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Here is the cookie monster of the 3.4 DOHC swap, the electrical hell. Well, there is no need to be scared or frustrated anymore nor pay lots of money to a shop to get it done. Here in will show you how to do it. DOHCFiero or any collaborator is NOT responsible for any damage of your vehicle, do it at your own risk. Sorry but we have to cover  of our asses. ;)

There are 3 different types of electrical systems for the 3.4 DOHC 91-93 are MPFI, 94-95 SFI, 96-up are ODB II. For the Fiero swap the best shot is the 91-93 harness and ECM. This "tutorial" is based on the 91-93 harness and ECM, actually I use an auto harness and manual ECM due my GT is 5 speed manual. Also there are 2 different engine styles 91-95 and 96 up. They differ in heads, intake, exhaust manifolds and the location of the DIS. Different people do same things in different ways, this maybe not the best way to do it but is one of them, and is better than none.


Hands On

First you have to consider if you will use an auto (not recommended) or a manual tranny. This will determine how difficult will be the process. Let me explain, if you have a manual harness & ECM and your car is a manual then your swap will be more easy going. It is preferable that you get the ECM, harness and engine from the same car, this will make it easier for harness distribution as I stated before auto or manual. But if you are like some people (my self included) that buy parts as they are available you may have a rough ride in the harness. I have a 92 manual ECM, 91 auto harness with some cut wires and a 96-up engine. I decided to use the 97 engine due the improved heads, intake and exhaust, but that is other story. Now, on with the process.

Grab the schematics here. Print them and look at all of them, you will need to be familiarized with them to make sense of it all. Lets move a little faster, the main mojo is in the 3.4 DOHC C100 connector, is the one that goes plugged into the firewall. In the schematics you can see the pin outs with its description, that is what we have to match and connects to the Fiero C203 and C500 connectors. Don't get dizzy,  the hard work is done already, that's is what is page is all about ;) .  Now that you realize that is not that difficult lets start the madness.

In the 3.4 DOHC harness are some cables that is not needed for the Fiero, is recommendable to take them off-depends in your config. In my case I cut some wires that is related with the auto tranny, and some equipment that the Fiero don't have or the circuit works in the remaining Fiero harness. The idea of cut those wires is to create electrical troll that can make your life a living hell!. LOL!! J/K Is for make the harness more easy to work on and more simpler. But in order to do so you will need to take the loom off; yes have to do so; and clean the harness.

For the ones that have to pass hell like me here is some preparation info. The 96-up engine use the DIS next to the upper intake, the 91-95 use the DIS below the front manifold. To use the 91-93 harness we have to move the DIS cables to the upper intake side. That will be in the Fiero near the battery location. To do this madness you have to take all the loom off and tapes. BUT beware to attach again the cables when moving the DIS ones to it new location, you don't want a spaghetti in there. The best way to verify the proper position is attach the harness to the engine while is out of the car.

Now that you have cut (not the plug, the idea is to remove the cables as far as possible) and re-route your cables we can start the cut and paste with the connectors. BTW, this is placing the ECM in the firewall where the battery is located. You can now move the battery to the front compartment, come on!! You know you want that for a long time ;) . If you want to place the ECM inside like the Fiero then you will have to cut all ECM cables and do some extensions (I'm not 100% sure but when I was measuring, the ECM cant go in there with the default harness cables). I reuse the Fiero connectors, to try to make it as stock as possible. I cut the C203 connector, is located under the center console between the seats. Its a clear plastic connector that is part of the Fiero engine harness hooked in the ECM. And also cut the C500 connector that is in the Fiero engine harness, is the one that hooks in the big dual block near the battery. Cut the plugs with enough wire to work on, if you cut too close to the plug the matching maybe more difficult.

Grab a big glass of Pepsi, some nice music and start to splice wires. One by one, take your time. If you cut all the 3.4 C100 plugs you will have a fast and nice trip to hell. You can cut all the Fiero C203 & C500 plugs at the same time, the plug is coded in the tip. In the following tables are the cross match cables reference. Is listed in the Fiero plugs sequence to make it easy, the ones listed are the ones used.

I have been talking with other PFF members about to take off the RS electrical center and full integrate the 3.4 DOHC harness to the Fiero. In order to do that we have to reuse some of the C203 and C500 Fiero connectors wires to do the trick, just remember that those circuits will do a new job. Doing this the new harness will me much likely like the OEM Fiero harness look. We will use the 3.4 DOHC Fuel and A/C relays and the fuse specs of the 3.4 RS electrical center fuse specs in order to keep the system work the default way it should do. We don't like electrical trolls here, at least I hate them. You will see the configuration of this trick in one of the following tables.

This is for manual transmissions, for auto transmissions you have to verify some cables not all. The C203 And C500 pin outs are here.

Grab the schematics here. Some of the schematics are extracted from and others are a contribution from Rock.


3.4 DOHC Harness Connectors

ECM Connector A - Orange

ECM Connector B - White

ECM Connector C - Green

ECM Connector D - Blue

DOHC C100 Connector Pin Out

Cruise Control Connector Pin Out

DIS Connectors Pin Out



Fiero C203 Connector

Is located under the center console between the seats. Its a clear plastic connector that is part of the Fiero engine harness hooked in the ECM.



