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Discussion Starter · #1 ·
I took the trailer to it's winter storage location and experienced a very odd problem. While just tooling along the trailer brakes would apply themselves and the truck and trailer brake lights would come on. This only occurred when the truck's light switch was on in either the park light or headlight position. With the lights off the trailer brakes worked as they should.

The wiring on the controler consists of:

-a constant hot direct from the battery (with an in-line circuit breaker)
-a hard ground from the controler to the truck
-a trailer brake energizing output wire
-a brake signal pickup wire connected to the non-powered brake light wire in the truck

In short, when the wire connected to the truck's brake light switch wire senses a power signal, the controler routes power from the hard 12v to the trailer brakes circuit.

To get to the storage location I simply turned the lights off, it was dusk but I just made it there before it got too dark. I've had issues with the trailer brakes and trailer wiring in the past so I assumed it was in the trailer and that I could deal with it later this Fall or next Spring. However, on the way home in the dark I noticed when the lights on the truck were on, the brake controler under dash kept lighting up intermittantly. With no trailer and thus no trailer brakes this didn't cause a real driving problem except that my brake lights kept coming on and off at random and/or staying on for long periods. The turn signals and flashers didn't really want to work properly when this was occurring.

I pulled over and experimented a little. With the lights off the turn signals and flashers worked as they should. With the lights on but during the periods the brake controler was not acting up the turnsignals and flashers worked fine.

To correct the problem so I could get home safely I disconnected the power feed to the brake controler. That cured the problem.

Today I was looking over the system to find the problem. I assumed I probably had a short somewhere in the trailer plug. Nope, everything is wired tight and secure. If the controler had power and the lights were on the problem continued. With the lights off or the power to the controler disconnected or the controler's connection to the brake light switched wire (where it gets it's signal to apply the brakes) disconnected the problem was cured.

At that point I assumed the controler was at fault and I removed it with plans to replace. But tonight it dawned on me that the controler has absolutely no connection to the taillight circuit whatsoever! The trailer's running lights are fed directly off the taillight wiring at the back of the truck and are simply fed when those wires are energized when the park lights or full-on headlights/park lights are turned on.

It makes sense the that controler is buggered and intermitantly feeding a brake signal to the brake lights and trailer brakes, but why would that only occur when the lights are on? How could the taillight circuit power this failure? If I had a short in the truck's wiring between the taillights and brake lights wouldn't this failure also occur when the controler was removed from the circuit?

There just has to be something I'm missing that will make this make more sense. I still feel like the controler is at fault, I just can't prove it to myself altogether!

Hopefully in a few weeks I can borrow the unit from my Dad's wagon (which is the same brand and model) and test it in my truck to see if it solves the problem before I buy a new one.
 

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It could be a bad ground at one of the tail/stop lights. If not grounded correctly when the tail light is on power will feed back to the brake light switch easily enough to fire the trailer brake unit.
 

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Discussion Starter · #3 ·
I'll check that! I've had a lot of trouble with one of the taillight sockets which I thought I had cured. Time to take another look I guess. I sure would like to cure the problem without having to buy another trailer brake control unit.
 

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Do you have separate bulbs for brake and turn signal? Can't remember on a '69 truck but late models do. If so, I'd think the trailer would need to be the same. Otherwise wouldn't the turn signal and brake signals bleed over somehow through the trailer. Might just need to add a separate lamp for turn signals on the trailer. If its the same bulb, you may need to add a relay to the brake light switch output to isolate the power for the trailer brake application so turn signals and flashers don't affect it.
 

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Discussion Starter · #7 ·
One bulb for both turn and brake. Besides, the problem existed without the trailer hooked up.

I think I solved the problem. I cleaned up the grounds at each tail light and side marker light. I reinstalled the trailer brake controler and everything seems fine.

I'm beginning to wonder now if it was a faulty bulb. When I was digging into the problem the other day I found one of the tail light bulbs was half bad. The brake filament was not working consistently. I found the the filament was not broken but had come loose from the mounting post it is supposed to be connected to at one end. Occassionally when the filament touched the post it would light, thus causing a flickering situation. I wonder if the filament was occassionally touching the tail light circuit inside the bulb and causing the feedback problem? Too bad a threw the bulb away!
 

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Rich I have had bulbs with the filament loose at one end like you describe. If the load isn't to heavy it will power the opposite circuit. They are usually tough to track down, but having experienced it once it is easier to spot. Must have been a tough piece of tungston to be able to apply the trailer brakes. I would bet it wasn't one of the newer offshore bulbs
 

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Discussion Starter · #9 ·
Originally posted by 64elcamino:
Rich I have had bulbs with the filament loose at one end like you describe. If the load isn't to heavy it will power the opposite circuit. They are usually tough to track down, but having experienced it once it is easier to spot. Must have been a tough piece of tungston to be able to apply the trailer brakes. I would bet it wasn't one of the newer offshore bulbs
As a matter of fact it was a cheapie from China. It was less than a few months old when I replaced it.
 
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