Good evening, my 1991 XJS coupe (LHD) trip computer seems to be working normally aside from every time I crank the engine the clock reads 88:88 momentarily then 00:00. If I reset the clock and go about my business, come home and shut her down for the night the next morning it will retain the correct time when the key is in the auxiliary position but as soon as the engine cranks I get the momentary 88:88 then 00:00.

Here are the steps I’ve taken as of today:
1. unplugged the TC and cleaned connections with electronics cleaner
2. Cranked the engine a few times with the TC unplugged
3. Replaced #15 5 amp fuse in main fuse box
4. Purchased another TC - yielded same result

The only schematic I’ve found for the TC is for the 88 XJS and it mentions the TC utilizes another ignition related fuse in the main fuse box at #4 and is 5 amps but my 91 handbook doesn't mention this fuse at all. Perhaps there was a change. BTW my TC part number is DAC4296 - there are a few.

Any guidance would be appreciated - this is driving me nuts!

-David

 

Sounds to me like the TC electrics are objecting to an excessive voltage drop.

The starter draws a lot of power. If weak/worn/failing it can really, really draw a lot of power and pull down supply voltage elsewhere. Has it been behaving OK?

Next.....

Poor connections may be compromising the system. If the TC is only getting (let's say) 11 volts then the additional drop from the starter may be more than it can tolerate. Check/clean/tighten the two "+" voltage junction posts on the firewall and see if anything changes. These posts are like Rome, and all roads (circuits) lead there .....eventually.

Various other connections could be weak as well, but I'd begin with these ^^^^^

Do you have a voltmeter? It would be useful here

Cheers
DD

 

In addition to Doug's excellent advice, check the ground points for the starter and/or the TC for excessive resistance.