all is well. Went for a very short drive and my fuel gallons only showed 2,7. Had a half tank on regular gauge. Trio meter was way off also. Do i need ti give it more time to correct or am i having other problem. It is a 1989 XJS V12 convertivble

 

Not sure what model year and engine you have.

 

Might be that the TC is simply giving up the ghost. Not long ago I tossed a half dozen dead or near dead units. I’m a bit amazed that any of these might still be fully functional after 30-35 years

Might be a faulty “Fuel Interface Unit”…..the module that converts all the inputs into a language usable by the TC.

Poor connections at the plug-ins are always a possibility.

Cheers
DD

 

The trip computer does not read like the fuel guage. It merely reads what has been used since it was last reset (in your case, disconnected). So the fuel guage, trip on the speedo dial, and the trip function and fuel readouts on the computer are not expected to tally.

 

Greg has got it right. The Trip Computer is not linked to any other display. Though clever for it's time, it is not smart. It only stores what has happened since the last reset. Until recently, I had no working fuel gauge, so relied on the Trip Computer to tell me how much fuel I had used since the last top-up ... meaning I had to reset it at every time I filled up to know how much fuel I had left.

When next you fill up the tank, reset both the Trip Computer and the Trip Odometer and you should see subsequent readings match (nearly!).