While trying to figure out why my boat trailers running lights would work, but not the brake lights with the running lights on, I stumbled across a blown fuse in the engine compartment. 20 amp fuse, replaced, thought I had fixed the problem, but created another one. My parking lights and dash light now stay on until I pull the fuse I replaced. Went to the store, replaced 1 relay, same problem. (It was the relay in the 201 location, if that helps.) I have to pull that fuse out (105 in the engine compartment I believe) to get the lights off, but they will still work if I need them to. Anybody have a clue?

On the way to the store, driver side sway bar link breaks, btw. They say things happen in 3's, so there was my 3rd. I should be winning the powerball tomorrow.

 

You need to get the actual fuse position in the BJB.

Fuse #105 is a Maxi Fuse, 40A for the front blower motor.

 

My mistake. It was fuse #3. The half relay I replaced was #201. #5 was the one that was blown and the one that I have to pull out to turn the lights off. I got my numbers a little confused. I edited my post above.

 

You now have 2 fuses listed. Both #3 and #5 were blown ? Fuse #3 is a 30A fuse, not a 20A.

Is this issue happening with and/or without the trailer plugged into the trailer tow adapter ?

If it is only with the trailer plugged in, are you using the 4 pin or the 7 pin trailer tow adapter ?

 

I may have messed up, again. I will redo those 2 fuses and let you know what happens. Sorry.

 

Let's see if I can get this right now. It is the #5 fuse, (20A TRAILER TOWBACKUP/PARK LAMPS), that I replaced. I also replaced that half relay that is 201. Without the #5 fuse, they will turn on and off when I want them to.

It is a 4 prong on the truck. It did it both with and without the trailer hooked up. I redid the ground on the truck, and on the trailer. (I did it backwards, instead of starting with the fuses, I gave myself more work). I can take care of the trailer lights, I just don't want to have to pull a fuse everytime I leave the truck.

 

New twist. If I leave the #5 fuse in, and pull the 201 half relay, my lights go on and off when I turn the headlight switch. Could this be a bad headlight switch causing this whole thing, or probably not seeing as the lights work when I turn the switch?

 

Sorry for the delay in the reply, had to let this one sit and simmer in the back of my head.

The Parking lamps for the trailer are power from Fuse #5 through the relay 201, and are isolated from the trucks parking lamps.

The Trailer Tow parking lamp relay coil is activate by the parking lamps on the truck



The only thing that is not isolated from the truck is the stop / turn lamps, they are the same main circuit, just fused between the truck and trailer.





If you were having what you thought ( or in part ) a ground issue, it could actually be the stop / turn circuit looking for ground thought the parking lamp circuit. Are the parking lamps at full brightness ?

The circuit I am thinking of is on the 7 pin trailer tow adapter.

Start by pulling the fuses for the trailer tow stop / turn ( BJB 19 & 20 ) and see if that changes the function.

If not, next is one at a time the trailer tow battery charge relay and then the trailer tow reversing circuit.

I am confused how the parking lamp circuit ( which is isolated with the relay ) can get back to the truck's parking lamps, even after swapping the relay with a new one.

The thing that has me questioning this, is the trailer tow battery & reverse are only hot in the run position, so when you turn the key off, there should be nothing to cause the symptoms with the truck off.

Did you check to make sure nobody added anything to the back ( assuming you are not the original owner of the truck ) from the trailer tow adapter to the truck's parking lamp circuit. I have read posts were used trucks, a blown fuse that people can't read in the OM will just have them splice in a jumper. Pull the tail lamp housing and check ( also check along the back bumper by the license plate lamps ).

Check the back of the 7 pin trail tow adapter for mashed pins and corrosion in the back ( it is screwed together, and can be taken apart ).
The relay you installed was new ? Might need to bench test both of them, to make sure, depending on what happens.

You can see in cell 95-3 above ( and your testing seems to confirm it ) that the truck & trailer should be isolated from each other...

Sorry this one is a bit confusing to me, so I do not have a quick easy answer to it, it is going to be SWAG & supposition time with some testing to figure this one out.

 

The parking lamps seem to be at full brightness. I checked BJB fuses 19 and 20, but didn't do any testing with the lights with them in and out. You are correct in saying I am not the original owner of the truck. There is a red wire running from the tail lights to the 4 pin adapter to my trailer. I redid the ground wire, and it looks like there are 2 more wires going into a harness near where the ground wire is. (Might be 3 wires, I am not sure). If that is the issue, might be easy. I will keep trying and let you know. I do appreciate all your help with this Sscully.

 

Easy fix on the truck light. Got under there, found a red wire running from the brown wire on my harness to the driver side license plate light. Got rid of it, now my lights on my truck are fine. However, now my brake lights on my trailer don't work at all. That should be an easier fix. Thanks for the help, Scully.

 

check fuses #19 & #20 in the engine compartment fuse panel.

Have someone press on the brake pedal, and check both sides with a meter for + VDC.

The fuses only have power when the brake pedals are on.

If the fuses test good, check for power at the trailer tow adapter on the truck both 4 in & 7 pin, prior to looking at the trailer side of things.

 

Got everything running. Thanks for the help, Scully. Got to get on the water and get a fresh seafood dinner.

 

Can I bring this back? I know it's 2 years later, but I'm having a very similar problem.
Just bought a trailer on Sunday and hooked it up. Parking lamps would not light up on the trailer. However, the turn signals and brake lights worked well. Ended up splicing a wire from the license plate light to the 7-pin connector for power for the parking lights on the trailer.
2 days later......
Parking lamps will not shut off on truck! I do have the Bambi Mod done on my truck and right now, I have the 202 and 201 relays swapped to ensure the parking lamps shut off when I want them to. Bambi mod is currently disconnected because I don't know what to do. Haven't had too much time to look at it. Any ideas on the Trailer park lamps not working and why my parking lamps wouldn't shut off?

Thanks for anything.

 

1st thing, undo what you spliced into on the truck.
- there is no reason to be taking a short cut like this.

Next check the 7 pin adapter both in the socket and the back of the socket for corrosion.

I seem to recall at least one thread where a member had corrosion built up in the back of the 7 pin trailer tow adapter and it caused something similar.

Your fuses for the parking lamps seem shot, so it is not via that path, but the one you added.

The trailer tow adapter not having parking lamps, did you check both the 4 pin & the 7 pin with a meter or test light or did you use your trailer as the test light ?
- Quick Tip, use a meter or test light when diagnosing things.

Not too sure why you are swapping relays, did you test the fuse that feeds the relay 1st ?
- Cab fuse panel Fuse #10, 20A. Test the fuse with it installed, using a known good ground to each of the pins on the back of the fuse. Both sides should have the same + VDC reading.

 

Will undo the shortcut as soon as possible and check for corrosion.

Checked both the 4 pin and 7 pin connector and, with the lights on, nothing was "hot". We checked all pins with a light tester. Turned the blinker on and we found the pin for the blinker.

I swapped relays before when I was trying to get the trailer lights to work. I think I forgot to put them back, but with the 201 in the 202 spot and vice-versa, my parking lamps actually turn off. So I left them. Fuse #10 was blown, replaced it and didn't change anything.