Well I have never been able to get the tach to work correctly. Typically the tach appears to be working when I first start the car but the speed that it is reporting is too high for idle and if I blip the accelerator the tach doesn't move. Eventually it just dies when I put the car in gear and go on my way. That's the problem.

In my research I have found that my 85 works differently than earlier versions and a simple cigarette butt in the tach amp doesn't apply. In fact I don't think my car has a regular tach amp on the driver side fender, that's not to say there is not identical looking plug there, it is, just doesn't have the tach amp part (very confusing...).

This is what I believe and please correct me if I am wrong.

I start with the ECU which is this black box on the passenger side kicker panel. The ECU is the brain trust for the environmental crap on the car. On the side there is a plug with 25 pins. Pin 25 I think is the one of interest for the tach. The manual says that it goes to the revolution counter? Is that the Tach? Pin 16 and 20 are inputs from the RPM Sensor which in my car test out okay, via multimeter testing. However the Pin 25 is dead and thus I think not powering my tach.

If this is correct, this would explain other things, for example when I plugged the EGR vacuum plug, I go no gain in performance. I think this is because the ECU never turns it on because it is not processing the information from the RPM sensor that indeed the car is on and operating at greater 500 rpms operate the EGR...

Long story short I think my ECU is busted, I'm going to try a resoder it this weekend to try and repair the tach. As anyone else done this, am I totally off in my thinking? I'm using section 14.8-100, -50 from the tech manual on this. It is for testing the EGR, but it also as a lot of other information that is important too that I can't find anywhere else in the manual.

IF I can't get the ECU working, I'm not so sure it's worth buying a new one, and I doubt I'll be able to find a good used one. Has anyone tried to by pass the ECU maybe using a tach amp (I'm drawing at straws here) to get there tach to work?


Thanks Jason

I have something similar, but for different reasons. My car's an '85, but the engine and transmission were transplanted from an earlier model year. As a result, the required sensor ("which is screwed in on intermediate flange to automatic transmission," according to my FSM) simply isn't there at all.

I'd like to try using the TDC sensor (front of engine) with an earlier model year tach amp. What this would require would be for the tachometers themselves, across these model years, to operate off of the same sort of signal. I don't know this to be the case, but I'm working on buying some parts from a guy who's parting his '83 out, so that I'd have some bits with which to play.

Depending on the year the of the engine/trans will determine what you can do...What year is it?

Ahhhhh.....the ECU looks a little intimidating for me to do a resoldering job on. Basically it's two circuit boards soldered together with the 25 pin plug. Lots of resisters and alike inside. Maybe I should rethink finding a used one or start retesting everything else....

I was told that the engine/trans came from an '83. I must admit that I haven't physically looked for the presence of the "other" RPM sensor yet, but I'm assuming that it won't be there on the older hardware.

It's not a tremendous concern to me, and not high on my list of things to do on the car. I'd just sort of like to have the tach working.

I could be way of bases here and I hope someone else chimes in here, but I don't think you have a big issue. Assuming your ECU is working, which may be a big assumption, I think you can simple "plug in" your RPM sensor into the ECU and you should have your tach back. You may need to splice it in, but if you reference the tech manual 14-50, and 14-100 you will find the schematics outlining the ECU and the RPM.

Again I believe the tach amp to be in the same as earlier versions it's just built into the ECU, so I think it should work.

Has anyone ever tested the ECU? I spoke to the dealer, and an Independent and both said that they don’t test them; they either leave them broken or replace them.