Position Wire Color   Position Wire Color
E1 Tan/Black Switched ground for up shift light (manual) Note 1 A Tan/Black
H3 Brown/White Service Engine Soon lamp C Brown/White
C3 Lt Green A/C on request D Lt Blue
F2 Tan

Oil pressure sender                 

E Tan
C2 Pink/Black Fused feed for ECM                      Note 2 F Pink/Black
H4 Dark Green VSS signal to Speedometer           Note 3 G Yellow
A2 Gray Fuel Pump relay activator L Tan/White
C1 Black VSS Ground M Black
B2 Black VSS Low                                      Note3 R Purple


1. In the auto harness this wire is missing in the C100 connector. You have to cut the Tan/Black wire from the auto transaxle plug. You can find it in the connectors section below. When you find the wire, cut it and move it to the C100 connector area to joint where it belongs.

2. The Fiero fuse is 10A, must be replaced with a 15A fuse.

3. In order to use the Fiero speedometer, a mod have to be done. You can find the info here.


Fiero C500 Connector

Is located near the battery, in the engine bay. We need to use the connector from the engine harness that bolts in the c500 connector block.

DOHC Description Fiero
Position Wire Color   Position Wire Color
K3 Black Ground A2 Black
E2 Brown Charge indicator B3 Brown
G2 Yellow Temp Gauge                         C2 Dark Green/White
D4 White Tach C3 White
A9 ECM Dark Green/White Fan Switch control                     Note 1 D1 Dark Green/White
G2 Dark Green Temp Light D3 Dark Green
C4 Light Green Reverse E1 Light Green
B3 Purple Feed to starter E2 Purple


1. Has to be connected to A9 on ECM


RS Electrical Center Removal Trick

Here is the config to eliminate the RS electrical center. You will have to move some cables to its new destination. In the schematic zip you will find a diagram of the RS electrical center to make this easy.

DOHC Description Fiero
Position Wire Color C203 Connector Position Wire Color
RS electrical center   Fuel/ECM Fuse  20A                                                  Note 1 B Orange
RS electrical center   DIS Fuse  10A                                                          Note 2 J Pink
RS electrical center   Injectors Fuse  10A                                                   Note 3 K Pink
Position Wire Color C500 Connector Position Wire Color
RS electrical center


Ign1 and Ign2 Fuse 15A and 10A respectively             Note 4 E3 Pink


1. In the Fiero fuse box this circuit have a 10A fuse, it have to be changed to 20A. It is labeled F Pump.

2. In the Fiero fuse box this circuit have a 5A fuse, it have to be changed to 10A. It is labeled TBI INJ1.

3. In the Fiero fuse box this circuit have a 5A fuse, it have to be changed to 10A. It is labeled TBI INJ2.

4. In the Fiero fuse box this circuit have a 20A fuse, it have to be changed to 25A. It is labeled IGN. In manual configuration only is needed to use the 15A wire, for auto both are needed.


There are 2 brown wires connected to the RS electrical center that no have fuse. This wires are for diagnostic purpose, you will go and identify both in the main and then splice them and solder them.  The same goes with a Dark Blue wire.

This is the hard part, you have to cut the relays cables one by one and use new connectors to attach them to the relay. Be careful to not miss place the wires or it wont work. In the RS electrical center you can see in the relays connectors that are numbered, 1 to 5, just plug them in the same way. Go and get the Fiero relays and take the "inner coil stuff" out, we going to reuse the outer case to hold our new custom made relays. Then place the 3.4 DOHC relays inside the case of the older Fiero relays, look the pics. You can place them in the engine bay using this method.


Or you can place them inside of the car, where the Fiero ECM should be. There is a kit (relay and connector) in Western Auto for $5.99. BTW, don't use the relays that comes in the package, use the 3.4 DOHC instead. You can use the connector plug to replace the RS electrical center connectors, maybe the use of some wires as extensions will be required. Do one by one and verify its location, if you miss connects the wires the fuel or a/c may not work.


Now that you have all wires matched, soldered and insulated, all that rest is wrap the harness with reflective tape and loom it. I use DEI Cool tape to wrap and protect the harness from heat. Then I loom it back with a nice red loom. Remember to he sure that all the plugs reach its destination before start to wrap it and loom it.


ALDL Connector

DOHC Description Fiero
Position Wire Color   Position Wire Color
J1 White/Black ALDL Enable diagnostic B White/Black
J4 Orange ALDL ECM serial data I/O E Orange



 3.4 DOHC Harness Plugs
Alternator and Knock Sensor Connector

Coolant Temp Connector




ECM Connectors

Fuel Pump Oil pressure switch

IAT Sensor Connector

Low Coolant Temp Indicator

O2 Sensor Connector

Power Steering Pressure Switch

VSS Connector

Auto tranny connector



Well, after all it was not that bad. You have made your own harness integration and save some bucks. We at hope you achieved what you cam here to look for. Also I want to give special thanks to Rock and Fierobessed for helping me out in this.


Author: DreX


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