87 2.5 with the Isuzu 5-speed. I bought the car about three years ago and it has been my daily driver ever since. The tachometer has never worked since i bought it. Being an 87, it is a DIS ignition, so i have no idea what is supposed to feed the tach its information. I've already changed the ignition control module, both coil packs, and the crankshaft position sensor. The "shift light" does work.
Yesterday, the tach decided to come back to life. It will ever so slowly creep to 6k RPM regardless of what the engine is doing. Sometimes it will continue to creep until it has made a full revolution, other times it will ever so slowly creep back down to zero. Is there any troubleshooting i can do to see if the tach can be made to work properly? Or is time to start shopping for a new gauge?
My '84 tach started reading 500 to 1000 rpm high. I got a tach assembly from a Pick & Pull for $10 & the problem was solved. (May not be your issue...) My '86 tach dropped to 0 rpm a few times while driving. It would come back on by itself or if I whacked the console. (Connection issue...)
My tach was going crazy, turned out to just be a loose connection to the HEI module in the distributor. I have an 87 GT.
I'd venture a guess that one of your electrical connectors (somewhere) is loose, and/or was totally not connected. Either that or it's corroded and you have to clean/di-electric grease the connector to get it to make up nicely and give a consistent signal. The corrosion story works for a connector that isn't working at all, then decides to work because something allowed it to (water or some other reason the electrical signal could make it through the corrosion)