Creating a wiring harness for your new engine
This page will outline some general techniques for creating a wiring harness for an engine conversion. This is a difficult area for me to explain. It seems like either you know what to do or you don't. If you don't, then this probably won't help you anyway. Before beginning, you'll need to gather some very important information:
Wiring schematic for the Fiero
Wiring schematic for the engine donor
Tools and materials for making connections
I have provided schematics for the Fiero and several other popular engine donors via the links below. You'll need to decide how you will be connecting the wires, and how you will be insulating each connection. I have also provided some wire splicing suggestions from GM below.
You'll begin by identifying each circuit on the engine harness. Any circuits that will not be used can be eliminated from the harness. There are some circuits that will or will not be used depending on the particular application. For example, If you are using a stock EPROM with the passkey option you'll need to retain that circuit, but if you are using a custom EPROM that has passkey turned off, you won't. You will find that most of the wires connect the various engine sensors to the ECM. Some will connect sensors to the instruments, some will connect the ECM to the instruments, some will connect the battery to the ECM, some will connect the battery to the sensors, and some will be grounds. You need to identify all of them using the diagrams.
You will only need two connectors from the Fiero engine harness. They are the C203 and C500 connections. I normally disassemble the Fiero engine harness and cut those connectors out with as much wire as I can. The pin definitions for these are linked below. These two connectors provide connection points for all of the car's instruments, the in-tank fuel pump, the A/C system, and the switched power.
You can arrange the harness any way you wish. In most cases I will completely un-loom and un-tape the engine harness in order to rearrange the wires. When I have come up with a suitable layout for the wires, I begin connecting them to what's left of the Fiero harness. In some cases wires need to be shortened and others extended. When you have finished with the Fiero connectors, there isn't much left. You may have some wires from the ECM such as heated windshield or power steering switch. Since these are not used in the Fiero, you'll need to figure out what to do with them. Many can be left unconnected, but some will need to be connected in some way to prevent error codes or adverse effects on drivability.
These diagrams are provided as general guides only.
Cadillac 4.5 '90
Buick 3800 sc/na '91-'00
Chevy 3.4 TDC '91-'96
Chevy 5.0/5.7 TPI '86-'92
GM wire splice suggestions