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Electrical Nightmare '72 F100

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Old 01-22-2018, 04:01 PM
Dans72
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Electrical Nightmare '72 F100

One of the previous owners had spliced into the 13A409 rear harness sometime during the life of the truck. I removed all the splices then reconnected all the wiring (I'm pretty sure it's right), but no brakes lights or 4 way flashers. All others lighting is working including the front 4 ways.
Also, the turn signals front and rear are working.


Any insight would be greatly appreciated!
 
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Old 01-22-2018, 04:05 PM
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Troubleshooting electrical wiring remotely over the internet is pretty much impossible. I will say loose or missing grounds, corrosion in block pin connectors etc, will cause weird stuff. Anything over 5 to 10 years old is a strong candidate for problems.
 
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Old 01-22-2018, 04:22 PM
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Thanks Tedster!

The only thing I didn't do was rework the grounds behind the tail light housings. All of the flat block connectors were cleaned and given a light coat of dielectric grease, but I will rework the grounds behind the housings.
 
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Old 01-22-2018, 05:59 PM
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Cleaning grounds and connectors along with new screws, failed to fix the problem. All other lighting works except stop and rear 4 ways. Fuse checks ok. Any other ideas would be appreciated from the forum! Thanks
 
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Old 01-22-2018, 06:40 PM
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Originally Posted by Dans72 View Post
One of the previous owners had spliced into the 13A409 rear harness sometime during the life of the truck. I removed all the splices then reconnected all the wiring (I'm pretty sure it's right), but no brakes lights or 4 way flashers. All others lighting is working including the front 4 ways.
Also, the turn signals front and rear are working.


Any insight would be greatly appreciated!
The '72 schematic is showing dual filament bulbs for the stop & turn signal functions.

If your brake lights don't work but your turn signals do, then half the power is not getting to the bulbs.

If the grounds were bad for any bulb position (right or left), you wouldn't get anything there - no brake light and no turn signal. The ground has to be valid for the turn signals to work, so if you've got turn signals, then it's not the grounds.

If you have an assistant, have them push the brake pedal while you chase power at the splice. If you do not have an assistant, disconnect the brake switch and short the connector (not the switch), in order to fake a "Brakes On" condition. This should apply 12VDC to the wire(s) you're looking for. Make sure to remove the short and re-attach the connector when you're done.

With the "Brakes On", use a voltage tester to find out which wire is (or wires are) supposed to be feeding power back to your bulbs.

I can't quite tell from the schematic which wire is supposed to feed the "Brakes On" signal and which is supposed to feed the "Turn" signal from the flasher. It shouldn't be too hard to figure out which is which, once you find the wire with the "Brakes On" signal.

If you cannot find the "Brakes On" signal anywhere at the splice, the problem is farther forward in the truck.

If the front lights are flashing when you put on the emergency flasher, then it's not the flasher that's bad.
 
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Old 01-22-2018, 07:36 PM
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Maybe you know this already, keep in mind the ignition coil and contact points will burn out if ignition switch is set to RUN position without engine running.
 
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Old 01-22-2018, 07:46 PM
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Assistant (good wife), held brake pedal while I used the test light. No power coming from main harness. There is power to the other rear lights, but no brake lights. Looks like I'll be heading up to the next connector in the 14405 harness. Something up front evidently didn't like me messing around with that rats nest in the back!
 
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Old 01-22-2018, 08:38 PM
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Originally Posted by Tedster9 View Post
Maybe you know this already, keep in mind the ignition coil and contact points will burn out if ignition switch is set to RUN position without engine running.

Point taken Tedster, but I changed to electronic ignition during the summer. Also, I'm using ACC position during testing.


Done for the night and will get a fresh start in the morning. It's nice to be retired!
 
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Old 01-23-2018, 06:33 PM
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Check for power at the brake light switch. If have power there does it go through switch? Then it runs through the turn signal switch in column before going to rear of truck. Possible bad turn signal contacts on brake side.
 
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Old 01-23-2018, 07:03 PM
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Originally Posted by moose4x4 View Post
Check for power at the brake light switch. If have power there does it go through switch? Then it runs through the turn signal switch in column before going to rear of truck. Possible bad turn signal contacts on brake side.

Just heading for the garage and saw your post. Before I get testing the brake light switch at the pedal, I need to ask another question. In the wiring diagram I just looked at, the Emergency Flasher switch also gets a signal from the brake light switch. Before I did the work on the rats nest in the back, I installed a new Emergency Flasher switch from Dennis Carpenter's. I thought I'd unplug it and test again, but didn't get past the computer. I'm old, but love the emoji's that apply!
 
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Old 01-23-2018, 07:57 PM
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I tested the switch, it's working. I went further and used a jumper at the connector for the switch. Still no brakes lights. This is going to be a slow process.


I just put in a new Emergency Flasher switch for Dennis Carpenter, but have no idea how to check it.


Dan
 
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Old 01-23-2018, 09:07 PM
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If you have power going through the brake light switch, it should then go through the turn signal switch. Jump out the brake light switch connector like mentioned, then start pin probing the turn signal wires under the column to see if brake light power is going into and coming out of the turn signal switch. One power wire going in, and 2 coming out, same 2 wires as the turn signal uses It's a very common problem for this symptom.
 
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Old 01-24-2018, 08:15 AM
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Did any of this stuff work before you fixed the patchwork?

I'm thinking it might be your hazard switch. If turn signals function, then the wiring is correct.

The hazard switch controls the rear brake/turn signals and if that switch is not wired correctly or broken you will not have brake lights. I know you mention you tested it, but if the wiring is broken going to the switch the lights will not work.
 
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Old 01-24-2018, 02:38 PM
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Originally Posted by Freightrain View Post
Did any of this stuff work before you fixed the patchwork?

I'm thinking it might be your hazard switch. If turn signals function, then the wiring is correct.

The hazard switch controls the rear brake/turn signals and if that switch is not wired correctly or broken you will not have brake lights. I know you mention you tested it, but if the wiring is broken going to the switch the lights will not work.

Freightrain, I'm getting to believe you are right. I have tested everything but, the brand new out of the box (Made in China) Emergency Flasher switch. I'll pull that out this evening and bench test it.
All of the other lights work, with the exception of the brake lights and rear Emergency Flashers.
I installed a new turn signal switch a few months ago and it has worked fine along with everything else until I redid the rear (13A409) harness and installed the new Emer. Flasher switch.


Is that a "B" model in your avatar?
 
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Old 01-24-2018, 02:52 PM
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Surprised no one has mentioned this yet. Check the flashers, there are two. My PO did the exact same thing to run an LED strip, once I cleaned up his mess I had no turn signals. Put the correct flasher in and it was all good .... well almost the PO also chopped up the left front turn signal housing and installed a late model turn signal housing which cased a "slow" turn flash.
 
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