Another year of winter storage has passed, and once again my tachometer reading in my '90 RS keeps getting higher! The tranny and converter seem to be working fine, but now the tach reads a whopping 4,500 RPM while doing 60mph in locked-up overdrive. (the actual should be around 1,600 RPM)

The only reasons anyone can give me for this are, "It's getting old", "they just do that sometimes", and "these cars have bad wiring".

I have driven some of the worst jalopies on the planet, and even the most neglected junk heaps have never given me gauge problems.

If someone has a real answer to this problem I would love to hear it. I would rather not change the gauge cluster, since this would be like throwing my documented mileage reciepts into the shredder.

 

Well unfortunately, these cars definitely do that sometimes. Most of the time in fact. The later-model GM tachs are all basically identical, except for how many cyls they're set up for; and when they fail, they always read high. And they all almost always fail.

Getting a new circuit board for the tach is about the only way to fix it, short of another V8 cluster.

Here's the circuit board, with the part that fails circled.... note the little slit cut in the left half of it, that's done with a laser to trim the resistor to the right value.

 

What is the likelyhood that a replacement cluster (used) will have the same failure?... and why would a failed resistor continue to change in value?

 

Rather likely actually. Out of all of these that I have, the one in my car is the most nearly accurate; it only read about 3% high when I put it in a couple of years ago.

I don't know... I didn't endow it with those properties, or even decree that they be so; I merely observe and report.

 

Hey. Those screws look familiar...

Ever tried to replace the DIP resistor pack with precision wirewounds? It would be a lot of work, with all the drilling and jumpering, but the tolerance wouldn't change since there would be no carbon block to crumble and break down. That might be the final solution to wandering tachs.

 

Vader, if you or anyone else has a tach that is readind reasonably correctly do us a fav and check the resistance across the pins of that chip. Maybe we could just unsolider the chip and put a DIP socket in there and make a "daughter board" of the correct values that could then be plugged in.
Maybe offer them for sale to others with the same problems.
Heck, if it could be changed that easily then you wouldn't have to use high buck precsion's.

 

When I break out the 'Bird for spring, I'll try to remember to verify the tach. If it's reasonably close, I may just need to remove the I/P to clean out the gauge faces a bit.

 

Can you even get a "true" reading with the DIP chip still in place? I have a Camaro tach I need to try to see if it is accurate. If it is I'll take the resistance readings on the chip and post them here (or make a new post with them)

 

Sorry to bring this old post up, but did any of you guys figure out the resistance reading on that chip. My tach is annoying me as well, and I want to fix it.

 

I came across an old post which answered my question indirectly. Didn't give me the resistance, but better yet, a part number.

In case anyone is interested, here you go... https://www.thirdgen.org/techbb2/sho...hreadid=114057

 

I think this explains it -- [GM]

 

 

Just ignore his comment. He'll be taken care of...

 

How does your tach read compare to your scan tool? I say this because both my tach and speedo agree with my scan tool; and the ratio of mph/rpm remains constant.

And I have verified the accuracy of my speedo with mile markers as well. It is within +/- 0.5% with the difference being tread depth on new and old tires. This was measured over 100 Kms (about 60 miles).