I need some help. I have an 02 F250 7.3 that I pull with and I have an issue. (All the trailers were working correctly, Checked with another tow vehicle). All lights on Truck work correctly.

The problem....
When a trailer is connected, I have no turn or brake lights. My electric brakes are kinda like my kids... They work..sometimes. On the trailer brakes, the green indicator light on the brake controller is bright green until I touch the brakes, then it goes out.

What I have done so far...
The trailer connection with a test light shows everything else works OK. (constant power, reverse, running lights) With a meter, The turns/brakes and elec. brakes get low voltage (3-4v). So I started with the rearmost connection and worked my way up to the front of the truck. At all the harness connections, I get the same results as I did at the connection point. I tested the trailer fuses (with a meter) and they are OK. I also tested the fuse block with a test light and it is getting power there. Where the wiring harness enters truck (Drivers side by seat) all the wires there so far look good. I did not completely take apart the wire loom (time running out that day) but I did not notice any of the green corrosion, or obvious marks to the loom that may lead to a broken wire.

My thoughts so far...
I am thinking that I have a corroded wire somewhere or some type of short. I am thinking my next step is to drop the fuse block to check the backside of that. Other than that, I am at a loss. I have been told that the trailer turn flasher is on the firewall and a pain to get to, But I am more than willing to look for it if I can get a better idea of where it actually is.
I bought the truck used, and with no history behind it But I just had to have a 7.3!! I can tell you, the truck appears to have been a Fleet vehicle (Maybe a mechanics truck) and there is a TON of dirt under the dash where the garage I purchased from had not cleaned. (Everything else was clean as can be!)

Sorry for being long winded, but I am at a loss, and quite frankley, need help.

Thanks
Hab
02 F250SD XL

 

I don't have my DVD with me, but if you could find relay box #4 in the diagram below, that would be a good place to start checking some things with your meter. If I remember I will look up the location of the relay box.

http://econtent.autozone.com:24991/z...52801e08c9.gif

In case you don't know, the turn lights and the brake lights are the same bulbs and wiring from the trailer up to the relays, and all the way to the turnsignal switch.

 

Franklin2, Thanks, But I am a little lacking when it comes to reading schematics. I understand that I have a relay for each R and L turn/brake. The relay #4, is that gonna be in the main panel under the dash?

Thanks
Hab

 

No. The fuse box under the dash is called the "central junction box" All components located in this box will start out with "f2".

The fuse box under the hood is called the "battery junction box" If I remember correctly, it's "f1". Then they have these little boxes sprinkled here and there and they are labeled #1, #2, .... I can't remember were the #4 box is located.

 

OK thanks. One thing that seems to throw a few people for a loop is, I do not have the fuse block in the engine compartment on the Drivers side wheel well. All that is there is one big relay and two smaller ones and they dont seem to have anything to do with the trailer lighting. I guess I have to look around for another fuse block then huh? In the central junction box (under the dash) is where my fuses are for the trailer lights, constant power etc. I do wish I had the box under the hood, that would make this so much easier..

I hate electrical gremlins, they are tricky little guys...

Thanks
Hab

 

Is this a regular pickup or is it a chassis cab? It might make a difference in the wiring.

http://econtent.autozone.com:24991/z...52801e08c9.gif

In the diagram in the link above, you will notice the fuse that feeds the trailer relays is "f2.6/20a". That means it's located in fuse box f2(underdash) at fuse location #6, and is a 20amp fuse.

 

Have you cleaned and checked your main ground cable from your harness?

 

Franklin2, It is a regular (Extended Cab) truck, The reason I think its a fleet vehicle is, its school bus/State highways yellow and on the bedsides, there are holes drilled where toolboxes would have been installed.

Heymrdj, You mean the main ground within the wiring harness? I have tried a few methods of ensureing a good ground from trailer to truck, with no change. I did look for an obvious ground but did not see one. I figured it would ground somewhere in rear of the truck, but all the wires are in the loom all the way to the front. (at least from what I can see, the loom goes along fuel tank in frame rail).

What do you guys think would be the best way to test that ground if I cant find the ground?
I did clean both sides of the connections.

Thanks
Hab

 

Ok for trouble shooting we'll make cliffs.

Truck ok when no trailer is on it.
Trailer is ok because of other tow vehicles not shorting when connected to it.


This leaves only the harness system that connects to your trailer, and the things that attach to it such as relays ect. So that's where to aim the troubleshooting. A simple 25$ multitester from Walmart automotive will test grounds, 12v and many other things.

 

I have tested quite a few things, except the relays. (at least the ones I can find) I have searched and found a few sites that tell me how to test relays... The problem is, I dont understand what are trying to tell me. If someone could give a quick easy testing lesson on those I would appreciate it.
To test a ground, I guess you would measure ohms, and one end on the ground pin, and the other to a ground to see if it ohms out??

On another note, I have read over 150 pages on this forum (One advantage to being off work due to injury) and I have opened anything that even remotley seemed related to trailer issues, and they are all the same thing, Hidden fuse or relay. All I gotta do is find them all I guess.....


Thanks for the replies and helping me out

Hab

 

Well, I had some time and was able to get out of the house today, so I went and purchased a Haynes for my truck, and low and behold, 2002, the "Other" fuse box is located........... behind the glove box!!!! I had gone to a friend who has the same truck and his owners manual and supplement, had nothing about this fuse box. What the heck??? I will get to the garage hopefully tomorrow and hopefully find a faulty relay or two....

Hab

 

So I did find some time today, went out and opend the glove box and saw what could possible be where some fuses and relays are. But upon closer inspection, there are a few screws holding it on... Didnt seem right, but I took them out anyway.... Yep, the Heater core.. No fuses anywhere behind the glove box. Now i am hoping the brake/flasher relays are with the trucks, which sound like they are above my 4X4 shifter behind the dash.... The search continues.....

 

Well, They are not with the trucks flasher...

In my fuse box, there is a fuse marked tow relay. Its a mini fuse. I am thinking that if I drop the fuse box and follow the wire it will take me to a relay somewhere. I do forsee that as being a real pain in the ... Guess I keep on searching, I have the dash half apart now, so I may as well keep on with it.

 

Pic of ... No Fuse box under the hood.



Pic of Fuse box behind lower dash on drivers side of truck.

Is this fuse box bigger than the other years? By the picture in the Haynes manual, is seems to be twice the size. So I am thinking that all the fuses are located here, and all I need to find are the relays for the turns.

 

Ok, I finally got to sit down with my DVD. What is in that box in the picture below? The dvd says relay box #4 is located on the driver's side rear in the engine compartment. You must realize we are not looking for a fuse box. You are looking for a small box that has plug in relays. I see a small box in the picture below.



If you find the relays, they should click when you operate the turnsignals. One of them should click for left and another for right. There are wires coming from the turnsignal switch that feed each one of these relays, and cuts the relays on and off. The relays in turn open and close the separate fused circuit that feeds the trailer turn lights. The fuse mentioned in a previous post, and is shown in the diagram in the previous post.

If you have trouble determining if they are clicking, hold a screwdriver onto the relay and put your ear to the other end and see if that helps.