The darn weather and my back have pretty much been keeping me out of the shop for the last couple of weeks. Rather than sit around and wait for better weather I started on a couple of small projects that I would normally do much later in the in the process. One of those projects was to figure out the wiring for the MSD 6A and backup Chrysler Electric Control Unit (ECU).

After I build a car the big test is usually to drive it from Arizona to Illinois and back (normally around 4000 miles or so), something I hope to do in the Valiant. About half of the trip is done at night and anybody who has driven through the Southwest knows there are section of the trip where there is nothing for miles and miles but miles and miles. I usually carry a box of “just in case” parts with me, the usual things like belts, hoses, bulbs and fuses ect ect. I also carry things that might be not be on the local parts store shelf that would a show stopper if I couldn’t find a replacement. Sitting in a motel in downtown nowhere for a day or 2 while I wait on something to be shipped in is not fun so I usually also carry a spare complete HEMI distributor, wiper motor and switch, alternator (1 wire 10SI) etc.

I like running a MSD 6A on these old HEMIs (which I have convert to Chrysler Electronic Ignition), it really helps with cold starts and drivability as the engine warms up. To me the 6A boxes are a bit pricey just to have one sitting in a box in the trunk “in case”, their also PIA to change out along the side of the road at 3AM. The compromise I came up with on my 57 Plymouth was to mount and wire both the MSD box and a Chrysler Electronic Ignition Module to separate plugs that can be connected to the cars wiring harness. If the 6A box goes out it’s a simple matter to unplug it from the wiring harness and plug in the Chrysler module. I’d usually run the car on the Chrysler module once in a while just to make sure there are no issues if I ever needed to use it.

I built the 57 Plymouth almost 20 years ago and used a 6AL box. I’m using a new 6A box on the Valiant and there is a bit of difference in the wiring between the 2 so I basically had to redo my wiring diagram. Of course the wiring on the Chrysler ECU hasn’t changed in decades, although I have noticed there have been changes in the color coding of the wiring harness that plugs into ECU depending on where you get the harness from. You might also notice that there are some circuits on my diagrams that don’t show a color code. That’s because I haven’t built the wiring harness yet and the color(s) I use will depend on what wire I have on hand (I’ll pencil in the color on my copy of the diagram once I get it all wired into the car).

These are the wiring diagrams for the 6A box and Chrysler ECU I started with.

wiring MSD CHRY org by M Patterson, on Flickr

And these are the diagrams that show the 6A and Chrysler ECU wired to the 8 pin plugs I used. I actually only needed a 7 pin plug but 8 pin plugs seem to be a bit easier to find.

wiring MSD CHRY 8 by M Patterson, on Flickr

Finally the wiring harness plug that connects either the 6A or Chrysler ECU to the cars harness.

Wiring Harness by M Patterson, on Flickr

If someone decides to use this as a guide for building a backup ECU system keep in mind IT IS ONLY A GUIDE. There are differences in the wiring between the 6A box I’m using and other MSD boxes depending on the model and probably age. As I noted there are also differences in the wiring color coding on the Chrysler ECU plugs depending on where you source them from. From my experience the wiring/plug that goes into the distributor have always been orange and black wires, but you never know what’s out there in the aftermarket if you use a replacement pickup assembly. Basically it’s up to you to verify the wiring for your specific car/components.

Anyway for me it’s worth the extra effort to go I through if for nothing else than just for the peace of mind when I driving far away from home and shop. It was also a good mind exercise on a cold rainy day LOL.

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