Brake lights not working right

[LoneRanger]

Hello. I own a 1994 Ford Ranger 4x4 with a 4.0 liter V-6, 35,000 miles. I decided to make it a show truck and took it apart right down to the frame.
I replaced, painted, or customized just about everything. Now the truck is back together and it's already won awards. The truck runs fine with no warning lights on. The one issue that I'm having is with the brake lights. I installed new LED tail-lights, but they are wired right and it appears to be related to the truck. When the truck key is off and the brake not depressed, the tail-lights are off. However, when I start the truck, brake pedal still not depressed, the brake lights come on-dim. I checked the service manual and at the brake pedal there's a wire with 2 lugs- it should have 12v all the time on one lug/side and nothing on the other unless the brake pedal is depressed. When I start the truck the other lug/side of this wire gets about 9v on it. This tells me that I'm getting feedback or interference from something and that's helping to go to the brake lights and power them before I push the brake pedal. I already checked a couple of things that are tied into this wire but it's hard to isolate each device. I even tried pulling fuses, with no luck. Any thoughts would be greatly appreciated as it makes driving the truck difficult because it looks like the brakes are on when the trucks running and when I hit the brake pedal you don't see much change=an accident waiting to happen. Thank you.

 

[MAKG]

Do you get that same 9V reading at the brake lights? Do you get the same reading at the switch if the brake lights are disconnected?

 

[LoneRanger]

Yes, I get 9 volts at the brake lights. In addition, I still have 9 volts at the switch with the brake lights disconnected.

 

[martin]

do the lights come on if the switch is disconnected?

 

[MAKG]

That's quite interesting. That rules out the brake lights.

Now, you get to find the voltage drop. It's 12-14V at the battery. Where is the extra drop? Trace the circuit with your voltmeter until you find it.

 

[LoneRanger]

Yes, the brake lights do come on still when I unplug that switch/connector at the brake pedal. For this reason, I'm wondering where on that wire I'm getting power because it's tied in with a bunch of things including the brake lights.......

 

[LoneRanger]

What's the best way to find a voltage drop? Do you suggest just checking each item in the circuit until one of them is below 12v? One reason this problem is difficult is because so many things are tied in together, it's hard to isolate them, especially since the truck is together. If I find the voltage drop, do you think that is the culprit for my brake lights getting fed voltage. After all, I don't want any voltage on the one side of the switch at the brake pedal, let alone 9v.

 

[MAKG]

How many things are you powering off that switch?

Sounds like you F'd up. Something is finding ground through another component. Everything on that circuit has to either be ground side switched, hot side switched, or isolated. You can't mix them without a lot of pain (isolators and all that).

 

[RobbieD]

One quick-test approach for a starting point is to unplug the brakelight switch (BOO), and probe both sides. If you have a full 12 volts coming into the switch your problem is after the switch (light side). If you ain't got a full 12 volts coming into switch, the problem is before the switch (battery side). Remember that besides the tail lights, the stop light circuit also goes to the ECM and the cruise control (if equipped). So you've likey got more than 2 wires at the switch, and hopefully you've got a wiring schematic to go by if you're troubleshooting this thing.