Every once in awhile, my speedometer malfunctions. For instance, tonight, it was working fine. Stopped for a sandwich, and it malfunctioned. Out on the freeway, at 70, per my gps, the speedo was showing 100 mph. Got off the highway and stopped at a light, the speedo was showing 40 mph.

It has done this quite a few times lately, and usually after a few minutes of the engine being shut off, it works as normal.

At 75 mph, the tach reads about 2100 rpm and gps shows 75. The tach never fluctuates.

I've searched these forums, and cannot find any treads with a similar problem.

Anyone have any ideas?

Thanks,
Darrell

 

Have your battery load tested.

 

Thanks, I'll do that, however, it was load tested about 2k miles ago when I had it tuned up. But, it could be failing, too.

 

The batteries on these trucks do some strange things as they age. One symtem of weakness is gauges bouncing around.

 

Dealer ran a full check of the electrical system this morning. Two problems.
1. Alternator shot. Replacement will be $600, but none in stock. Getting one overnight from Denver.
2. Instrument cluster shot. $720 to replace that. Not gonna happen, I'll just put up with the goofy speed readings, and use my GPS for accurate readings.

 

Battery is fine. Alternator was bad, so had it changed, and what do you know? Speedometer has not messed up in 800+ miles!