My 87 IROC (170k miles, 5sp)has an eradic tach reading and running rough (I have two other postings on the subject-thanks for replies). Today I took the distr. off and it was badly corroded and the rotor was shot, also the plug wires had rusty end connections. I cleaned it up (will get replacement wires and rotor and cap Monday) and put it back together. Car runs smoother but the tach is still going crazy. I have a replacement pickup coil as I thought that it might be the problem (ran an ignition coil check and it checks out ok). To get the old one off I will have to take the gear off the end. I will do this when I have the new rotor cap. So, I notice in the book that you need a special tester to check the "module") that is located in the base (my coil is remotely mounted).
What I am wondering, is could my problem possible be the module (is it a tach sender unit? and is there a way to test it with a multimeter???
As long as the distributor is going to be apart I would like to test it.
Thanks for any replies.

 

Autozone and discount auto(I work for az, so im kinda partial)can test the module cold on the bench. the only problem with that, is it doesnt simulate the heat of real world engine temps. any obvious problems should display. as far as the tach cutting out, your module or coil could be to blame. stock(or OEM replacements) cu out around 4500-5000 rpm. accel or other aftermarket manufacturers make better modules for this purpose. more than likely its the module, but a resistance check is only good for that, not function. good luck,
mike