Brand new circuit board and speedo still erratic above 70mph or so

As the title says, just finished putting a brand new circuit board in my cluster, tested all connections at 0 Ohm, amps are now rock solid (were flaky), everything is as it should be, apart from motorway speeds when speedo begins bouncing, at first a little between 70 and 80mph and then goes berzerk the faster I go. I thought new circuit board would solve this but it didn’t. Ideas please?

I had some issues with my speedo on my 88, thinking it was the circuit board, ended up working again by replacing the Speedometer Transducer at the trans.

What year car do you have? If yours is 89 or new, it would be at differential. Perhaps your transducer is flaky, or the little parts turning are not right at high revolutions.

Could also be the speedo itself.

Anyone rebuilt a speedo? What’s in there?

What is the model year of your XJ-S a what engine does it have?


So signatures are gone on the new forum I guess?

'91 xjs v12 5.3 early facelift in BRG

Yes, there are no signatures on this new site and they can’t be supported. If you don’t include your year and engine in your subject line or in the body of your post no one will have a clue what car you have and whether or not they can help.


As Greg mentioned, the 91 uses a transducer on the diff cover which feeds another small black box in the boot (DAC4591 or similar) which feeds the trip computer and speedo.

Was the circuit board an attempt to cure this problem with the speedo? Or did the problem show up only after the new ckt board?

Any obvious weirdness with trip computer? That could indicate something further up-stream.

Is the transducer in the diff cover snug? I suppose if it was vibrating or loose some strange readings could result.