I have a 1990 xjs 5 speed converted and the trip computer is intermittent, when you have it on instantaneous MPG is updates every 2 or 3 seconds it appears to read near correct (hard to tell) and then it fluctuates in units of about 5mpg up or down or sometimes you just get 0 Mpg. I have tried replacing it with another unit and same thing so it must be the senders, I think the speed sensor is in the diff but the speedo doesn’t fluctuate (although it does read slow) so I assume it must be on the fuel side but not sure where the fuel sensor is. I have had the ECU off recently to get xk6 engineering upgrade done and hoped that disturbing that plug might have improved things but alas. Any suggestions greatly appreciated
BTW for those interested when the trip computer appears to read correctly I can get figures in the early 30’s (2.88 diff and getrag box)
Hello Matt J - go to the free site www.jagrepair.com which is a pdf, then follow to XJS, then 1990-1991 electrical, then scroll to Fig. 19.1 and 19.2 - this will give you the location of the speed interface unit and the electrical schematic - you may have corrosion on the trip computer plug leads since you say that the speedometer is okay - you will also see the fuel interface unit that connects with the speed interface unit and the trip computer - Tex Terry, II - 1991 XJS V12 Classic Coupe, 1986 XJS V12 Coupe - sent 1/27/2021 2139hrs. EST USA.
I saw those fluctuations once, on the way back from le mans, thought something was up (4mpg or something) then realised the problem was with my right foot when after 20 minutes I had to stop and refill both tanks…
I once had a scare with falling MPG on a long run, tried every trick I could to save fuel, but it only got worse. Then figured out I had hit the Miles/Km button and the L per 100Km figure being displayed would only get lower the more economical I drove.