Ok, I've got a couple of other threads going, trying to figure this thing out, but this question is a little different, even though I'm working on the same problem. Working on a '96 E350, 460 gas, w/dual batteries and trailer tow package. In the Power Dist. Box, there is a 60A fuse that is labeled "Trailer tow and aux battery power feed". Can anyone tell me what trailer tow functions this protects? There are other fuses for all of the trailer functions; stop/turn LH & RH, running & backup, running lamps, trailer battery charge & electric brake controller. As such, is this a redundancy? If it is a seperate circut from all of those others, what does it do?

 

I know the super dutys have seperate fuses for trailer lights. On my truck, there is a hot wire that keeps battery charged on the trailer. there is a constant hot (with key on) on the 7-pin connection.

 

Yeah, that's what I'm working on. In the manual, the PDB diagram shows this fuse as "trailer tow & aux battery power feed". However, on the fuse locator list, it lists it as protecting the "trailer tow & aux battery RELAY". So, there is a hint there that this circut has a relay in it somewhere (which I have already determined is NOT in the PDB). If there's a relay involved, IT could be my problem. I'm hoping that someone out there can tell me if this is the case, and where I can find it to check it.

This fuse in question is blowing. The 30A and 40A fuses for the trailer tow battery charge and the trailer tow running & backup lamps have very low voltage at the fuse (test lamp in the fuse socket with the fuse removed) telling me that there is a short to ground somewhere. If I melted down a relay somewhere, that could be my problem. But, I don't know where to look.

 

Ok, I'm a bit smarter now. There is a aux battery relay mounted near the left headlamp that controls power into the PDB. We are checking that, and I suspect that it is our problem. More to follow when I know more. Again, if anyone has any other things that we should check, let me know.

 

In a class two configuration with a auxilary battery, the 60a fuse "T" feeds the aux battery only, via the aux battery relay.

If you do not have a aux battery, fuse T 60a feeds fuses "J" 40a trailer charge fuse relay and fuse "K" 30a which supplies power to the trailer running lamps relay and the trailer backup relay.

 

Interesting. We DO have an aux battery. Upon exploring the aux battery relay, removing it and reinstalling it cured our problem. Or at least we now have full power at fuses J & K. Perhaps we just had some corrosion or a bad connection? Or maybe we have a intermittently bad relay? We are going to replace the relay, just to be safe. But for now, we have everything working again (except the alternator, now). I suspect that the short that we had at the trailer tow plug overworked the alt. and fried it.

If T feeds J & K, and the relay for the aux battery was causing the problem, this all makes sense. But, you are saying that T doesn't feed J & K when we have the aux battery, so I'm kind of confused here. We'll see what happens when we replace the relay.

Thanks.

 

Well, I thought we had this thing licked, but NOT! Today we installed a new 130 amp alternator from NAPA. They tested the old one and it tested out bad (198k, and it appeared to be the original motocraft alternator!). We also replaced the aux. battery relay. When we hooked it up to the trailer...no lights. We unplugged the trailer, just in case there was a short there causing problems, but no go. We found the 60A fuse "T" blown. With the ignition off, we unplugged the (new) alt batt relay and replaced the fuse. When we started the van, the fuse blew again.

Fuse T definately does supply power to J & K. We have traced the trailer running lamps circut (fuse J, I think) back to the PDB, and we are confident that it is not shorted out. We have not done the same with the same yet with the trailer tow battery charge circut (fuse K). Tomorrow (when NAPA is open so that we can get some more fuses) we are going to unplug both relays (anyone know what the other one is for??) as well as both J & K fuses and see if fuse T blows again. If it blows, I have to believe that we have a problem in the PDB. If it doesn't, we'll start adding the relays and fuses back in, each one by itself and see if we blow the fuse again. That's all that I know to do.

We will try to trace the trailer tow battery charge wiring back from the rear to make sure that it is not shorted/melted somewhere, but I would expect that if that were the problem that fuse K would blow before fuse T, wouldn't it?? I also would expect that if the short were in that circut, that we would identify that when we start isolating things by pulling both fuses and both relays.

What do you guys think? Has anyone got any other ideas here??

Thanks!

 

Oh, I should have said that there are two relays up above the driver's side headlight. One is the the aux battery relay that we were focused on, but I don't know what the other is for. If anyone knows, please share!

Thanks.

 

It's hard to figure this out from the diagrams. It's shows the aux battery relay in the headlight area, but doesn't list any of the others, except in another diagram is shows a group of three relays sort of in the same general area. Standing in front of the van and working from the center to the driver's side fender, they list these as trailer backup relay, trailer battery charge relay, and then the trailer running lights relay.

 

Hopefully these diagrams work, and you can try to figure out what's going on.

 

Perfect, thank you! If the other relay is for the trailer battery charge, then maybe we replaced the wrong bad relay. Crossing my fingers. Will let you know.

 

Ok, with both relays and fuses J & K removed, I installed a new 60A fuse in T. Fuse T and the smaller yellow power supply wire coming into the PDB got hot with the ignition turned on. Fuse T did not blow, but I suspect that it would have eventually if I had left the ignition on. I will try to unwrap the yellow wire and inspect it visually as far back as I can. But, short of that (pun intended), I'm not sure what else to do. Any suggestions?

 

I must be missing something here. Fuse T has power with the ignition off and on, and does not heat up or blow unless the ignition is turned on. J & K are not energized until the ignition is turned on, so there has to be a relay between T and J/K. However, I am not seeing that in the diagrams above.

I am wondering about the aux battery (not the trailer battery) jumper and how that all works. Presumably, the intent here is to power the trailer functions from the aux battery instead of the main battery. Is it possible that my aux battery is bad/shorted out such that when the ignition is turned on, that short is introduced into the circut? There has to be something between T & J/K switching the power, but what and where is it located?

 

If you suspect your aux battery is bad, take the wire off it and see if the fuse heating goes away.

Those diagrams are pretty confusing, since there are so many options. I assumed the aux battery jumper was there when there was no aux battery option installed?

 

Two steps ahead of you Dave! After studying the diagram for some time, it does appear that the Aux battery jumper enters the circut with a spice (S172) somewhere between fuse T, and fuses J & K. The Aux battery relay, where ever that is located, allows the Aux battery to be charged by the alternator, presumably when the ignition is on...

Thus, if the Aux battery is internally shorted (which it was) then the load on the charging circut blows fuse T.

The aux battery, a 5 yr. old interstate, was dead. When I removed the + cable on the aux battery and put my test light on it, it was so dim, I couldn't hardly tell that it was acutally lighting up. A battery test showed that it had 4 volts. When we tried to charge it, it started smoking...

The meltdown in the trailer plug probably shorted out the battery and/or the alternator (which tested bad as well previously). Once we got the short fixed, and the alternator replaced, the new alternator had enough power to blow the fuse as it tried to charge the aux battery. I suspect that though it doesn't specifically say so, that fuse T is designed to protect the alternator in just this situation.

We have new battery ordered (no one stocks a C50-XHD around here). With the + cable off of the aux battery, everything seems to work fine. I'll be reinstalling the relays and fuses J & K to see if I have output in the trailer tow running lights circut without the aux battery installed. If not, I may have to wire up a work around for trailer running lights until the battery arrives.

Thanks again for all of your help! I will post again once we have the new battery in, just to make sure that all is finally working as it should.