Tach not reading correctly.

FrankieD61 · Jun 28, 2022

Hi,

So, recently, I just replace my ICM and my coil, and now my tach is not reading correctly after a short drive (basically, after things heat up). Which one of the two is the most likely culprit?

Thanks,

FrankieD61 · Jun 28, 2022

Sorry, just realized I left out some info (bad day).

I used NAPA parts both the coil and the ICM. The tach works perfectly when the truck is cold or warm after sitting. But, once things heat up, it just reads incorrectly, jumps, and falls below zero if I'm not hitting the gas. The tach worked fine before this.

User_name · Jun 28, 2022

Most likely icm. Parts like that should either be acdelco or delphi.

Did you use the paste on the back of the icm?
what engine do you have??
Tbi? Vortec? Is your buddy sitting on the hood feeding it with a coke bottle and turning the distributor rotor by hand?
We need basic info on your truck dude

FrankieD61 · Jun 28, 2022

Shannon400 said:
Most likely icm. Parts like that should either be acdelco or delphi.

Did you use the paste on the back of the icm?
what engine do you have??
Tbi? Vortec? Is your buddy sitting on the hood feeding it with a coke bottle and turning the distributor rotor by hand?
We need basic info on your truck dude

Sorry, like I said f'ed up day. I'm usually pretty good about that.

I used the heatsink compound. They give such a small amount, though. I have a TBI, 350. The truck is a 91 Chevy K1500. Roughly, 5 to 7000 miles on the motor.

User_name · Jun 28, 2022

Check the reluctor and the magnets in your dist. My magnets cracked in my 95 (tbi350) and caused an erratic tach signal. I scratched my head for months on that one

Frank Enstein · Jun 28, 2022

Don't use too much heat sink compound. Just a thin film to fill in the minute imperfections between the two metal surfaces.

+1 on the module. Aftermarket ones can foul up with an MSD box and that erratic tach output (white wire).

Check the grounds. I always cross that off the checklist first.
Checking the grounds first will save you banging your head on the wall a bunch.

FrankieD61 · Jun 29, 2022

Frank Enstein said:
Don't use too much heat sink compound. Just a thin film to fill in the minute imperfections between the two metal surfaces.

+1 on the module. Aftermarket ones can foul up with an MSD box and that erratic tach output (white wire).

Check the grounds. I always cross that off the checklist first.
Checking the grounds first will save you banging your head on the wall a bunch.

Yeah, like I said, it only happens once things heat up. This morning, for instance, it was a little chilly and the tach worked fine. But once the motor heated up, it went wacky. Does this sound like an ICM? I used the NAPA Echlin ICM.

PlayingWithTBI · Jun 29, 2022

FrankieD61 said:
Does this sound like an ICM?

It does to me. I had stumbling when cruising at ~2200 RPM when warmed up. Turned out to be the Spectra Premium ICM and the Pertronix Flame Thrower coil. I caught it when data logging and my RPMs went up to 4775!?! YMMV

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Schurkey · Jun 30, 2022

1. Do you have a tach set up for 6-cylinder when used with the transplanted 8 cylinder?

2. If the tach reads screwy, but the engine isn't misfiring...I'd suspect the tach, or the tach wiring, not the ignition system.

FrankieD61 · Jun 30, 2022

Schurkey said:
1. Do you have a tach set up for 6-cylinder when used with the transplanted 8 cylinder?

No, I'm using the tach from the 6. I had never been a problem, though. It always reads a little off but nothing serious.

2. If the tach reads screwy, but the engine isn't misfiring...I'd suspect the tach, or the tach wiring, not the ignition system.

Yeah, this all started with the new ICM and coil from NAPA. It has never been a problem.

Tach not reading correctly.

FrankieD61

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