I went to U haul a couple weeks ago hooked up to the trailer and both my 4 post flat and my seven pin don't have right hand turn signals. I don't know if it would be my harness or just the connector but I am affraid it's the entire harness, I am affraid that with the muffler delete and hummer wheels I can't take it to the dealer but I have 18,000mi and 2yrs before my warranty is up and it should be covered im not sure any help would be great.

 

There is a separate fuse for the towing tail & turn lamps have you checked them as well??

 

I had the same problem with my pickup. I thought it was the fcm but I ended up replacing the fuse box and that fixed my problem.

 

Do a search and you'll find all kinds of posts regarding this issue.

In short (no pun intended) if it isn't the fuses then it's probably the relays in the PDC (Power Dist. Center aka fuse box) or a blown cucuit in the FCM (silver box on the side of the PDC, making it an IPM or Integrated Power Module). In either case, you're out several hundred dollars with no guarntee that it won't happen again .

So, just do what I did and drop $40 on a modulite and be done with it. Here's how:

https://www.dieseltruckresource.com/...light=modulite

 

I have a problem similar to this with my trailer. I have turn signals and brake lights but no running lights or tail lights when the headlights are turned on, absolutly nothing. I am leaving this weekend for a fishing trip and hope to get some help asap to look for. Lights work fine on neighbors truck and it does the same thing on the flat connector as on the round 7 pin with an adaptor. Thanks

 

I also had this happen on my 03 and it was a broken ground wire, took it to the dealership and in about 10 minuted they found it and fixed it. you might want to try that.

 

Okay I called the dealer today and he said it would be around $100 just to look at my truck and we all know it gets pricier from there. I have confirmed that it is somewhere between the fuse box and the connector. Would a modulite fix this problem?

 

If I get the modulite what kind am I going to need? the one for a 2 wire system or one for the 3 wire system? or am I going to need the HD? I would appreciate the help.

 

If it's not a fuse (or ground wire I guess- but that would stop all trailer lights from functioning) then a modulite will likely fix the problem. Most who have run into this problem either have a bad relay (in the sealed PDC) or blown circuit in the FCM.

The one I bought was for both 2 or 3 wire, so it didn't matter. I went with the HD because my trailer has lots of marker lights. There is some rule of thumb on the modulite website, but if you spend a few dollars more for the HD then you won't have to wonder about it.

 

Okay well I didn't get the modulite I went to napa and got their version of one instead, I needed one fast as I might be leaving on a moments notice and need to get it fixed, with the modulite does it still enable you to use the seven pin or will I need an adapter?

 

by the way, the one I got from napa looks just like the HD as it's for the same purpose as an HD.

 

Would I wire this in at the trailer harness or where the tail lights are?

 

Okay well I looked over the post from SOHappy about his modulite project and noticed I could splice into the seven pin, when I do this will it allow the four pin pigtail in the harness to work again as well?

 

Since I just had this problem with my truck (and I've read some of the other threads), if you did splice it into your 7pin, did it fix your 4pin?

Or maybe this depends on where you spliced it - (I'm guessing since I have not done this yet) - at the joint where the 4pin splits out? Or farther back?