I've looked on some other threads but couldn't find a situation exactly like mine.
When I got the car the tach would only read when the car was cold, and only under 2500 rpm, sometimes jumping between on and off while driving until warmed up. Now it doesn't read at all anymore (after sitting over the winter).

I've tried a different tach and it still isn't working. I've checked and double checked the wire going from the ICM to the C100 firewall connector and that's all fine. I don't really wanna have to pull the dash off to access the wiring going from the firewall to the tach.

Is there a way to test for the correct signal getting to the cluster?
Is it possible that a bad ground is causing this? If so, which grounds should I check? Engine grounds have all been checked over.

All the other gauges in the cluster are working correctly.

 

Check the connection at the coil. The tach signal comes off the negative side of the coil. My 91 had a wire break at the connector. I'm not sure if your 89 uses the same coil setup, but replacement pigtails are available if needed.

 

You can easily plug a known working tach into the connector at the gauge cluster, power and ground will be in that same plug, to test for a signal. Also using a DMM set to AC voltage will indicate if there is a signal getting to the cluster or not.

 

As mentioned I'm on my second tach now because I thought it would have been the tach at fault. I think it's the connection at the ICM since the problem was present before and after an engine swap