A few days ago my tach started bouncing all over the place while at idle and driving. I checked the wire, and it's still firmly on the coil.

Any thoughts on what's going on?

 

My Speedo does that now probably just wornout or needs the spring to be rewound or whatever but don't take my word for it yet still more possible solutions.

 

My car started doing that once, it would happen randomly. Then my alternator went out. After i switched it stopped.

 

Same thing happened to me. Alt failing made my tach jump around and then all the idiot light flickered too.

 

Usual cause is voltage regulation problems in the alt. A bad ground at the cluster can do it, too.

 

Well, my ALT is a used JDM FD unit. Maybe its about to fail any day now. I will just have it rebuilt, before it goes.

Thanks

 

Water or other crap messing with the connection can do this as well. Happens to me every time my road floods and I'm stupid enough to drive on through it.

 

A bit unrelated to the tach, but my voltmeter does this every time it rains or there's humidity or the bird flies too close... I've double checked that everything is ok with an external voltmeter. Must be a ground somewhere. Slightly more related, are you still running off the trailing J-109? When my tach died, it went out completely. But maybe a loose wire in the ignitor could be bouncing around? Just another thing to try.

 

My tach wire is wired to the -neg on the Leading coil (SA). I'm running the DLIDFIS mod with HEI modules. I too have an external volt meter but it's working fine at 14v constant. I'm going to put the wire on the Trailing coil to see what happens.

 

for the hell of it, check your distributor rotor. that happened to me once out of the blue. tach needle bounced around while idling - would work fine while driving though - and it persisted like that for a while. i can't remember why, but for whatever reason, i had to pull the cap at some point and what i found was a broken rotor. the damnedest thing ... a true WTF moment.

needless to say, i still have it to this day.

 

I will check it out. After all, it is also a used 85 dizzy given to me by a friend when I converted to DLDIFIS. I was supposed replace the cap and rotor at the dealer but I completely forgot. Thanks