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Ignition switch power gap between run and start.

EFI Wiring Fundamentals

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I’m wiring EFI into a 70s Dodge. The factory ignition switch has one RUN circuit and two START circuits in the key cycle. It does not provide 12v in both RUN and START at the same time. Many people splice one of the START circuits into the RUN circuit to solve this issue. This is not working for me, as the key goes between RUN and START the ecu shuts off due to the gap in power. How can I solve this? It seems like it should be simple but I’m not coming up with any good ideas. Any help would be appreciated.

The ignition switch is not faulty, the original and a new switch have the same gap.

I'm not specifically familiar with the Chrysler switch of the period, although I did have a sixties' era valiant many decades ago, but that said...

How many terminals does the switch have, I expect there to be 4 - one main terminal for power in, one for accessories, one for ignition, and one for the starter circuit. Sometimes the ignition and accossories get mixed and this can cause problems because normal practice for switches is to turn off the accessories contact when in the 'start' position.

Could this be your problem?

@Gord

you are correct with the four different circuits. It is possible that the circuits are mismatched but I don’t think they are.

i will double check, but I don’t think that’s the issue. I’m surprised no one makes a replacement ignition switch with the circuits combined. If anyone has the capability to easily make them, the market is there.

Simplest thing, if you're not set on using the key switch and it is an actual switch design problem, is to use a separate momentary contact starter button - that way the key can be turned to the ignition position and the starter then pressed. It would also act as a basic anti-theft device if one wasn't aware of the separate switch.

I have thought about this. I would like to use the key if possible.

Oh, forgot...

It would have used a ballast coil where the ignition goes from the switch to the coil through a relay, there would also be a lead from a second terminal on the starter solinoid which would bypass the coil resistor to give more voltage to it on starting.

This may be the problem, if I understand it correctly, as the coil may not be powered by the key switch in the start position in the expectation it would be powered from the solinoid.

Some applications may have the ignition terminal on one side of the ballast, with the start position powering the other end which is connected to the coil

Might help to understand if this link works http://classicmini.weebly.com/uploads/4/8/0/0/4800178/8783566_orig.jpg

Almost 4am here, my excuse for not thinking. You could possibly use a relay switched by the start circuit, to maintain power on the ignition side from the start position while keeping it isolated?

Duh, slept and fed now - if switching, rather than powering, the ECU you could just run a suitable rated diode between the 'start' and 'run' terminals/wiring. In run the diode would block the start, and in start it would power the run, there will be a 0.7V drop across the diodewhich shouldn't cause an issue, but if it does there rarer (more expensive) germanium (?) diode is only, IIRC, 0.3V.

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