My tach sometimes when at idle, will click on and off, it looks like the end of a fishing pole when you get a bite, jumping around. I hear a clicking sound under the dash when this happens. Would this be the relay? It does not do it when Im driving, seems only at idle. Any ideas?

I have checked and cleaned the connections on the trailing ignitor and it still does this.

 

I've seen the same thing on mine from time to time, absent the clicking. No idea at this point what causes it; most-likely cause would be a short-duration miss.

 

YEAH! My car isn't the only one that does this!!

Seriously, though - check the plug on the back of your Gauge Cluster behind the dash. You can reach up under the dash by getting your back into it's most uncomfortable position imaginable while laying on the seat and floorboards and barely reach it. It's a socket plug connector that sometimes gets corroded or rattles loose. If you can reach it, you're not in enough pain, but wiggle it back and forth and maybe spray some contact cleaner on it, that might help.

Mine is intermittent and tends to act up more in the wintertime when temps are lower. Summertime, it rarely does it, as the temps here in AZ desert tend to keep things stuck together pretty well.

You mentioned checking your ignition system, which could be the other common cause and brings a lot less back pain. The same white wire that drives the fuel injection system also sends the feeder signal to your tachometer. Check that wire for a solid connection on the trailing coil, but that would also come with some intermittent 'missing' which may or may not be noticed since trailing performs clean-up of ignition rather than primary burn.

 

My car also does this sometimes. I'd like to find out why.

 

My car did this until I switch the tach to come off the leading ignitor instead of
trailing. I have an 80 SA BTW. I suspect my trailing ignitor is going south slowly.
Does it the most when it is under heavy load and high heat.

I'm going to switch them out for the GM HEI ignitors which I understand work
fine and are really cheap. I'll just buy 3 or 4 and keep the spares in case one blows.

 

On the 80 SA, the tach is supposed to go to the leading... since the trailing doesn't run all the time, on stock wiring build.

 

Yes, when its below 60 degrees, mine will not work at all till it warms up.

I will check the connection behind the dash by trying to reach up in there. I had experience in the position you talked about by having to re-install the cluster in my kids 81 Z28.

I have also cleaned up the connections on the trailing and leading coils, but I can check this again.

Wonder why it is not doing it when the gas is pressed, during normal driving? It only does it at idle or if I put the car in neutral (thus engine at idle) and coast.

 

Thats sounds like a loose connection to me.

 

Yeah it did until I installed the 2GDFIS and then I put it on the trailing and
unplugged the connection that could cause the trailing to cut out. Worked fine
for quite awhile. Anyway, moved it to leading and its fine for now but I think both
ignitors are getting ready to blow.

 

You think at the coil or the back of the tach???

 

Could be at the coil, the back of the tach, or even at the plug at the rear of the alternator.

Don't laugh - it happens. The Alt is mounted to the engine, which sees a lot of vibration. Eventually, if that plug gets corroded, it can shake loose and then give a weird signal from the alternator through the battery charging circuit, possibly enough to cause weird tach signals as the coils are directly wired through ignition to the battery.

Worth looking at, anyway - and the Alt is right on top and easy to reach...

 

Well, at a guess, at an idle of 750 rpm, that's only 12.5 pulses per second triggering the tach; one missing pulse is long enough for the mechanism to detect. At higher RPMs, the mechanical lag in the guage response is such that it can't react visibly to an individual missing pulse.

Another possibility: engine low-frequency vibration amplitude is greatest at idle, so if it's a loose-connection issue, there's more chance for it wiggle at idle.

 

Interesting, I noticed my tach bouncing again and it turned out whenever I turned
on my fan for fresh air out of the vents the tach jumps around. Like Divin says at
higher rpms than idle it goes away.

Sooo, I think theres a bad connection or ground happening with that fan motor.

Yet another way to make a bouncy tach

 

Hey guys, sorry to bring this up again but this started to happen to me on my 12a recently.

First I never had a steady rpm signal on my tach but since a few days ago the thing is bouncing way more than before. I always read my rpms signal from my trailing coil, yesterday I recorded this vid so you can see what I am talking about. I really want to get this fixed so please help me out on what I need to start checking, I have stock coils, stock dist.

http://www.youtube.com/watch?v=cLR0i74fZD8

 

i seem to remember this happening to one of my old cars, and at the same time the alternator went. once i replaced the alt, the tach worked fine again

not sure if its related, but seemed to be in my case

 

this happened to me on my 85 and all i had to do was clean and dress up my connections going to the coils

 

I will try that, maybe I should try changing the coils too. I bought them used and I dont know how old they are

 

bad exhaust leak there brah!!

 

mine will do this and as stated above specially during winter, but it's not very violent.
the thing that does have me somewhat worried is that my tach at idle reads rpm higher than it should, but its not running at the rpm it shows, for example my idle is at about 850rpm and it'll read 1100 on the tach, and this happens after it warms up. when i turn the car off my needle won't drop all the way down it'll stay at around 500rpm. give it some time and the needle drops crank it and it acts fine but slowly climbs up again.

 

I've had similar "tach jumping" issues with other cars too. On one it was a grounding issue, I ran new chassis to motor grounds for a stereo I was installing and suddenly tach worked fine. One was igniter related, never figured out why, but when I tried a new module, the tach was fine. Car ran fine with either. (wierd!)

 

im tellin u im 99.9999999999% pos that it is either ur coil is bad (since u said it was old)
or a dirty connection

 

I agree with you, maybe I have both scenarios . Will check on Saturday carefully and see whats up

 

I agree that it,s contacts. mine does that but only when it,s real cold. But stopped when I changed coils.