Brake lights work but don’t work

[Francis89]

Ok so I’ve scoured the threads a while and haven’t found anything relating to my issue. I bought a bobbed deuce a little while back and have had quite a few issues. I’m working them back to get this truck safer to drive.

So the issue is my brake lights. I don’t have the pressure or the air switch. I have a switch mounted under the brake pedal. I’ve adjusted it to where it turns the lights on with the slightest of braking pressure. That’s where they work. Once I start the truck up about a minute later the brake lights don’t work. I don’t have turn signal wires going back but the night time running lights still work. For some reason they just stop working after I turn the truck on. I’ve tried moving the light light switch to all the positions and it’s the same. They work fine with the engine off.

Thanks guys


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[Francis89]

This is what the switch looks like

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[cattlerepairman]

Ok, so what changes with the engine running? On a stock truck - nothing. In your case, kinda hard to tell. Why would one go through the trouble of installing a brake pedal switch? Does your truck retain the hydraulic pneumatic booster (air pack)?
It appears as if the feed supplying the lights is only available on battery power. You need to figure out what switches over with the alternator coming online. This is not a stock truck problem!

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[Francis89]

Thanks for the reply. I still have the airpack installed. The pneumatic sensor in the cab is still there.
460,461,22 is the red wire from the switch. Routes all the way back to the tail lights and splices between the two lights
21 is oem from the harness and splices between the two lights also.
Black wire from the switch goes into the cab and plugs into an air sensor which then turns to wire 85. Which then is wrapped in the OEM harness. This is what I have figured out so far. I’m trying to decipher where this sensor is on the wiring diagram.

Sorry for my shitty initial analysis. I had to pull back a bunch of wire tape to figure out the actual wires going back. The sensor on the airpack is removed


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[Bill Nutting]

I hate to hear stories like this. Someone decided the stock wiring needed to be improved. I’ve seen this before. Someone that doesn’t know what they are doing cobbles up the wiring and leaves it to the next guy to sort out...
Turning on two break lights seams so simple. It is if you don’t want to have turn signals too. The lamps should not be spliced together at the back of the truck. They are kept separate and route back to the turn signal switch. That switch, through make and break contacts, allow a turn signal to flash one lamp while the break switch lights the opposite side steady. If no turn signal is selected, both sides light steady. The only way to achieve this is by contacts in the turn signal switch.
My suggestion would be to use the wiring diagram, start at the lamps checking every wire tag to be sure it is connected right. Then move further into the circuit towards the turn signal switch. I fear the electronics genius that modified your truck may have cut and spliced some wires. That will make following the diagram all that much harder because you won’t be able to believe the wire tags. You may have to use a meter to trace out each wire from the back of the vehicle to the turn signal switch and break light switch. I like to break it down to one wire at a time. That way I don’t get overwhelmed by the symptom and I can better focus on the solution.
Good luck, I hope you find a simple solution.

 

[Francis89]

Thanks bill. I agree something so simple as stop lights. I’ll go through and check all the wires in the back. Maybe look for a new harness also. Looks like one of the wires going from the brake switch goes to the turn signal switch. Would it be possible that the issue is the air sensor in the cab. I’m thinking once the truck is turned on, pressure accumulates through the lines and that sensor may be sticking. Just a thought as to why it stops working a few seconds after turning the motor on.


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[cattlerepairman]

Francis, that is a good thought. If you are right, it may be ...dare I say simple...by moving the power supply to the correct side of the low air buzzer.

 

[Francis89]

So I switched the wires on the sensor in the cab. Is that the low air buzzer? I can’t find what it is. Same issue but I was able to find out that the lights work till the compressor reaches 60psi with engine running after that the brake lights cut out. So could that sensor be the issue? I can’t find any trouble shooting steps in the TM’s about this issue. It definitely has to do with the air accumulation though. After I turned the truck off I pumped the brakes till the air pressure went down to 60psi low and behold brake lights started working again.

Thanks guys


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[Francis89]

I just found it on the google. Description is correct to my symptom that contacts should open at 60psi while mine are actually closing. I’m going to put some air oil in it and see if it frees up


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[Jeepsinker]

This may help.

 

[Jeepsinker]

That pressure switch should not be involved in the brake light circuit at all. Someone really botched the wiring on your truck.

 

[Francis89]

This switch. I tried ordering a new one but got the wrong part so I sliced the two wires together. Now the brake lights work fully. Just need to figure out the turn signal now. Haha


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