Strange LT1 Tach problem- works until 4200rpm, then goes to zero
Thread Starter
Joined: Dec 2000
Posts: 2,361
Likes: 2
From: Buffalo, NY
Car: 1984 Trans Am
Engine: LT1
Transmission: T56
Axle/Gears: 10 bolt w/ 4.10 gears
Strange LT1 Tach problem- works until 4200rpm, then goes to zero
I have a 1995 LT1 swapped into my T/A. I've tried two 3rd gen tachs now, with the same result. I've used a 8000 RPM tach from an L98 car that I know for certain worked perfectly with the original L98, and I've also used a 6000RPM tach from a 305 TBI car. For both, I will slowly accellerate and the tach will move up smoothly until 4200 RPM, then it drops off the table to zero and stays there until I drop below 4200 RPM again. The engine doesn't get choppy or anything when this happens, it appears to just be the tach that acts funny. Does anyone know what is causing this?
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iTrader: (2)
Joined: Nov 2001
Posts: 920
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From: AL,USA
Car: 1990 Formula
Engine: 95 LT1
Transmission: T56
Axle/Gears: Weak 7.5 with 4.10s
Re: Strange LT1 Tach problem- works until 4200rpm, then goes to zero
Do you have it hooked to the tach output from the PCM? Mine did the same thing. I ran the white tach wire directly from the C100 connector to the coil and it fixed my problem.
Thread Starter
Joined: Dec 2000
Posts: 2,361
Likes: 2
From: Buffalo, NY
Car: 1984 Trans Am
Engine: LT1
Transmission: T56
Axle/Gears: 10 bolt w/ 4.10 gears
Re: Strange LT1 Tach problem- works until 4200rpm, then goes to zero
Senior Member
iTrader: (2)
Joined: Nov 2001
Posts: 920
Likes: 0
From: AL,USA
Car: 1990 Formula
Engine: 95 LT1
Transmission: T56
Axle/Gears: Weak 7.5 with 4.10s
Re: Strange LT1 Tach problem- works until 4200rpm, then goes to zero
Let me know if it works. It took me a while to figure it out.
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