Tach

[94 formula]

What wire does the tach attach to, to get its reading?

[wolfdogs]

What wire does the tach attach to, to get its reading?

depends on the year. the spawn from the PCM. Autometer has the info for your particular year... just tell em when you call you bought a shift light and you need to know....

[94 formula]

Ok. I was wondering this because I'm having low rpm stumble and I'm pretty sure its caused by the opti. I noticed driving today that if it stumbled the tach would jump up and down about 500 rpms, and was thinking maybe if the rpms were read from the opti that this could be a good way of telling when an opti's ready to go out the door.

[Injuneer]

The PCM reads the engine RPM from the cam/crank position information, supplied in the high and low resolution pulses from the Optispark optical sensor. The PCM feeds the RPM signal directly to the tach. So..... a jumpy tach, accompanied by a stumble, could be a direct indication of a problem with the Opti optical sensor, or of corrosion in the Opti harness connectors.

But it could also be a sign of a faulty engine ground. Do any of the other gauges jump around, or do any of the dash warning lights, like the ABS INOP or LOW TRAC lights come on.

[94 formula]

Everything else is working fine. I'm guessing its the opti I just turned 110k and its the origional opti. So is my theory probably right about the jumpy tach being cause the opti. Also I hooked my car up to freescan this weekend and got no codes from it so if I'm not getting the resolution pulse code (forget if its high or low) am I alright to go with just a cap and rotor or should I replace the whole opti? It has never had water damage due from the water pump and has just started acting up.

[Injuneer]

Also check the dash mounted voltage gauge when it starts to jump around... that might indicate a low system voltage condition.

If you replace the Opti, I would do the entire unit, and use only the genuine GM replacement. The price is going to be high ($300), even from the online discount parts sources, because you have the "unvented" one. There are too many things that can go wrong.... the cap/rotor is just 1/2 of the unit. The back 1/2 includes the bearing, which is a very low quality item (mine was hard to turn at 50K miles), and the optical sensor. If your sensor is starting to "blind" because of the rust that forms on the bearing retainer plate, it needs to be replaced..... so, if you are going to the work of replacing part of the Opti, bite the bullet and do the whole thing.

But don't forget to check the harness connector at the Opti, and the other gray connector on the bracket on the passenger side of the intake manifold. The one on the Opti will corrode pretty badly and screw up the signal.