I've searched around the "bad tach" threads, and most of them are diagnosed as a bad distributor. Well, my distributor seems fine, and my symptoms are a little different.

 

My tach used to work fine, but once in a while would start jumping around between 0 and the actual RPMs. The jumping became more frequent until the tach would just rest on 0 all the time. However, the weird thing is the tach will come back to life if I turn the interior fan on full blast. In fact, the higher speed you set the fan for, the more active the tach is. I'm guessing this is probably some bad wiring for the gauge cluster. Any tips on what to look for?

 

Thx

I'd still check out the distributor. Check for wobble in the main shaft where the rotor is and make sure all the wiring to the distributor is well connected (along with the coil). Don't ignore the easy stuff before you get into the more "esoteric" reasons for this symptom.

After reading this post I can think of a couple of things that may cause this to happen. First thing I would do to check into this would be to run a ground jumper from the negative battery post to the dash ground to see if there is a grounding problem. Along with that, check the wire connection between the disty and the tach to make sure it is good. If those things are ok then I would suspect that the tach is picking motor noise generated by the blower motor. Checking the power to the motor with a meter set to the AC volts function may show the noise up.

Yes this is a symptom of a bad ground wire, but i've never located the particular one. My old wagon used to do this, along with a number of other strange symptoms such as switches turning on things they arn't supposed to. It happened after i had the dash out, then fixed itself, so i never located where exactly the problem was.... Try adding an extra ground to the instrument panel and see if it fixes it...

I'm still not sure why the fan affected the tach functionality, but replacing the distributor fixed the tach issue.

I'm sure the issue has since been resolved, been almost a year.

 

Well, he was just updating his post. He's the original poster. It's good to follow up your posts so when someone has the problem, they know what's been done.

Johnson']Well' date=' he was just updating his post. He's the original poster. It's good to follow up your posts so when someone has the problem, they know what's been done.[/quote']

 

My mistake.

I'm still not sure why the fan affected the tach functionality, but replacing the distributor fixed the tach issue.

 

The fan motor may be generating some noise into the power buss and the tach is picking it up.

My mistake.

 

Hey, I wasn't trying to be rude or calling you out. Sorry if I seemed like I was being so.

Johnson']Hey' date=' I wasn't trying to be rude or calling you out. Sorry if I seemed like I was being so.[/quote']

 

None taken. I feel embarrassed when I do that crap.