I couldn't really find anything in the search, so I figured I'd do a mini write up, or rather a conformation that the 90-92 tach fix does work the same on the 82-89 tach. It is a little harder to pull off because you don't have a removable board to play with. You don't have to disassemble the tach though, I did it all from the back side.

The concept is the same, read the other thread for deatails. Suffice to say you need to change resistance across pins 4 and 10 of the resistor pack chip. If you use a 10 turn precision pot you can fine tune or even recalibrate a tach. I turned this broken V6 tach into a functioning V8 tach with this method. After adding the pot I just hooked up Tunerpro RT and monitored RPM while I adjusted the pot so that the tach matched.

There is a very convenient place to add the pot on the top corner of the board. This way you don't have to worry about cutting the legs on the resistor pack chip, just cut traces. And you can solder the pot to two solder joints that are close enough together that I didn't even have to add leads to the pot. Here's a really bad pic of where I put the pot.

If anyone has questions and can post a better pic of the back of their tach, I'll be glad to photoshop directions onto the nicer pic.

Sorry if this has been covered but I couldn't find anything and would like to help others out. Also if this is in the wrong spot, feel free to slap the FNG and move it.

Hope this helps,
~Tyler

 

Hey, quesiton for you.

My Tach (89 GTA) seams to be displaying low... and the higher it revs it seams the slower it climbs. when it gets to around 4k, the needle bairly moves. I will try to acquire a tachometer to hook up and verify, but yeah.... I live in Minnesota and the car is in the bay area, so all my tools aren't around here.

Will this fix these simptoms too? most things I read are about the tach displaying too high.

 

Hello, ive got an 89 iroc 5.7 tpi. Needle jumps everywhere. Usually 4x what it should be. Thank you for showing us what to do. Can I ask what size potentiometer you used? And do you know what resistance your final adjustment is set at? Thank you so much.

 

FWIW, the op hasn't been on the site since 2012.
"Last Activity: 01-31-2012"

Here is the other thread he referred to and may have the size needed.
https://www.thirdgen.org/forums/elec...meter-fix.html

And another thread with more information.
http://www.fieros.de/en/articles/tach.html


I don't recall what size to use but you may be able to find it by searching further. The needle jumping around may indicate more than just a resistor and I'm guessing maybe a capacitor or something else like a cold solder joint.

...looking through he thread I see mention of an 1meg purchased, and also people using a couple of 100k resistors. So I might try a 500k ohm variable resistor. You can use a 1 meg ohm, but it may not be as accurate. It would still work for the most part though.

Also this:
https://www.thirdgen.org/forums/elec...ml#post3004390