Tachometer not working

Well, a check engine light came on the last time it happened, but it did this erratic behavior for quite a while before the light came on. I put the car into the mechanic's shop yesterday and they tuned it up (it was due for that anyway), changed the tansmission fluid/filter, etc.

I described the problem to the mechanic and he said somehting along the lines of ``Oh it just needs a tune up''.

Thanks for the help!

Well they hooked it up to a computer, and it told them to clean the mass flow sensor, and replace the cam shaft sensor. This seemed to solve the problem, and the transmission shift into the passing gear on the freeway is almost seemless.

As an aside, can anyone tell me what the cam shaft sensor does?

Ok. Here's an update for anyone interested.

After I replaced the camshaft sensor/tune-up, my truck ran better than it has in years. Then the problem came back. Sent the truck to the dealership, and they replaced the same sensor again. ``You must have gotten a bad part'' they said. Right.

I drove the truck off the lot, turned a corner, and the check engine light came on. The dealership had just closed, so I drove the truck to AutoZone, and the code read a misfire on cylinder four.

Long story short, the mechanics at the dealership had to call some experts somewhere at GM. Aparently there is an issue with the bearings (?) That hold the camshaft in place. On high mileage vehicles, the bearings can wear unevenly and cause a bit of a wobble in the camshaft, which will break the sensor, and may confuse the computer into thinking that there is a misfire.

Again, the symptoms are a VERY bad intermittent miss, and the tachometer going nuts. (I think the tachometer goes nuts because it takes its signal from the camshaft sensor, not because the instrument panel is bad---GM has a recall in place for the instrument panels on some trucks I think.)