92 tachometer drops then regains

[merc Dod]

The tach on my 92 bonneville will suddenly fall and just as suddenly regain to correct rpm, over and over. Anyone have any insight into this?

[HotZ28]

Any engine hesitation during this event? If not, check the white wire on the ICM cable for corrosion, or loose terminal.

[Jrs3800]

Agree with HotZ28 completely.. Check that white wire... and as he wondered does this cause any hesitation or momentary stall when the Tach drops.. almost like the engine dies for a split second and comes back?

[merc Dod]

The tach just drops and regains but what I thought was an unrelated problem (the car maybe twice a week will suddenly jerk or shudder. I thought it might be a transmission problem. Could it be related? I assume the ICM is the rectanglar object that the spark plug wires originate from. Is this correct? If so do I somehow dissconnect a plug and spray it down inside with contact cleaner. Is that the best procedure?

[HotZ28]

I assume the ICM is the rectanglar object that the spark plug wires originate from. Is this correct?

Yes, that is correct. Remove the bolt from the front of the terminal and pull out on the the terminal to remove. Clean with contact cleaner and apply dielectric grease, then reinstall. The sudden "jerk or shudder" could be the early signs of ICM or crank position sensor problems, both of which can cause erratic tach display, or you could have simply a bad plug, coil or wires causing the "jerk". Last but not least, you could have a problem with the I/P cluster or tach. BTW, the white wire we were referring to is terminal "E" or 121.

[merc Dod]

I removed ICM, noticed one of the ends the spark plug wire slips over was partailly ate away by spark. (I think plug wire was not all the way down) Cleaned bracket where ICM attaches to it just in case a good ground is needed there. Sprayed both sides of connector and pluged back in. Tomorrow I will test drive to work (30 miles one way). Might have to replace ICM due to where it was ate away, and plug wires. We shall see. Thanks for your help so far. But I doubt Im done with you yet.

Forgot to mention: was getting a ses light (cam or crank sensor) and noise on right side, tach problem, and shudder. So I replaced the timing chain, timing gears, timing cover gasket, oil pan gasket, water pump, interrupter (magnet), cam sensor, crank sensor, serpintine belt, chain tensioner, harmonic balancer, oil filter and oil, and oil pump rebuild kit. Now I only have the tach problem and shudder. Hope the cleaning takes care of it.

[merc Dod]

All is well, thank you.