Trailer brake lights on but brake pedal not depressed.

[steegie]

I just installed a new trailer wiring harness in my 18' boat trailer. My 2023 F-150 truck has a 4-place light switch....
Position 1 = "Off." All lights are off on truck and trailer.
Position 2 = "Parking Lights." All truck and trailer lights are on but left and right turn signals are inop on trailer only (ok on truck).
Position 3 = "Auto High Beams." (tested in day). Left and right turn signals ok.
Position 4 = "Headlamps." On. All lights on truck and trailer are on but right and left turn signals are inop..
So......the only position the turn signals work is in position 3, automatic.
Is this a Ford problem or my poor wiring skills problem??? Please help.
P.S. ---the truck end checks out ok with an LED tester.

 

[rustybronco]

The only thing I can think to offer is to power the boats taillights directly off a 12 volt battery and see what happens. You might want to use an inline fuse just in case.

 

[airshot]

No electrition here, but...it sounds to me like the wiring is messed up. Never saw where a light switch controls brake and turn signals....but it might be something new on a new truck. New vehicle??

 

[guy48065]

4-wire trailer harness?

 

[04fxdwgi25]

Not 100% sure I understand the 1st post.
1. You say only time your trailer turn signals will work is when your headlight switch is in "auto", correct?
2. You are doing this testing during the day (running lights off) ? If that is the case, then the turns should work in the " #1 OFF" position, also.
3. You are saying your turns only work when headlights / running lights are off?
4. Your brake lights (same connection on harness as turns) work same as the turn signals?

I would say in either case, you either have a bad ground, or a short, in the harness or one of the rear lights. Check the rear light fixture grounds.

Here are the wiring diagrams for the 4 pin connectors. Turn signals and brake lights are on the same wire for each side.

Sounds like your trailer is miswired, or there is a grounding problem. Ensure the white wire is securely grounded.

PS: if tester says truck is correct, that should be a clue that problem is in the trailer harness, because the turn/brake lights won't work and/or depend on other lights not using the ground path.. You aren't using the trailer ball as your ground, are you?

 

[Bondo]

I just installed a new trailer wiring harness in my 18' boat trailer. My 2023 F-150 truck has a 4-place light switch....
Position 1 = "Off." All lights are off on truck and trailer.
Position 2 = "Parking Lights." All truck and trailer lights are on but left and right turn signals are inop on trailer only (ok on truck).
Position 3 = "Auto High Beams." (tested in day). Left and right turn signals ok.
Position 4 = "Headlamps." On. All lights on truck and trailer are on but right and left turn signals are inop..
So......the only position the turn signals work is in position 3, automatic.
Is this a Ford problem or my poor wiring skills problem??? Please help.
P.S. ---the truck end checks out ok with an LED tester.

Ayuh,...... Welcome Aboard,...... If you've got 'em wired like the diagram above, try hookin' yer jumper cables from the trailer frame, to the truck frame to see if they'll work,.....

I've seen wirin' harnesses that don't pass the ground,.....
'n no ground causes some pretty wonky stuff to happen,....

 

[steegie]

4-wire trailer harness?

Yes, a Y style. And its new. According to the diagram this hookup should be a hoot.

 

[steegie]

Not 100% sure I understand the 1st post.
1. You say only time your trailer turn signals will work is when your headlight switch is in "auto", correct?
2. You are doing this testing during the day (running lights off) ? If that is the case, then the turns should work in the " #1 OFF" position, also.
3. You are saying your turns only work when headlights / running lights are off?
4. Your brake lights (same connection on harness as turns) work same as the turn signals?

I would say in either case, you either have a bad ground, or a short, in the harness or one of the rear lights. Check the rear light fixture grounds.

Here are the wiring diagrams for the 4 pin connectors. Turn signals and brake lights are on the same wire for each side.

Sounds like your trailer is miswired, or there is a grounding problem. Ensure the white wire is securely grounded.

PS: if tester says truck is correct, that should be a clue that problem is in the trailer harness, because the turn/brake lights won't work and/or depend on other lights not using the ground path.. You aren't using the trailer ball as your ground, are you?

View attachment 389723

Thank you for this diagram. I have this and its very easy to read and understand.

Not 100% sure I understand the 1st post.
1. You say only time your trailer turn signals will work is when your headlight switch is in "auto", correct?
2. You are doing this testing during the day (running lights off) ? If that is the case, then the turns should work in the " #1 OFF" position, also.
3. You are saying your turns only work when headlights / running lights are off?
4. Your brake lights (same connection on harness as turns) work same as the turn signals?

I would say in either case, you either have a bad ground, or a short, in the harness or one of the rear lights. Check the rear light fixture grounds.

Here are the wiring diagrams for the 4 pin connectors. Turn signals and brake lights are on the same wire for each side.

Sounds like your trailer is miswired, or there is a grounding problem. Ensure the white wire is securely grounded.

PS: if tester says truck is correct, that should be a clue that problem is in the trailer harness, because the turn/brake lights won't work and/or depend on other lights not using the ground path.. You aren't using the trailer ball as your ground, are you?

View attachment 389723

No, not using the ball as the ground. Routing the ground screw from the 4-wire connector coming off the truck, to the trailer frame about 3' away. Thanks for your interest.