Last fall I came out after not using the truck for a week and found the battery flat dead. The battery was almost new. With another good battery I found that even with everything off, there was notable (.3a) current being drawn on the battery. It appears I have a phantom load. By removing fuses one at a time I found that it has developed at Fuse #21 which the manual says is for the trailer tow battery charge relay. I have verified that if the fuse is pulled then relay #9 no longer is has power at the relay switch terminals. I have verified that the relay is working. If ignition is off, then there is no 12v power at the trailer plug connection. If ignition is on, then there is. However, even with relay #9 pulled, the load still exists.

I have had to use the truck all winter with fuse 21 pulled but now I want to use the camper. Does anyone have a schematic for the main distribution box? Is there another load or relay besides #9 that fuse #21 feeds?

 

I would take a good look at the fuse box for corrosion.

 

It looks pretty pristine in the engine compartment but I haven't taken the distribution box apart. Assuming the power is disconnected, is this a safe procedure or something you don't try at home? My preference would be to review the schematic first to see if there is any other pathway, but maybe not. Has anyone removed the distribution box top from the body where the wiring enters?

 

Yes, it's something you can try at home. The fuse box just pops apart. Something that should take 20 minutes to look at.

It will also give you a better idea of wiring colors.

The other suspect might be corrosion in the 7 pin trailer plug or it's harness somehow completing a circuit to ground. Take a gander at that, and see if you see anything.

Sometimes it just the obvious.

 

Thanks, I will give it a try. Yesterday I tested the trailer plug extensively and found no leakage to ground. I even took it apart and the inner contacts were pristine. Note that the problem occurs even with the trailer battery charge relay removed which cuts off the 12v supply going back to the trailer plug.

 

Can you explain what that means? Do you mean the current draw (300 mA) still exists with the relay physically removed?

Nope, nothing.

 

Yes, the load of 300ma remains even after relay #9 is removed if fuse #21 is in place.

 

The current would be classified as a "draw", not a "load".

I'd also recommend what ManualF150 did, there's nothing else that makes any sense (assuming all wiring is unmodified). Perhaps damage to the wire denoted in the attached schematic as SBB21.

 

Agreed, now that I see the schematic. What is the battery junction box (BJB 11-1)? Is this internal or external to the distribution box?

I am hesitant to remove or open up the distribution box without knowing precisely the procedure. Although I am familiar and fairly competent working on electrical issues, I don't want to inadvertently turn the truck into a brick, or to fix one issue only to cause another. by improper disassembly or reassembly.

 

Yes, that is the battery junction box. The one that is in the engine compartment.

There are clips all the way around the BJB. Carefully spudge the unit in half. You may need to unbolt the cable that's connected to it and you might need to remove all the fuses/relays.

If you do need to remove the fuses and relays, I highly suggest taking a couple pictures.

Just take it easy when taking it apart.

 

OK, looks like I may need to by a fuse puller first. They don't come out easily. Good tip on taking a picture first. I have been trying to do that now with my cell phone. It would be good to know whether the fuses and relays need to be removed. There are almost 100 of them!

How does is the battery junction box different from the distribution box, or are these one in the same?

 

Same thing, slightly different names.

The "11-1" is an in-application hotlink to another page that provides an alternate or comprehensive diagram, in this case, the diagram of the physical layout of the fuse box.

 

Thanks for the assistance. I have the original manual. The manual doesn't include electrical schematics. What source do you use?

It's a mystery to me how a jumper from fuse to relay can cause this issue in a protected location like this, but I guess I will find out soon.

 

I have the factory service manuals.

 

OK. With great trepidation and a little cursing I got the distribution box detached and have examined the underside. As expected it looks factory. I found no visible issues with corrosion, no bare insulation, nothing at all that would hint at an issue with this simple circuit. It is really hard to see the underside of fuse 21 but I managed in spite of bifocals. It looks clean, just like its neighbor. I examined and felt all along the grey/red jumper wire from fuse 21 to relay 9 and found nothing. This really has me perplexed. It's a real rat's nest of wiring under this box, but nothing appears to be out of order. Before attempt to button this back up is there anything else to look for?

I This is looking under the rear of the distribution box. It is clean and dry.