The tach in my 90 iroc goes from rest to straight up when cranking and falls to the stop (past 7000) while running. I have the plastic cover off and flipping the needle back to the other side only lets it sit on the stop (past 0). I have tried another guage cluster with the same results. I have cleaned the connection and put a smalll amount of dialectric grease, but get the same result. All of my other guages work fine. My next guess is an ignition module. Any other suggestions? I do have a stock replacement msd coil but have no idea if the tach worked correctly before or not. I bought the car dead and have managed to get it running again. Last inspection expired 8/98! There are no other electrical problems.

 

Did you check the lead coming off the coil for the tach? It sounds like it's grounding out on something. If the tach was off, it would still move around with the engine, it would just be off in terms of the number it was pointing to.

I also think there is a minor thing you have to change when installing a MSD coil if you want your tach to be correct. I might be wrong though. I'm sure someone with more knowledge on the subject will step in and give you some advice.

Welcome to the forum

 

I hate to peel the factory wire loom off but may not have a choice . While this car has been ignored it hasnt been abused. Im the first hamfisted grease monkey to lay paws on her. Think ill hit the msd website and see if they list any special instructions for this. I dont remember anything in the included instructions other than how to modify the factory bracket to use the supplied bolts.

 

The website is really vague but does mention a tach filter. What and where the heck is that? I sent an email to msd tech but havent had a response. Guess Ill have o call.

 

The tach filters (if you even have one, a lot of cars don't) is only a concern with the entire MSD ignition box - not just a coil.

There is no "extra" procedure for installing an MSD coil, just plug it in, make sure it's grounded, and go!

 

Its definitely not the coil as I just tried a factory type and it is still acting stupid. If I can find a module from one of my old projects, Ill give it a try. I just dont think its a wiring problem as the car hant been messed with by anyone. Thw riring looms are all still on the car and nothing looks cut,but I havent crawled all the way under the dash either.