1989 XJS 5.13
My speedometer doesn't start to register until about 20mph, then it stays there until around 40mph but it registers about 30, at 60mph it shows 45.
With my tech showing 2200rpm my speedometer should show closer to 60 as on my other cars. Any ideas??
Dick C.

 

I have the same problem on my 90 XJS, have you had any response in the last 3 years. Today's date is 06/24/2017

 

Your trip computer can help you diagnose which part in the speedometer circuit is the source of the problem.

Take the car on a stretch of road where you get it up to steady speed and set the trip computer to AVG SPEED and press RESET. If once it resets and it registers a speed more or less equal to what you were doing, it is most likely the Speed Interface unit mounted in the trunk(boot) that is at fault.

If it registers zero or some speed other than what you were traveling at, then it's the Speed Pickup unit that attaches to the differential.

Let me know your test results.

 

Thank you for your advice. I will do the test and report back.

 

The speedo and TC use different outputs on the interface unit and the pulse rate is different. So use the ODO not the TC to check accuracy of distance.

I blew my speed interface unit up when I was installing my cruise control and had to repair it, so I got a good view of how it works.

 

Having driven at freeway speeds (California) as recognized by the speed of the traffic alongside, the speedometer registered around 30 mph. The average speed on the TC showed 25. After the reset the TC showed 0. Seems like the speed pick-up unit. How would one test this unit?