I have a 1992 XJS V12 early facelift car so it has the newer gauge cluster. I noticed the speedometer is working accurately up to 75 mph but if you go above that it starts acting erratically. Then if you slow down it returns to normal and reads correctly. Any ideas what the most likely source of the problem is?

thanks for all the help!

 

sconce01,

Are you in a position to verify readings against the Trip Computer (assuming that you have one)?

If so, see if you can verify the average speed on the Trip when the Speedo is malfunctioning. If the Trip seems correct then the problem probably lies in the Speedo Head.

If not, then the problem may lie in the Speedo Interface unit or within the shimming of the Speedo sensor in the rear diff. The latter is less likely as it seems to perform ok up to 75mph.

Good luck

Paul

 

Maybe it needs a drop of watch oil.

 

Thanks for the replies everyone! The speedometer is actually only malfunctioning when the headlights are turned on so I’m guessing it’s a bad ground.