Tach wire causing no spark?

[TPI84GMC]

Hey all,
I have an 84 with a 350 and HEI distributor with the coil in the cap. I have had the truck just shut off a couple times while running and has no spark. Then it will just magically start again later like nothing happened. After fishing around I found the tach with from the distributor is somehow like 15’ long under the hood and was laying on the headers with the insulation melted. If the tack wire is grounded will it stop the distributor from creating spark or is this just a coincidence and I need to keep chasing it?

Thanks!


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[JohnnyPopper]

bd pry has the definitive answer, but my first thought is that any signal from the distributor that goes to ground will cause a problem. Safety it off or reconnect it, bet the prob goes away.

[square81]

Had the exact same issue a few years ago. Green tach wire rubbed on a bracket shut me down on the highway. Run wire a different way no more issues.

[TPI84GMC]

Thanks for the replies! Got it re-routed and covered in high temp wire loom. Hopefully that was it and I’m out of the woods!


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[JohnnyPopper]

So it started running again?

[TPI84GMC]

Yeah, it has been running for quite some time. It was an isolated incident and I haven’t really trusted it since. After looking at everything I could think of this was the only thing that seemed off. Just wanted to confirm grounding the tach signal wire could have been the problem.


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[JohnnyPopper]

Yes, from my understanding, the signal is generated by the pickup module in the distributor.

Grounding it would shut down proper operation of that device.

You can always duplicate the issue and prove the theory...

[TPI84GMC]

Lol thought about that but seems like a bad idea to intentionally ground it. Thanks for the help I think this is as figured out as it’s gonna get. For now anyway...


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[frotosride]

I know you may have put this to rest but if you care then here's a little bit of info on what was happening.
To keep is simple and not worry about dwell time here are the basics. So on any ignition most ignition systems the coils ability to send a spark is controlled by grounding the coil to charge it then removing that ground to amplify/increase power through multiple windings and sending a spark. With the tach wire melted and grounding, the coil was never able to discharge and thus no spark.
Not every coil functions exactly the same but controlling primary coil charge and discharge is a constant in all of these systems.
Take away. If this lasted for any period of time don't be surprised if that coil goes bad anytime soon. Glad you found the issue!