My 84 H/O had the engine wire harness ate up by rodents. I got the harnesses from an 83 H/O, it was cut off at the firewall. As I was getting ready to sit down and soldier them together I found that the car has 30 wires in the bundle and the 83 has only 25. I thought that these 2 cars were the same, I know 85 roller engine have differences. Anyway only thing I can think to do is join them together and see what is left over and make that by hand. Another idea is to get an 83 service manual to compare with the 84 SM. I am good at wiring, I like to repair old stereos, but all of this is making my brain hurt. I thought I was golden with the replacement wires.
Any ideas or information is appreciated! Thanks Jackson

 

Cruise control is one thing that's going to be different for sure if both cars had cruise. The K34 and K35 RPOs were decidedly different on wiring. I believe 84 has more wires in their cruise setup under the hood. I'm not a wire-biter, so I'm not exactly sure what all runs with the engine wiring harness under the hood off the top of my head.

But you are correct. The best way to do it is compare wiring diagrams as far as routing of all the terminations. Find someone with 83 wiring diagrams that can share. There may be some enlightenment there.

 

Every G body I have had used a separate cruise harness that is independent of the vehicle harness. Same with engine management, they plug into the main harness but are separate.

To answer your question, you are probably going to need a factory service manual. While GM is very good about using the same wire color for function (light green is high beams since at least the early 60s, and to the best of my knowledge that way still today) they may have changed the firewall pinout location.

Take detailed notes, make some diagrams, don’t cut anything until you have identified each wire. It might be easier to just move each wire from the intact harness into the correct cavity for your vehicle instead of making a bunch of splices.

 

X2. I found this out the hard way when I tried to use a 1970 forward lamp harness on a 1969 442. Wires are the same but pin locations in the firewall connector are different.

 

Square body GM trucks are notorious for this!! I have a buddy that is constantly wheeling/dealing those trucks. He has acquired some extremely Frankenstein trucks (different cabs, front end swaps, etc) that took some wiring changes to get running.

 

Unfortunately I did cut the wires because I thought they were the same and I have the fender off and pulled up a table to sit at to work. The reason I was trying to save this stuff was that I thought the sensors for the oil temp volts etc. for the dash gauges were part of the CCC harness. But reading the service manual and looking at the car shows they are on a separate harness I believe. The cruise harness is in good shape, and the gauge harness can be fixed up without to much work. I've got a 1971 350 for it, and I it built envision up to 400 HP. Then I go out to the garage and try to patch this emissions garbage back together. Since it can't be both ways I think I'll put the smog stuff in a box and save it in case it's needed someday. Also I noticed the intake didn't look right, turns out it's silver painted cast iron and the numbers says it's a 1973 piece. The block #s says 80-84 307 , and the heads are 5A. So maybe the original Aluminum Intake cracked or something. Goal for now anyway is just to get it running, fuel system, brakes, etc. Thanks Jackson