My tach is twice as high as it should be. With the computer monitor plugged in i noticed that my tach was twice as high. I don't understand why or know how to go about fixing it. Does anyone have a thread or maybe an idea of how to help? I have a 90rs that uses to be a 305 tbi but is now 350. Same t-5 as always but still this issue.

Thanks,
Brandon

 

Sounds like it may have something to do with pulses on the VSS. It can either be 4000 pulses per rotation or 2000 if my memory serves me right.

Example...5 rotations at 4000 per rotation = 20,000 pulses

5 rotations at 2000 per rotation = 10,000 pulses

If you are using a computer or changed to something using 4000 pulses, on an originally equipped 2000 pulse system, it might double your speedometer reading, because the speedometer/computer or whatever is receiving twice as many pulses per rotation.

Hope it helps you out.

 

my speedometer is fine, my tach is off. When you pass by those reminders of your speed it says the same as my speedo says but i am pulling 3500 rpm @ 70 in fifth gear with 3.08's? This is how i knew something was wrong. The computer confirmed it. I can not see how it woud be different as this car was originally a v8, no computer changes were done before i owned the car, and everything is original besides the block, heads, and cam.

 

anyone?

 

I believe this has something to do with a faulty component in the tach assembly found in 90-92 camaros. My 91 is the same way. If I'm not mistaken, there is a resistor or something that fails. Do a search on tachs being way off and you should find a number of threads on the subject, but I don't know if anyone has found a fix for it yet, though. This is probably why the 90-92 speedo/tach assembly sells so quick when someone puts one up for sale here.

 

yeah, i did some searching but did not see any kind of fix for this. Does anyone else have any experience with us?

 

i'm gonna guess no-one knows. Guess that leaves it up to me then.

 

I have the same issue on a 1991 Z28. I swapped in a good cluster and tach read correctly. That means the tach driver board is the problem.
I am also trying to fix the tach and found these links:

https://www.thirdgen.org/techbb2/sho...hreadid=279765

http://www.fieros.de/en/articles/tach.html

I have not attempted any fixes yet, but I will be starting the process now.

Have you had any luck so far?

Thanks
Mike