Engine Harness Rework


Active Member
Jan 22, 2014
Reaction score
Pittsgrove, NJ
Pulled the engine and front end lighting harness out of my '68 300. Every wire in the car seems to have been cut and taped back together. Luckily no evidence of a fire but it looked like a real possibility. The firewall connectors look good so that will be reused after being cleaned up. Since I don't want to buy a new harness and I have the time I figured I would clean or replace all the connectors and make some adjustments while I have it on the bench.

- Add HEI module with new coil, removing the extra ballast resistor wiring and splicing the IGN1 and IGN2 together.
- Add electric choke wire for new Edelbrock Carb from IGN2 wire
- Remove mechanical voltage regulator and replace with solid state (D7018) to convert to a "one-wire" type alternator.
- Run new wires for the cornering lights as they were completely removed from the car.
- Add relays for the headlights
- Relocate the Headlight Door Cover relay into the engine compartment

Every wire I touch will get a heat shrink label to help me or others in the future with troubleshooting. Wire colors are great until they all turn black or brown from dirt and grime! I have learned after doing the first plug for the temp lamp switch to not put the heat shrink so close to the end of the wire as it makes it difficult to reinsert into the connector.

Once I finish the engine compartment I will turn my attention to the wiring under the dash. Would like to get that done before I reinstall the heater box. Enjoying the wiring stuff way more than I thought!


I've done this. I used a scrap of plywood and some finishing nails to hold it all in place.

Progress today, good feeling indeed. If you're a Mopar or Nocar kinda person maybe stop reading here...

While I had the harness apart I decided it was the right time to replace the ignition box and ballast resistor with the GM HEI setup. It just seems like a better solution from everything I have researched. Time will tell.

Also deleted the wiring for the alternator and converted to a Delco 1 wire. The existing alternator wiring was in rough shape and the pulley had a slight wobble which I'd guess was the reason the pulley would get hot after running just a few minutes. I had to drill out the broken rusted bolt in the head mounting hole and put in a helicoil to get it mounted square. Probably why the old alternator got wobbly, it wasn't running square due to the mount being a bolt that was poorly welded to the head. Or running overly tight, maybe both.

Also relocated the starter relay back to what I believe was the factory location and not in line with the oil dipstick. No more sparks checking the oil!

Temporarily reinstalled the harness to test everything and she fired right up and held voltage around 14.5v

I'll pull the harness out and finish the front end lighting bits before I wrap it and mount the HEI module.

you may want to bypass the bulkhead connectors and/or convert the ammeter to a voltmeter while you're at it...
Not going to bypass the bulkhead connector yet. Definitely going to disconnect the ammeter (which is the one thing that seems to work in the dash!) though I am considering converting it to a volt meter when I make my way to the dash wiring harness.
Got the HEI mounted with the new coil. Will finish the loom work on the bench now that it is mocked up.

I highly recommend doing the bypass now. It's so easy to prep the new wires needed to do the bypass (and it's WAY safer), especially with the harness out of the car.

Please consider doing it now - it's super easy to poke both the black and red power wires through the speedo cable grommet, and use a nice safe terminal block to connect them to the fusible link etc.

Those bulkhead spade connectors are such a poor design for high amperage applications.
I believe my terminology is wrong. I plan to keep the bulkhead harness but I intend to bypass the ammeter and wiring through it. I'm also thinking I only need a single wire through the firewall since I upgraded to a one wire alternator, which would include a slight change to the wiring in the dash, splicing in the accessory circuit breaker differently.

I haven't fully formalized this plan yet but I do intend to remove the 12gauge wires from the bulkhead connector!
Mocked up a panel for the relays, planning to install where the old mechanical VR was mounted as I hate to drill more holes in the firewall. I absolutely love the Epson label maker that lets me print on heat shrink. If you plan to do any rewiring I can not recommend it enough!

Tested with some jumpers and everything works as expected :thumbsup: . Now I need to figure out why someone deleted the wiring for the cornering lights. Hopefully the correct switch is installed to run these when the blinker comes on. I read two different switches exist (red and white), red works with cornering lights, white does not. Will deal with that when I get under the dash.

I am using 14 gauge wire for almost everything. The source running from the starter relay to the headlight relays is 10 gauge to match the 30amp circuit breaker.

Headlight relays and circuit breakers mounted to firewall. Will remove and put in a loom for final install.
Headlight Relay Panel Mockup.jpg
Any reason I can't use a Bosch style relay for the horn? The original horn relay is shot (although I was able to resurrect the horns) and I have a Bosch relay I could use.
Any reason I can't use a Bosch style relay for the horn? The original horn relay is shot (although I was able to resurrect the horns) and I have a Bosch relay I could use.
No problem. Just remember the wire coming from the steering column is ground and the relay is powered all the time.