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ECM location?


Well-Known Member
Local time
9:00 PM
Jul 10, 2020
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Houston Texas
Hey guys,
I’m prepping a Hellcrate motor for installation into a 69 Charger. Starting to layout the wiring harness and had a couple questions:
If the ECM is installed behind the dash, what size firewall penetration is needed and how would it be closed around the wiring harness or is it best to install it on the firewall? Anybody have photos of their installations?
Another ignorant question not covered in the Mopar instructions is how to wire the keyed starter switch. I know MOPAR uses a push button but I’d like to stay with the stock switch.
I’m using the American wiring harness to re-wire the body and dash.

Any help would be greatly appreciated!
Sorry I don't have any pictures for you but I typically mount the ECM under the dash, typically on the passenger side so I can get to it if needed. As for the wires, I like to run them to the drivers side and use one of the existing holes. There are a number of wire grommets out there that can be used ranging from the stockish type plug that you cut the center out, the actual round grommets and then the plates that have some type of rubber or material in the center that will conform to the bundle of wires.

Where ever you mount the ECM make sure you can get to it.
You need a hole pretty big for the Main harness plug. Try a hole in a piece of cardboard or thin metal to figure hole size. The problem with the stock switch is you need power to the run while cranking that the stock switch does not have. There are ways to do it but we have used an aftermarket switch.
Make sure to look at your connections and their size, don't just rely on the size of the bundle of wires. Most of the time you can get away with a 2" hole. I always make support plates to go around the hole with flared ends instead of a grommet.

You need a hole pretty big for the Main harness plug.
You would get away with a 5/8" diameter hole if you feed to loom in the other way around.
In my cuda i installed the ecu in the engine bay. I used kwik wire to re-wire my cuda and im going to use it to rewire my 68 charger also. It shouldn't be hard to wire the original key style

I mounted my ECU on the radio. Didn’t have to make a bracket and it was centrally located and was a perfect spot for my car.

I put mine here for ease of service. (If I remove the engine in the future, easy to access/unplug).


On my 70 Challenger, I mounted my Holley HP ECU about the brace under the driver's side fender. I looked at many locations but did not want it under the hood or under the dash and this was the best place I could find. I made a platform which the ECU mounts to. It is protected from rocks and debris pretty well.
The only way I've heard to make the stock switch work is with a time delay relay. I think they did it on roadkill. The stock switch cuts power to run when you move the key towards start. As soon as run loses power the asd relay shuts it down. If you connect the ignition on wire to a time delay relay that last long enough for you to start it then it would work. Sound German was experimenting with a diode between start and run. This would allow start to send power to run but stop run sending power to start. The issue with that is that between run and start no power is being sent to either so the asd relay cuts out. I think the best solution would be a time delay relay that only lasted long enough to get you from run to start AND the diode. I just need to get my hands on a 1 second time delay relay and I'll be able to test it out.
Some of this stuff I understand....or oversit. Brain going into mush mode now. Do any of y'all know how to deal with points and condensers? :D JK JK. Gotta be an electrical engineer for the stuff now a days.....
Good stuff here in this thread. I'm working on trying to figure out the engine and trans harness routing into the pass compartment on the wife's roadrunner. I really don't want to mount it inside the engine bay so it has a cleaner look, IMO. I'd prefer to have all the holes and mounting ironed out before I put the car on the rotisserie to start finish body work.

Since this thread is a couple years old, anyone out there have any other examples and pics on how they routed their wiring through the firewall?

Thanks in advance.
Added my ecu under the radio area and the wires are ran like almost to blend in with the factory bulkhead connector



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