Extending EFI Wires

[DaveSL33]

I need to install the engine PCM in the trunk area, which now requires extending the wire harness from the engine by an additional 8 feet or so. I looked into a new harness, but for the $900 for a new complete system, I thought I would try extending what I have. Has anyone ever extended the factory engine wire harness, and if so any issues?

[carlewms]

Disclaimer: I have not extended one but I have done several modifications and extension on my Fast EZ EFI harness and my InfinityBox system on my roadster:

1. i would not even attempt it unless you have a very detailed wiring diagram showing all the wires in the harness;
2. Do you know if the PCM is normally extendable ... what I mean is the new harness just a plug in for the old one without any additional changes or additions to make sure the PCM can still communicate to the engine;
3. The connections have to be good; I used Delcity butt crimped connectors with built in heat shrink and a glue that melted upon heat application to make sure I had good connections. For me this method was more reliable than soldering because I cannot solder worth a darn. If you do not want to mess with the connector at the other end of the cables you will need 2 butt connectors per wire in the harness;
4. The harnesses I have seen have different colored wires for different circuits but getting those same color or color combinations from a supplier is not easy for the hobby level person to procure (I found a few places that carry different combinations but the quantity sold was way too high for my budget);
5. Because of #4 you need a solid way to label each wire. What I do is take a label maker with a relatively small but readable font and then seal the label on the wire using transparent heat shrink;
6. If you decide to make up a new connector at either end you will need to have the right pins and crimpers to crimp on the pins and insert them back into the OEM connector; and,
7. I would recommend getting a 4 by 8 piece of drywall or plywood to create the new harness. You layout the harness including the branches required on the board with nails at the beginning, the end and at each branch. You then use this to create the loom or harness.

Last but not least if any part is under warranty you will probably be voiding that warranty by creating your own harness

Just a few thoughts for your consideration ... I am sure others will pipe in here as well.

Carl

[DaveSL33]

Thanks for the detailed response and recommendations Carl.

The engine harness a new from Ford specific for my engine, I am using a new Ford Racing Hot Rod control pack M-6017-463V. Everything is made to work together. My plan was to use the ends "as is" and put a splice in the middle to lengthen the harness. I admit, there are a lot of connections, and the key is a good connection. I need to look into your recommendation on the Delcity butt crimped connectors. I have not used them before, but they sound a lot safer than the old solder and shrink tube method. My biggest concern is on the reliability of the signal from the PCM to the engine with having lengthening the wires.

[Big Blocker]

Not knowing what harness you are extending (year of application), I'd say to be careful of any wires that have a "specified" resistance in the length. In the early ('89-'93) harness, the TFI harness part has a resistor placed at a specific distance from the ECM. This was called out in the FORD manual that I used to "play" with my harness as I re-routed a few things. FWIW, I now have my TFI module mounted in a remote location, approximately 3 extra feet from its original location of mounted ON the distributor. The resistor is still in the factory location . . . I also extended the O2 sensor wires to accommodate the new location of my O2 sensors. No issues for 13 years.

That being said, I would solder everything and shrink tube when the joint has been verified as a good joint. Ohm the wires before cutting and again after soldering - your readings should be [extremely] close to what the original reading was, if not the same. Use a meter that reads to at least a couple of places (.01 or better). Try to stick to the same wire "color" if at all possible - nothing worse then tracing a wire that changes color half way thru to its end. MARK/LABEL EVERYTHING. Keep a log (or modified wiring diagram) of every wire you extend.

Good luck . . .

Doc