Tachometer not working

2002 CHEVROLET SILVERADO
100,000 MILES • 6 CYL • 4WD • AUTOMATIC
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BENTHEMAN
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My truck typically cruises around 2000-2200 RPM on the highway at 70-75 mph. Recently, on a long road trip, when I went to pass or to go uphill (when the transmission would normally down shift), the engine ramps up to around 3000 RPM, and then the tachometer goes nuts, jumping erratically between 3000 and 4000 RPM. If I don't immediately let off the gas, then a very fast clicking noise starts. After this, occasionally the truck will shudder once or twice.

Otherwise it runs very smooth. The transmission shifts almost seemlessly when accelerating below 3000 RPM.

Thanks in advance!

Added in edit: I don't think it has to do with the transmission, because I sat in my driveway yesterday and revved the engine up---it did the same thing in park.
Dec 18, 2007 at 2:56 PM
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2CARPRO JACK
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I have seen this before.I take it there is no check engine light ? When was the last tune up? Last one I had, an ignition coil took care of it, but the plugs and wires need to be in good shape as well
Dec 18, 2007 at 4:33 PM
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BENTHEMAN
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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!
Dec 18, 2007 at 5:03 PM
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2CARPRO JACK
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He is thinking along the same lines...ignition signal to the tach, that is why it could be the coil as well. Definitly need to start with the basics
Dec 19, 2007 at 7:35 AM
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BENTHEMAN
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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?
Dec 20, 2007 at 11:31 AM
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2CARPRO JACK
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Tells the PCM where the camshaft is in relation to the crankshaft.
Dec 20, 2007 at 2:49 PM
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BENTHEMAN
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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.)
Jan 6, 2008 at 9:07 PM