Electronic Ignition and your Borg Warner Overdrive

The question often comes up when a customer upgrades the original mechanical points ignition system to a more modern electronic ignition...how to I make my overdrive kick down switch work with my new HEI electronic ignition? Well... there is no simple answer as it turns out. It depends on the kind of electronic ignition you have.

The most common upgrade appears to be a GM style HEI ignition from the aftermarket or from a salvage yard. They are cheap and fairly simple to adapt to most any application. The problem is using it with a Borg Warner overdrive... how do you ground out the ignition of an HEI type electronic ignition using the kick down switch so the overdrive can shift without being under load.

Originally the ground wire of the kick down switch went to the negative side of the ignition coil. Because that side of the coil was not fused and there were no solid states pieces and parts involved, all worked well.

But because and HEI uses a solid state module to provide the spark, that solid state module that is very voltage sensitive. So if you suddenly ground the ignition out on the output side of the ignition module just as you did with the old mechanical point ignition, you will create a huge voltage spike.

Because there is no condenser to absorb the voltage spikes (because solid state electronic ignition systems does not need one) there is nothing to absorb the voltage spike created by grounding out the ignition...except...you guessed it, the solid state ignition module, which quickly zaps your ignition module and your vehicle is dead in the water because you have no ignition spark.

Some HEI ignitions have an inline fuse that will protect the module from excessive voltage but the end result is that you will blow that fuse every time you hit the kick down switch. I know what you are thinking....I will just take out that fuse or put in a bigger one. Then you will for sure burn out the ignition module which is why the fuse was put there in the first place.

Some customers have tried to use the tach drive terminal on the HEI ignitions to run the kick down switch. While that terminal is a ground terminal, that typically does not work either... A tach creates no voltage spike unlike the kick down switch which does. It is that voltage spike thing again.

So basically any electronic ignition that has the coil built into the distributor will likely be damaged when you engage the kick down switch from the overdrive.

One solution might be to setup a way to interrupt the battery current going to the electronic ignition on the incoming (battery) side to the the electronic ignition itself. That is something you will have to engineer based on your application.

The other kind of electronic ignition you see is the Pertronix style that fits into the original distributor and replaces the mechanical points with an electronic ignition module, and the coil is still external just as it always was. Your kick down switch wire is connected to the external coil not to the Pertronix. You are grounding out the ignition at the ignition coil ahead of the Pertronix ignition module, ahead of the distributor. Those seem to work ok with the overdrive kick down switches, because the coil will absorb most of the voltage spike. This setup seems to work well with the Pertonix negative ground applications.

The positive ground Pertronix electronic ignition applications seem to have some issues with the kick down switch in the circuit. The manufacturer is not sure why there is a difference. Most positive ground applications work ok, however... a few do not. Something to be aware of.

I have talked to all of the aftermarket electronic ignition manufacturers and they are all telling me the same thing. We don't recommend grounding out the electronic ignition even for a brief time, because of the possible damage to the ignition modules from voltage spikes.

If it makes you feel any better we are not alone. Some boat transmissions also require a pause in the ignition the same as we do, to allow the boat transmission to shift from forward to reverse and from reverse to a forward gear. The reason was the same as for our overdrive transmission, so the transmission itself did not have to shift under load. Like our antique vehicles...those boats were manufactured when a contact point ignition was the standard.

So if you are thinking of adding of adding electronic ignition to your antique vehicle, you need to be aware of this issue with the Borg Warner overdrive transmissions.

If I figure out a good working solution I will let you know here.

UPDATE - 
It seem the best solution so far is to put a relay into the battery wire that powers the electronic ignition, that way you can safely interrupt the battery power going to the electronic ignition (before it gets there) so the overdrive can shift and you will not damage the module in your electronic ignition. This way should also not generate any voltage within the electronic ignition because everything is happening BEFORE the electronic ignition.