84 to 85 harness (Page 1/2)
cbouissey12 OCT 02, 01:22 PM
Looking for some advice. I swapped a 2.5 from an ‘85 with a bad frame into an ‘84 chassis. I kept the engine harness and c500 connector right on the engine when I pulled it from the 85. I know there is some difference between the 84 and 85 chassis harness including location of the c500 and the rear lighting harness. I can work with that, but what mods need to be done to make this work? When I plugged the c500 connectors together, and used the little distribution block from the 85, I get no power anywhere.
Dennis LaGrua OCT 02, 04:32 PM
The 84 harness should fit the fuel injector, distributor and sensor connections to the engine.You may not need to do more work to rewire for the 85 harness.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

cbouissey12 OCT 02, 06:23 PM
So the existing harness on the 85 engine should function with the 84 chassis wiring with no modification? Just plug in the C500 and PCM and away we go?
olejoedad OCT 02, 07:08 PM
Your best bet is to get two Factory Service Manuals and figure it out yourself.

There are many differences between the two years concerning the chassis wiring.

Do you still have access to the interior harness and tail light harness from the 85?

[This message has been edited by olejoedad (edited 10-02-2025).]

Dennis LaGrua OCT 02, 08:56 PM

quote
Originally posted by cbouissey12:

So the existing harness on the 85 engine should function with the 84 chassis wiring with no modification? Just plug in the C500 and PCM and away we go?



No. If you have an 84 stay with the 84 harness
cbouissey12 OCT 02, 10:03 PM
I have access to both harnesses, the 84 engine harness, and the 85 chassis harness. Im assuming it would be easier to switch the engine harness to the 84?
Dennis LaGrua OCT 02, 10:52 PM

quote
Originally posted by cbouissey12:

I have access to both harnesses, the 84 engine harness, and the 85 chassis harness. Im assuming it would be easier to switch the engine harness to the 84?


What can't you just leave ALL the 84 wiring intact and just use that?

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

cbouissey12 OCT 02, 10:56 PM
Prior to the swap, I was under the assumption that the wiring is the same so I kept the harness intact when I dropped the engine. Just fed the pcm connectors through the firewall and clipped the lighting harness long enough to splice to the 84 chassis.

Now that it’s done I find out that there are several differences in the wiring so it sounds like I’ll switch to the 84 engine harness.
jelly2m8 OCT 03, 04:19 AM
Just use the 84 Engine harness, install the 84 Distributer and Coil and be done with it as the easiest route.

Unless Like Oldjoedad asked, if you have access to and are willing to take the time to install the 85 interior/cabin and tail light harness, that would be better long term, but I guess.... you know, that 84 harness has been fine for 41 years now, it'll keep working

[This message has been edited by jelly2m8 (edited 10-03-2025).]

82-T/A [At Work] OCT 05, 08:23 PM
x2 on using whatever harness is already in the car. 1984 has one additional temperature sensor though, so you'll need to account for that. There is a high and low fan switch temperature sensor. The 85 also has the connectors for both, but one of them doesn't connect to anything. On the 84 though, it uses both of them.