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Dear Ole Mechanic;Wiring Loom

I got a good deal on an old 1978 International Harvester Scout, so I thought that I'd fix it up as a hunting vehicle.  My problem started with the gas tank.  At one time, it must have leaked or got rusty, as it had been disconnected--as had the fuel pump on the engine.  I was able to find a source for a new poly plastic gas tank and the parts needed to install it.  As I was pulling the old gas tank out--after removing the rear bumper to get the skid plate out so I could get to the gas tank--I just cut the old hoses as I knew they would need replacing anyway.  Unfortunately, what I thought was an evaporative emission hose going to the tank was in fact the wiring loom going to the rear of the Scout.  The loom has the gas gage wire, tail light wire, turn signal wires, back up light wires, and brake light wires.  At first, I didn’t think it would be a big deal to fix.  Just match up the wire colors and reconnect them with crimp connectors.  Then I unwrapped the loom covering.  There are five black wires and two green wires!  There is no way to tell which black wires go to which black wires or which green wires go to which green wires.  The Scout has been out of production so long that no wiring loom is available--as if I could afford it if they were.

Now what do I do?   Help!

Un Wired

 

Dear Un Wired;

Usually, when a manufacture uses one color of wire (Freightliner uses all white wires), they number each wire about every foot so that the circuit number becomes the way to trace wires.  Unfortunately, International Harvester only numbered their wires near the ends, from what I can find out.  Why would they use only one color of wire and numbers?  Because the percentage of color-blind mechanics is about the same as the percentage of color-blind people in the general population--ten percent.

With that answered, on to your problem.  No, you are not going to be able to sort things out with just a test light.  Using a test light and a battery could lead to melted wires.  You are going to need both a test light and an ohmmeter.  Since an ohmmeter provides a small amount of power, it can be used on the severed rear part of the loom without melting wires.  With the loom wrapping pulled away from the wires, use a wire stripper to expose a little bit of the copper of each wire.  An ohmmeter will have a high resistance reading if there is no connection and a very low reading if there is a connection.  Hook one lead of the ohmmeter to the gas tank sending wire that came off of the tank.  With the other lead check each wire that you have stripped in the rear loom.  One of them should show no or very low resistance.  Take some masking tape and mark that wire "Gas Gauge."  Now take the lens off of the back up light and remove the bulb.  Hold one lead to the center connector button in the socket and use the other lead to test the other stripped wires coming out of the rear loom.  The one with little or no resistance should be labeled "Back Up."  Now remove the license plate light lens and bulb.  With one lead on the socket’s center button, test the other rear loom wires.  The one with little or no resistance should be labeled "Tail Lights."  Do not remove the test lead from the wire.  Now remove the right tail and brake light lens and bulb.  There are two connector buttons in the socket.  The one that shows little or no resistance is the tail light, so the other one is the turn and brake button.  With the lead on that turn and brake button, test the other wires in the rear loom.  The one with little or no resistance should be labeled "Right Turn and Brake."  Repeat the last test to locate the  "Left Turn and Brake" wire.

Now it is time to strip and test the wires coming from the front of the loom with a test light.  The plan is to turn on one thing at a time and then test each wire to see which one has power for that thing.  One thing that you can eliminate is that a front black wire will not go to a rear green wire, and a front green wire will never go to a rear black wire.  With nothing turned ON--not even the key--test each wire in the front loom with a test light.  With nothing ON, none of the wires should light up the test light.  Turn ON the key and put the gear shift in reverse.  Test each wire in the front loom.  The one that has power and lights up the test light can be connected to the rear loom wire that is labeled "Back Up."  Next, turn ON the left turn signal and see which wire lights up the test light.  A note here: The left turn signal may not blink as the only load on the flasher will be the dash and front turn signal bulbs, and they may not be enough load to cause the flasher to blink.  The wire that does light or cause the test light to blink can be connected to the rear wire labeled "Left Turn and Brake."  Then turn ON the right turn signal, test the wires and connect the one that lights or causes the test light to blink to the rear wire labeled "Right Turn and Brake."  Next, turn ON the parking lights, locate the correct wire in the front loom and connect it to the "Tail Lights."  That leaves the gas gauge, which can be a little tricky.  A full 12 volts could cause a spark in the fuel tank sending unit, so most manufactures use a voltage limiter to power the gas gauge.  The lower voltage may not turn ON the test light.  This calls for a little common sense.  If the gas gauge reads full and then starts to move towards empty when the test light is connected to the correct wire, you found it.  Connect that front wire to the rear wire labeled "Gas Gage."  To my knowledge, the circuits mentioned above are the only ones used in the rear of the Scout. 

Is this exercise a little tedious?  Time consuming?  Yep, it sure is, but it will fix your problem and besides, I didn’t cut the wrong thing, You did!  Isn’t playing mechanic FUN?

Herr Professor Nuzanbolts