If you are looking for a harness for your TPI project car, you may have noticed that there are several options available. Some options may be better suited than others, depending on your application. Basically, there are three possible choices from which to choose: a factory GM harness, a new stand alone harness, or to have your vehicle's original harness converted (this is an option on a limited number of vehicles).
Using a Factory Harness
When you purchase a TPI intake, it often includes a factory GM TPI harness out of a Camaro, Firebird, or Corvette. Although it may seem worthwhile to use, its not always a good idea.
Factory TPI harnesses are now 15-20 years old, and most are in very rough shape. Typically, they will need repairs before they can be used. They can vary tremendously depending on what the harness originally came out of. The original year of the vehicle, the type of vehicle, the transmission type, and factory installed equipment will all cause variations in the connectors used, routing, dimensions, component locations, and in pinout layout. (the location of circuits where the engine harness connects with the rest of the car at the firewall bulkhead connector, and where it meets the dash harness).
In some cases, problems will be obvious at the time you attempt to install the harness in your car. Unfortunately, sometimes it isn't so obvious, and the problems occur when you attempt to start the car. For example, if you try to use a factory harness in something other than the vehicle it came out of, you may find that the harness has far more connectors than what you will need, there is nothing on your vehicle to mate with the firewall and dash connectors, and these harnesses will require alot of time with wiring schematics to wire into your vehicle. These problems are all obvious when you try to install the harness.
Suppose you have a 1990 Camaro RS with a 5.0 TBI engine, and you are converting it to run a TPI engine. If you try to install a 1989 Camaro TPI harness, it will all plug in without any obvious problems. However, when you try to start the vehicle, it will not run. The firewall and dash connectors that mate with the harness will look the same, but the circuits are in different locations.
By far the biggest concern here with a factory harness is the layout of circuits at C100 and C207 (the firewall bulkhead connector, and the engine harness to dash harness mating connector, respectively). Throughout the years, there were some variations in the connectors used, but this is a relatively minor problem in comparison with the C100 and C207 pinout variations.
Using a New Harness
This may seem like its the best way to go to avoid problems. After all, you wouldn't have to deal with the 15 year old factory TPI harness if you order a new one, right? Yes, and no. Depending on the vehicle you are installing it in, it may actually make for more work. For example, suppose you have a 1950's Chevy that you are installing a TPI engine in. The engine in the car currently has a carburetor, and a wiring harness to operate the starter, alternator, senders for your gauges, and a variety of other circuits specific to the vehicle. A new harness will be without a doubt the best solution in this case. The vehicle's original harness will remain in place, and the new TPI harness will be used in conjunction with it. It will be very simple and straightforward to wire up properly.
Now, suppose you have the same 1990 Camaro from before that you want to convert to run TPI. A new TPI harness will be a major headache to wire up on this vehicle. The reason is simple: the factory engine harness that is in the car includes the starter, alternator, senders for your gauges, power supply for your car's interior, and various other circuits. When you remove the TBI engine harness to make way for your TPI harness, you are removing all of these circuits, none of which are present on a new TPI harness. This means you will have to sit down, and analyze the factory TBI harness, one circuit at a time, to determine what you need to use, and what can be removed so you can use the new TPI harness, and whats left of the TBI harness together.
The reason why those circuits aren't included on a new TPI harness, is because ALL new TPI harnesses, regardless of the manufacturer are standalone systems. This means they include only the circuits needed to operate the ECM, and the items the ECM controls. If its not directly related to the ECM, its not part of the harness. The reason why its not part of the harness is because new TPI harnesses are intended to be installed in any vehicle you can fit a SBC into. Building a new TPI harness with all the connectors and circuits needed for a specific vehicle, will make the cost of the harness seem unreasonably expensive.
Having Your Harness Converted
The last remaining option will apply only to a limited number of applications. If your vehicle originally had a TBI engine, the TBI engine harness is similar enough in most respects, that it can be modified to run a TPI engine. On these vehicles, this is the easiest solution available. Properly done, a converted harness will look and operate like the vehicle originally came with a TPI engine. There are no problems with the connectors that mate with your firewall or dash harness, because the location of each circuit remains unchanged during the conversion. The problem that is present with a stand alone harness regarding the starter wiring, alternator wiring, etc... goes away as well, because these circuits are part of the harness before and after the conversion, and remain unmodified. The drawback with this option is that the service is only available or possible for a very limited number of vehicles. Having your harness converted will require you to remove the engine harness from the vehicle, and mail it in for service. Currently, we are one of the few places that offer this service.
The most important thing here, is that the harness that will be converted MUST be the vehicle's original harness. If you purchase another harness and mail it in for service, there is a chance to run into problems when you try to run the vehicle. Because the converted harness is not the vehicle's original harness, there is the risk of having different pinouts on the dash or firewall connectors.