Digital Tach acting up

[1984C4]

My 1984 digital tach is acting funny. A couple of times it read zero at all rpms with no corresponding bar indications either. After the car was shut off and restarted it worked normally. These occurences were months apart. Took it out today for a rare winter drive since the roads were dry and I have spring fever and noticed the rpm indication is all over the place from zero to 4000+ rpm. At idle the indication will drive up and down to 2000 rpm and while driving it will go well past 4000 yet the engine is certainly not doing that. It idles and runs fine and smooth. Since the tach input comes from the electronic spark timing distributor, wouldn't this cause the engine to run improperly? Isn't it the same signal sent to the computer to fire the plugs? Lastly does the digital cluster have a microprocessor that could be bad? Thanks for reading. Mike

 

[77-85]

I don't know if your 84 is the same are an 85 when it comes to the wiring for the Tach, but my 85 4+3 was doing exactly the same thing. My problem turned out to be a poor ground on the tach filter which is located near the distributor. In my car it is a small silver cylinder about 1 inch long and 1/2 in around, and it is bolted to the back of the driver side cylinder head (wiring goes to the harness connected to the distributor).

In my case I simply cleaned the ground, and made sure that the bolts were tight. The tach worked perfectly after that minor adjustment. Your situation may or may not have the same cause, but from your description it almost certainly is related to a grounding problem of some sort.

 

[1984C4]

Thanks for the reply. Sounds like what I was thinking also and it is plausible since I pulled the cap off last year to check the contacts on the distributer. I may have loosened something up. I'll check it out on a nice, warm, spring, Maine day.