Points -> HEI -> Holley EFI/Dual Sync Wiring - Impala Tech
Electrical & Wiring Troubleshooting electrical problems

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post #1 of 5 (permalink) Old 04-08-2019, 07:07 PM Thread Starter
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Hello all,
I previously switched the points ignition on my 68 396 to HEI. Easy and straightforward. Now, I'm adding a Holley Sniper EFI, dual sync distributor, MSD 6AL, and MSD coil to the new 496 and I'm stuck on two things.

The american autowire engine harness I bought previously is modified for HEI by AA and has a dropout with a 12 GA pink circuit and 20 GA yellow circuit
terminated together in the connector (see pic) that plugs into the BATT side of the HEI distributor.

First issue, the yellow circuit. This is the coil-to-starter circuit on the OEM schematic and routes from the HEI plug to the starter on the AA harness. Nowhere in the holley EFI, dual sync, MSD 6AL, or MSD blaster coil is this circuit mentioned. The coil states specifically that only the circuits from the 6AL connect to its terminals. What do I do with this circuit?

Second issue, the pink circuit. This runs through the bulkhead connector on the frontwall and ties into the ignition switch. I suspect this is a switched ignition source. My issue is regarding the size of the circuit. The dual sync distributor requires a +12V switched ignition source, but it uses a much smaller 22 GA circuit. Can I splice this into the 12 GA pink from the harness or will I fry something? Do I need to replace the 12 GA pink all the way to the ignition switch with a matching smaller circuit? It seems like going from 12 to 22 GA would cause something to heat up that isn't supposed to.

There is a lot of info here, I know. Thanks for any replies
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Last edited by 68WASAGOODYEAR; 04-08-2019 at 07:35 PM.
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post #2 of 5 (permalink) Old 04-08-2019, 10:21 PM
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First thing unplug the battery when working on the car's electronics. Only way save buying another ignition box from MSD.

The MSD ignition is a Capacitive Discharge system. It has large capacitors in the box that build up a charge (like the old style flash on a camera) and fires the plug using the coil as a transformer (voltage multiplier; not an inductive flux concentrator that HEI and points use). As such the ignition box has a heavy wire that runs to the battery for power and uses the ignition switch to trip a relay in the box to energize it. You can not use an HEI module with a MSD CD ignition. All you use in the distributor (an HEI distributor) is the reluctor wheel to trigger the discharge of the capacitors (purple and green wires). Because the coil only multiplies the CD discharge you should by an open E-I coil (no oil or oil cooled wires) instead of using the stock factory coil. It will over heat otherwise.

All of this is laid out on the instruction sheet with a pictorial wiring diagram. You do not use the Bat terminal or the Tach terminal on the HEI distributor. If it isn't on the MSD wiring diagram it doesn't exist and adding a circuit with voltage in it will burn something up.

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post #3 of 5 (permalink) Old 04-14-2019, 03:59 PM
Join Date: Nov 2018
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Curious as to why you wouldn't go with the Hyperspark distributor/coil/ignition box.

1970 Impala Custom coupe
400ci SBC
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post #4 of 5 (permalink) Old 04-14-2019, 07:12 PM Thread Starter
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Originally Posted by FearlessAZ View Post
Curious as to why you wouldn't go with the Hyperspark distributor/coil/ignition box.
In order to utilize the ECU on the Holley EFI to control engine timing, you have to use the holley dual sync distributor. Holley also owns MSD, and they have a YouTube video about how to set all this stuff up and wire it up too. Much easier.
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post #5 of 5 (permalink) Old 04-15-2019, 03:41 PM
Join Date: Nov 2018
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This can also be done with the Hyperspark distributor. It is actually made for that specifically.




1970 Impala Custom coupe
400ci SBC
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