My 79 has never had a very good tach but in recent it's gotten much worse, it reads exactly double the Rpms it's actually at until it hit a bump in the road or smack my dashboard hard over the cluster and then it comes back to normal, it also snaps back to normal readings under full throttle but as soon as I let off it jumps back to double RPM. I Figured it was a bad connector or solder joint so I took my cluster apart cleaned all contacts and reflowed all of the solder joints on the tach pcb, now it's just stuck doing double RPM, jostling it will not "fix" it anymore. Any ideas would be appreciated I'm at my wits end with this issue and I'm not paying $500 for a replacement cluster

 

I had an issue on a 79 where the tach would bounce around and act wonky and the issue ended up being a faulty external voltage regulator. I don't remember what I did to test it to confirm the issue, as it was so long ago. I do have the 79 FSM and I can post a picture of the section on how to test the voltage regulator if you would like. Its always best to test it and do a proper diagnosis before pulling out the parts cannon.

But if you're considering shooting the parts cannon at it, the voltage regulator is a cheaper part than a cluster. If you want to try another cluster or just the tach, you could always look in the marketplace section here on the forum and see if someone has one available. $500 for a cluster sounds like a crazy ebay price to me.

 

The 79 clusters are in shorter supply because back in the day we would acquire them to get the 120mph speedometer. This is why you see crazy prices for them now.

 

I did recently do the relay mod and deleted my voltage regulator since I'm running an FC alternator now. Was hoping that would fix it and it didn't. Going to recap it after work today but I'm not hopeful

 

also if the car still has points, and the points are on the way out, the tach will switch between being a voltmeter and a tach, or the needle jumps.

 

Any chance series 2 tachs or 80 s1 tachs are the same as my 79? I don't need a whole cluster

 

Are you sure you need a new tach?

 

Pretty certain since aftermarket tachs work normal, it's just the one built into the cluster that's having issues, I can use an aftermarket tach but I'd really like the factory one to work

 

That bit of information would have been handy in the original post.

 

Forgot I used to run an aftermarket tach, it's been a few years

 

I switched to an 84 dizzy with ignitors years ago, didn't change anything either

 

Sounds like a tach is in order then. The 1979 and 1980 are the same. I would imagine the 1981-1983 would also work but not sure. The parts manuals on Foxed.ca should help.

 

I have an update, unplugging the signal wire from the coil to the tach doesn't change anything. It'll still get tach signal somehow, but unplugging the trigger wire for the coil from the ignitor does stop the tach signal if unplugged, so I'm completely lost on how that works

 

You need to trace your wiring from when you switched the dizzy. Somethings shorting out I suspect and giving you that signal. Start from the ignitor since it kills the tach when unplugged.

 

I figured out more things, unplugging the ignitor completely kills my tach signal, if I unplug the tach signal wire and keep the ignitor wire plugged in and keep the end of the tach signal wire close to my coils I still see a reading but the reading is normal, not doubled like when it's plugged in. If I move the tach signal wire further away from the coils it drops to 0, so I guess I should try shielding the wire with aluminum tape? Not really sure what else to do, also not sure why this wasn't also causing an issue with my aftermarket tach and dwell meter

 

Did the tach work when you had points?
Did it work after the switch to the 84 dizzy?
Does it double the RPMs if you switch the leading and trailing igniters?
Does it double the RPMs if you put the tach signal wire onto the leading igniter?

tg's comment makes me wonder if somehow the leading timing signal is also making its way to the trailing igniter and doubling up the signal there?

 

Check the boots on the coils, in the dark, to see if they are arcing out. That may be your issue. How old are the coil wires?

 

The tach did not work before I went for the 84 dizzy, the previous owner put a single MSD 6AL-2 in and bypassed the points. swapping to the 84 dizzy fixed the tach and I didn't drive it very much at the time but the tach was working perfectly for a year or so after doing the swap to the 84 dizzy. I let the car sit for a couple years and it's my daily driver now and it has not been good since I started driving it again. I've cleaned all my grounds and reflowed every solder connection I can find, I haven't tried swapping which coil the tach signal is pulling from but I'll give that a try after work

 

Coil wires are pretty new, about 3k miles on them and I've seen no arcing in the dark, I thought about putting dialectric grease on the innards of the boots to better shield against possible arcing that I can't see though

 

Also not sure if it's related but my rpm buzzer also doesn't work and I confirmed the "speaker" for it at least is fully functional