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Old 08-10-2009, 10:46 AM
willhf1011 willhf1011 is offline
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brake lights not working

The brake lights on our tow rig are not working. Its a 2002 F-450. The lights don't work on the truck or at the trailer so that tells me the problem is in the truck. Both rear turn signals work so the bulbs are good. I checked both fuses on the fue panel slotted for brake lights (18 and 34 i think) and both fuses are good and there is current to them. The brake switch by the pedal is good because the electric brake box for the trailer reads when i depress the pedal. At this point i'm stumped. I'm hoping there is a brake light relay somewhere that could have gone bad or more fuses than just the two on the panel beneath the column. I would think that a wire was bad going back but i thought current for the brake lights ran through the same wires as the turn signals and operates the same bulb. Anyone have any ideas?
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Old 08-10-2009, 11:26 AM
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bill.pounds.1
I think you'll find another fuse/relay box under the hood. Somewhere on the driver side, fender well.

Brake lights and turn signals are all the same wires and bulbs, as you said, so I would look for that underhood box.

I would have guessed brake pedal switch, but you seem to have eliminated that already.
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Old 08-10-2009, 02:31 PM
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i found a small relay box with 3 relays in it but nothing beyond that under the hood, maybe i'm not seeing it. i would have thought that the brake lights would have a relay but there isn't one for them under the dash. If the electric brakes on the trailer work then the brake pedal switch has to be good right?
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Old 08-10-2009, 02:40 PM
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bill.pounds.1
Quote:
Originally Posted by willhf1011 View Post
i found a small relay box with 3 relays in it but nothing beyond that under the hood, maybe i'm not seeing it.
That's the box I spoke of. Did you try swapping the relays around, one for another?


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Originally Posted by willhf1011 View Post
i would have thought that the brake lights would have a relay but there isn't one for them under the dash. If the electric brakes on the trailer work then the brake pedal switch has to be good right?
If the switch is a single pole switch, then yes. Sorry, I don't have that year truck so I just don't know.
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Old 08-10-2009, 03:06 PM
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bill.pounds.1
I suggest you take another look at that brake pedal switch. I think you will find it is a multi-pole switch and the PCM is involved. I'm looking at an '05 schematic, so no guarantees. But I do not think you can assume that switch is good just because your trailer brakes work.
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Old 08-10-2009, 03:21 PM
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bill.pounds.1
If it's not a relay, and it's not the brake pedal switch, then it may be your multi-funtion switch (turn signal lever).
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Old 08-10-2009, 03:31 PM
willhf1011 willhf1011 is offline
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do you by any chance have a diagram or specs on what the purpose of each wire on the brake pedal switch are? you are right in that it is a multi-pole switch so i'm going to unplug it and try to bypass the switch, just need to know what wires are which
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Old 08-10-2009, 03:53 PM
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Okay, remember, this is an '05 schematic. They could have done anything with wire colors.

The hot wire will be LT GRN/RED (means light green with a red stripe). The output wire will be LT GRN. This LT GRN wire should be hot when the pedal is pushed. The LT GRN wire goes up to your multi-function switch, through normally closed contacts, and comes out as ORG/LT BLU to the right side stop light, and LT GRN/ORG to the left side stop light.

By the way, does your high mounted stop light work (on the back of the cab)? This is important, so make sure it has a good bulb and see if it works. Because that is the same wire from the brake switch, but does NOT go through the multi-function switch. So, if the HMSL works, but the rear brake lights don't, that is a strong indication that the multi-function switch is bad.

If neither HMSL or rear lights work, then it's more likely the brake pedal switch. Also, the LT GRN/RED wire should be hot at all times, even with the ignition key off.
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Old 08-10-2009, 03:53 PM
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