Sometimes when I start the XJS the speedometer needle pegs at 160 or shows a fluttering high rate of speed while the motor is cranking. When this happens, the speedometer doesn't work at all when the car is actually moving.

Let the car sit awhile, start it, and the needle stays at 0 while cranking and then the speedometer starts working normally.

Also, the digital dashboard computer fails to log average speed during the times when the analog speedometer doesn't work.

Is this a speed sensor problem? Where is the speed sensor located on a 1984 XJ-S HE?

I have ruled out an instrument cluster problem, having spent all weekend doing a detailed repair of all lamps, sockets, connections, plugs, and receptacles on the main cluster. The two locking tabs on the plugs had lost their plastic spring tension, allowing the main vertical plug to come loose. I mashed some foam rubber under them and fixed it. Now, all my gauges and lamps are working great except this intermittant speedo problem which has been around for a long time. Would sure appreciate any feedback.

Thanks,

Lou

Submitted by fastxb2003@aol.com on Sun, 12/27/2009 - 20:54

Thanks Doug! Will check the wires and go from there.

Re the cluster...The ground "buss bar" at the top was severely rusted. I cleaned it up and applied progold G100 to all connectors and screws etc. ad nauseum. I also noted the presence of two grounds; one on either connector, but electrically unified by the "buss bar".

Since replacing the cluster, strange things (in a good way) have occurred. The headlights started working normally (one side was out completely at hi-beam) and the wipers, that a local shop said "could not be repaired" (!! don't go there anymore) started working perfectly.

More proof of the feline spirit of these cars...pay them some attention and they sometimes return the affection...or bite!

Thanks,

Lou

Submitted by dougdwyer1@com… on Sun, 12/27/2009 - 20:46

The speedo going nuts while cranking usually suggests lack of voltage ....poor connections, low battery charge, that sort of thing.

However, transducer failures are common, and since your symptoms go beyond just the "while cranking" part, it couldn't hurt to replace it. It's on the left side of the transmission.

First, though, make sure the wires are not being burned by the left exhaust pipe.

You're probably not anxious to dig into the cluster again but I'd suggest adding a redundnat ground just on general principles.

Cheers
DD