Hi Everyone,

I have a 2003 F350 PSD and I have lost the trailer Turn/Brake lights. The fuse under the dash checks okay. Under the hood, near the brake master, there are three relay boxes. Do you know which one if for the Brake/Turn lights for the trailer? Help! Thanks, Ed...

 

Did you check the fuse with a tester? Sometimes they look good but are bad. Its unlikely both relays went at the same time. The diagram I found shows fuse #6 (20A) feeding them. Can't help you out with the relay location though, the 03 is different then my 01 and the only pictures I found were for 6.0's. One box should be for the ESOF motor, one for the trailer charge/reverse lights, and the other for the turn signals.

 

In your Owners Manual it shows the relays for trailer tow. Click the link and scroll down to page 178.

Here is a diagram of your system. You should look for Relay #4 which will be one of the 3 you have. It should be the only one with three relays in it. The others should only have two positions. Verify your fuse checks out good though. Should be fuse #6.

 

Thanks for the info. The owners manual with this truck does not cover the relays under the hood. (I could not find them in the online manual either)
Under the hood there are three black relay boxes, one has only two relays, and the other two have one large relay and two smaller relays. The #6 fuse under the dash for the Trailer turn/brake relay checks as good, and it does have power to it. I am still kinda lost, but possibly I could pull the #6, de-energizing the relays, and see if one set of relays no longer have power? Thoughts? Guidance? Help!

 

If neither work, you are probably not getting power to the relays. You can pull the relays and check for signal on #1's. Then see if you are getting power on seeing power on #3.

 

Look at the inset box (lower right) in the diagram Ken posted. It shows the left and right lamp relay in aux relay box 4.

 

After more testing I found that the relays are working (at least clicking when energized with a turn signal or 4-way signal), and I still have nothing at either the rear trailer plug itself or at the connector that is about 2FT up from the rear trailer plug extention. Does anyone have a drawing of the rear plug extention? I do still have running lights, trailer brakes, 12V power back to the trailer plug.....just no turn or brake.
If is there another plug connection between the relays and the rear plug extentions I should be looking for? Thanks a million guys, I really appreciated it!

Ed

 

Exterior light wiring schematic for a 2002. It should be the same for 2003.

2 of 2 has the trailer wiring. There is one relay for left and one relay for right trailer turn lights. There is also a trailer parking lamp relay, a trailer reverse lamp relay and a trailer battery charge relay. So there are 5 relays total. 7 pin connector is also shown.

 

I jsut went through a similar exercise. I have an in-bed camper. And didn't have any brake or left turn signal. I checked the camper wiring for continuity and the bulbs. Crawled under the truck where the RV dealer installed the wiring harness that's in the bed where my camper plugs in and it didn't have any power to the left signal/brake. My wiring harness at the trailer hitch was dead as well.

On my 2001 I have a seperate fuse box in the engine compartment and kept blowing the 7.5 amp mini fuse for the left brake/turn signal. I figured I had a short somewhere in the system. I finally found the short. The ground wire had worked loose inside the pigtail that the RV dealer made to go from the plug in the bed to the camper. It was intermittantly touching the left brake/turn wire. I re-wired the plug and problem solved.

Short story... it wasn't the relay at all just a short and bad fuse.