Tach goes crazy and engine starts missing

I will be driving along and the engine starts missing and the needle on the tach will go from 3000 to over 6000. The tach is all over the place while the engine misses even though the rpms dont really change. I am running a mopar electronic distributor with the chrome ignition box. The box is about 10 years old now. I dont know what to try first.

bad coil

Sounds like pick up, although they usually either work or they don't. Swap in a known good box would be the second choice. Look at the wiring for damage or shorts.

I had that happen with a msd system and the tach was bad. First and easiest check just take the tach wire off the coil and see if it quits.

Sounds like a ground wire issue. Try running a ground wire from engine to body. I had the same problem, but I forgot to hook up a ground wire from my intake to the firewall.
Hope this helps.

Thanks for the help. Sounds like it could be about anything. I guess i will just start with the easiest thing first. Start swapping in one part at a time until i figure it out. Its been 25 years since i put it together so anything could be bad by this time.

Hopefully you aren't still running the factory ammeter/ampmeter in the car? If so, it could be going bad b/c when mine went, that's how it started...right before it burned up the wiring harness.

Also seen a bad tach cause weird misfiring issues. Unplug it and try.

Disconnect the tach first and try it... thats
the easiest thing

When this happened to me my first thought was bad tach. Disconnected it at the coil and the ignition was back to normal. I was ready to pull the trigger on a new tach when it occurred to me that there was also a rev limiter connected to the same wire. Unhooked it and hooked The tach back up. Yeah it was the rev limiter.

Just posted this a few days ago on this thread

https://board.moparts.org/ubbthreads/ubbt...tml#Post2233534



Sounds like a lot of MSD Coils are failing lately from the research I did the last few days

Had my new 432 Stroker on the road the other day for the first time in over a year

Cruising on the highway after about a hour the engine started missing at high rpms

Turned around and headed home , where it started getting worse even at lower cruise

What I first noticed was the tachometer needle was bouncing all over the place , had a mind of its own

Went to touch the MSD 8202 Red Coil checking the tach wire , was super super hot

Just swapped in a old Mopar Resto coil that I had laying around along with a .8 resistor

Ran it for a few hours problem solved

This MSD 8202 Coil I had for about 5 years , ran fine with MP Chrome ECU , MP Distributor prior

First day with the new motor , had me wondering now what - The Tach issue was the only thing that pointed me directly to the coil


Anyways , this MSD 8222 Epoxy Coil - Better ? Same ?

Is there a better coil in factory location bracket on aluminum intake ?

Or is it time for a FBO Upgrade or Rev n nator Upgrade for this new motor

Street Use

Thanx Scott

I can see an internal short or failure causing that, if the tach signal is being picked up right at the coil primary terminal and not the port at an MSD box.

This is what I had happen back in the 80's with my Fiero GT. same symptoms . 1 spark plug wire had a crack and for some reason every once and awhile it would start throwing sparks to the block making a miss and screwing up the tach. Took a while to figure it out but a change of wires did the trick.

I agree with disconnecting the tach first,no parts needed.Then checking plug wires and beyond.

What engine are you working on BTW?

Working on a 70 dart with a 340. It has a mopar performance electronic ignition conversion kit. I finished it in 1993. It rained this weekend so i have not had a chance to work on it anymore. I did replace the ignition box, resistor and plug wires about 10 years ago but everything else is from 1993.

10 years is a long time for plug wires.. since
it was raining out I would look under the hood
in the dark and see if the spark is jumping
under there.. but I would start with wires

Use a spray bottle with water in it and spray a fine mist on the wires after dark. You'll find out pretty quick what condition the wire insulation is in.

I unhooked the tach and it didnt help. So i changed the coil and noticed a plug wire loose on the distributor cap. It runs fine now but i am not sure if i made the plug wire come loose while i was messing with it or not. Either way it seems ok now.