The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post - 3 speed overdrive with an HEI distributor

ray_mcavoy · 12-13-2018, 08:33 PM

As you know, the stock system momentarily grounded the negative side of the coil to make the ignition cut out just long enough to take the load off the trans and allow it to drop out of OD. It's my understanding that grounding the negative side of the coil with HEI has the potential to damage the module.

So instead, you could momentarily interrupt the power feed to the HEI. That could be accomplished by adding a relay into the HEI power feed wire (connected through the normally closed contacts). Then trigger the relay to open during kickdown by using the "ground-out" wire from the OD trans wiring that would have originally connected to the coil.

Now, I can't say for sure that momentarily interrupting power to the HEI will be 100% safe for the module. But it's much less risky than grounding the coil.

12-13-2018, 08:33 PM	#2
ray_mcavoy Registered User Join Date: Nov 2009 Location: Sherman, ME Posts: 2,354	Re: 3 speed overdrive with an HEI distributor As you know, the stock system momentarily grounded the negative side of the coil to make the ignition cut out just long enough to take the load off the trans and allow it to drop out of OD. It's my understanding that grounding the negative side of the coil with HEI has the potential to damage the module. So instead, you could momentarily interrupt the power feed to the HEI. That could be accomplished by adding a relay into the HEI power feed wire (connected through the normally closed contacts). Then trigger the relay to open during kickdown by using the "ground-out" wire from the OD trans wiring that would have originally connected to the coil. Now, I can't say for sure that momentarily interrupting power to the HEI will be 100% safe for the module. But it's much less risky than grounding the coil.