I'm now ever so close to being ready to do my swap. I have the complete 94 z28 (whole car) with the auto tranny. I would first like to better understand the wiring in doing this swap. I have read thread after thread and what i don't understand is if the ecu for the 91 is under the dash assuming the wiring from it runs out through the firewall on the driverside. How can I simply tap into the wires at the firewall on the driverside. I'm sorry if this sounds dumb but I'm trying to better understand before i start pulling it apart. Does the old computer stay in there or what? The 94 has a short harness requiring me to mount in the engine compartment somewhere. Does the old ecu run to the fuse panel under the steering wheel then to the engine compartment and what I'm doing is leaving these wires dead or what? Does anyone maybe have some detailed photos of what they did through the process? The help is much appreciated as I can't wait to have the added ponys under the hood. Thanks...
ACCLR8N
Senior Member
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Mar 3, 2026
This is what I did:
1. Old Computer in the garbage.
2. Old Computer wiring gave me the two pink/blk stripe wires for keyed power to new PCM, rest garbage.
3. Old firewall pass through connector is 2-pieces, smaller is lights. Bigger outboard portion I kept two red always hot leads to run stuff in car, and pink lead for keyed power under hood. Two red wires still go to battery, pink will go to new fuse/relay box.
4. '94 computer and engine harness are simple enough.
5. '94 larger forward connector (C100?) to under hood relay/fuse center will give you injector power, AC compressor, Air pump and dual fan control.
6. '94 under hood relay center will need battery power to 3 or 4 red leads and keyed power from the pink wire coming from old firewall pass through.
7. '94 inside the car you will want AC temp sensor, all the wires to gauges, starter enable relay, fuel pump relay, VATS module (pita to remove), back-up light wires, AC request, fuel pump prime and diagnostic port. I used every wire that came into the passenger side of the car from the PCM.
8. '94 radiator, AC (condensor/evap? radiator lookin' thing) and fans.
9. '94 power steering resevoir.
1. Old Computer in the garbage.
2. Old Computer wiring gave me the two pink/blk stripe wires for keyed power to new PCM, rest garbage.
3. Old firewall pass through connector is 2-pieces, smaller is lights. Bigger outboard portion I kept two red always hot leads to run stuff in car, and pink lead for keyed power under hood. Two red wires still go to battery, pink will go to new fuse/relay box.
4. '94 computer and engine harness are simple enough.
5. '94 larger forward connector (C100?) to under hood relay/fuse center will give you injector power, AC compressor, Air pump and dual fan control.
6. '94 under hood relay center will need battery power to 3 or 4 red leads and keyed power from the pink wire coming from old firewall pass through.
7. '94 inside the car you will want AC temp sensor, all the wires to gauges, starter enable relay, fuel pump relay, VATS module (pita to remove), back-up light wires, AC request, fuel pump prime and diagnostic port. I used every wire that came into the passenger side of the car from the PCM.
8. '94 radiator, AC (condensor/evap? radiator lookin' thing) and fans.
9. '94 power steering resevoir.