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Before I start tearing apart my wiring harness, I figured I’d consult the Ford Brian trust. I have no brake light on the passenger side. Checked voltage at the socket and it’s 7 volts. Traced that wire to the 4 prong, rubber connector just behind the gas tank and that’s at 7 volts as well. This connector is before the trailer wiring mess, so it’s not that. I have twelve volts at the brake light switch on the pedal. Is there anywhere else in between I can check before I start the search for a short? I’m not looking forward to tearing apart all that old wire loom.
Sort out or get rid of the trailer wiring mess and see if that helps the situation. That main harness works is way from the pass taillight cross over the dvrs side and then on up the inside of the dvrs side frame rail all the way.
does the RH turn signal work or the hazard flashers work on the RH side? The turn signal switch is between the brake lamp switch and the brake lights...I would confirm there is 12 volts coming out of the green wire on the steering column horseshoe shaped connector...
Just checked, I have no signal or hazard at that light either. I wouldn’t also think dirty or bad ground at the socket, but there’s low voltage at the connector by the gas tank. I’m now also thinking the horseshoe connector. I’ll pull the connector and check that as well.
Is there dual exhaust on the truck? And because this is a '78, isn't there always a saddle (side) gas tank? Or do you only have a rear tank.
The point I'm eventually going to get to is that over such a long run of wire, it does not take a full short to drop the voltage that far. Even in it's best days there was probably at least a moderate drop by the time it reached the tail lights.
I would try to follow along and inspect closely the entire harness between the lights and where it comes out of the firewall. Very easy for some part of it to have been damaged by work being done, or hot exhaust melting the wires, or any number of things over the life of the truck. It's actually quite common to find damage to the rear harness along the driver's side frame rail.
The question about the side tank was just by way of wondering how hard it's going to be to inspect the wires fully.
If anyone ever removed the tank to do some work, or replaced it with a different one, it's possible that some wires were damaged during the job.
Yes, poor ground connections will give you fits all over the vehicle. Seem to be fairly common at the back end, but could be anywhere. Do our lights ground through the housings, or have their own dedicated ground wires? Seems like the '78 would have a plastic housing, so I'm guessing it's got separate grounding. If so, do they ground to the body or the frame?
Either way, it's always good to make sure you have at least the stock original grounds, if not updated/uprated ones. The body and frame need grounding too. Not as much as the engine does, but the rest of the system needs good connections everywhere.
Do these trucks have a separate connector after the firewall? Been awhile so I don't remember if there is a connection under the hood that might be compromised.
Basically all that is by way of avoiding messing around with the turn signal switch for as long as possible! Getting under the dash and separating turn signal switch connectors is no longer my idea of a fun time. Used to be, but not so much anymore.
And speaking of those connectors, are they really called "horseshoe" connectors too? I've only heard the one at the ignition coil called that before. Because the column connector is a half-moon shaped thingy too, I would not be surprised to hear it called horseshoe. Just never noticed it called that before.
It actually ended up being a loose horseshoe connector, or whatever it’s called! Easy fix...after I pulled my steering wheel and dash. . On a good note, it was also why I had no right turn signal indicator that I thought was a bulb! Thanks for a the help!
Glad it was an easy fix...always check the simple stuff first!
Originally Posted by 1TonBasecamp
And speaking of those connectors, are they really called "horseshoe" connectors too? I've only heard the one at the ignition coil called that before. Because the column connector is a half-moon shaped thingy too, I would not be surprised to hear it called horseshoe. Just never noticed it called that before.
Paul
at least one other person referred to them as horseshoe connectors recently
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Getting under the dash and separating turn signal switch connectors is no longer my idea of a fun time. Used to be, but not so much anymore.
Easy fix...after I pulled my steering wheel and dash. 
. On a good note, it was also why I had no right turn signal indicator that I thought was a bulb! Thanks for a the help!
