(Question) HEI Internal to external Coil Conversion
 

Hello everybody! I have a question about wiring a external ignition coil. Right now as you can see i currently have a HEI distributor with the normal internal coil and am wanting to switch over to a external Coil so I can use my stock tach that doesn't work with hei. I found this part that allows me to switch to the external Coil but am not sure of what else I would need and how to wire it. I tried looking up a how to but can't really find what I'm looking for. I will link all the parts that I have selected down below.

Internal to External Coil Adapter: https://m.summitracing.com/parts/msd-8401
External Coil: https://m.summitracing.com/parts/msd-8202
Balast Resistor: https://m.summitracing.com/parts/msd-8214

I also have a coil wire ane mounting bracket selected as well.

Re: (Question) HEI Internal to external Coil Conversion
 

Maybe I'm missing something here, but why can't you wire your tach to the hei? Every hei I've seen has a TACH terminal right next to the IGN terminal ( where the green wire is hooked up now ). A female spade terminal on the end of your tach wire and you're done.:smoke:

Re: (Question) HEI Internal to external Coil Conversion
 

I know... I tried that and it doesn't read the signal that the hei puts out. I read on a older thread that there were two different ways that the tachs read. At this moment I have a aftermarket tach on my dash and I'm looking at removing it and actually getting the cluster tach working. I just have the feed wire up in the gutter wrapped up. I know it's hard to see. (See photo)

Re: (Question) HEI Internal to external Coil Conversion
 

Are you absolutely certain your tachometer works? I'm pretty sure I've seen factory tachs hooked up to an hei before. Something I learned that may or may not be helpful is on my ls engine my tachometer wouldn't work at all after the swap all the wiring was proper but it would raise to zero and that was all. After some digging I found that they don't have a strong enough signal to run the tachometer so you have to add a 10k resistor between a key on hot and the signal wire to boost the signal. It worked like a charm maybe consider that before buying all this stuff that might not work.

Re: (Question) HEI Internal to external Coil Conversion
 

I don't think you're going to solve this with an external coil. Even if it is the HEI upsetting the tach, it's going to be the ignition module (transistorized, maybe capacitive discharge not sure) that's throwing it off, not the coil.

So while you certainly can likely rig up an external coil to an HEI if someone makes the part you need, it's likely not a good idea and likely not going to solve your problem.

I had a tach that didn't work with my Pertronix, so I grabbed the "next one off the shelf" and it did work. Seems really hit or miss, and I don't know why.

Re: (Question) HEI Internal to external Coil Conversion
 

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I have never had an original tach that would not work with an HEI distributor.... Where is your distributor hot wire connected?

LockDoc

Re: (Question) HEI Internal to external Coil Conversion
 

It's connected to the bottom port on the back of the ignition switch.

Re: (Question) HEI Internal to external Coil Conversion
 

Double check that you actually have:

- GROUNDED the tach case. A lot of people miss this. It does not ground through the cluster.
- Verify 12V at the (+) terminal of the tach

Then worry about the signal. But verify the ground for sure, since it's such a common cause of problem.

Re: (Question) HEI Internal to external Coil Conversion
 

The tach connection is on the negative side of coil primary winding on both internal and external coils. What you have there is a pulse generated by the HEI module.

Also if you do convert to external coil, you do NOT want or need a ballast resistor. Its purpose is to reduce current through the ignition wire and points.

Re: (Question) HEI Internal to external Coil Conversion
 

I currently use HEI with factory tach, no problems.

Re: (Question) HEI Internal to external Coil Conversion
 

I'm thinking that my tach is dead. I've tried grounding the tach itself to the dash and no response from the key or anything. I've been looking at brother's trucks reproduction tach and it looks nice. Has anyone tried it?

https://www.brotherstrucks.com/67-72...tinfo/TACH100/

Re: (Question) HEI Internal to external Coil Conversion
 

Yes. It was as advertised, looks good, fit fine, works fine. Spendy though.

Re: (Question) HEI Internal to external Coil Conversion
 

Cool. The reason why I ask is I was also looking at classicparts tach as well and it states that it does not work with hei ignition..

http://www.classicparts.com/mobile/1.../#.WDp03-mIZ74

Re: (Question) HEI Internal to external Coil Conversion
 

GM offered tachs with HEI from 75 to mid 90’s. So they do work together.