No power to 12V constant pin - Diesel Forum

khokhonutt · 10-29-2006, 03:25 PM

Hi All,

I thought I'd post here to see if I'm missing something simple before I start digging further into my truck wiring.

I've got a 2001 F250 PSD with the factor towing package. I've been using the 7 pin plug for a while to tow my trailer, but with that trailer I primarily use the lights and trailer brakes.

Yesterday I was towing a box trailer for my kids' marching band. That trailer makes use of the 12V constant pin to run lights on the inside of the trailer. When we went to load the trailer last night, we realized those lights did not work.

I've checked the pin that should be the 12V power with a test light and it appears to be dead. Testing the wire (heavy orange wire) upstream, it appears to be dead as far as I can reach at the back of the truck.

This morning I checked the fuses that appear to be related to the trailer battery charge circuit (what I assume is the 12V line). Both the 30A fuse under the hood and the 10A fuse in the cab are fine.

Looking at a wiring diagram in my Haynes manual, it looks like there's a relay for the battery charge circuit. If that's the case, what activates that relay? Am I incorrect in assuming that line should be hot at all times?

Any info anyone can provice would be appreciated. Worst case, I'm going to run a direct line with a switch, but I figured I'd check here and see if anyone has any tips before I start crawling under and tracing wires.

Thanks,
Jeff

enginebill · 10-29-2006, 04:14 PM

Did you test the pin with the key in the on position?

khokhonutt · 10-29-2006, 04:28 PM

[ QUOTE ]
Did you test the pin with the key in the on position?

[/ QUOTE ]

Yes. Last night when we were trying to get the lights to work I tried with the key in the on position, with the truck running and with the running lights turned on. None of them seemed to provide power to that pin.

overfly · 10-29-2006, 04:56 PM

When I had my '03, I had the same problem. Turned out even though it had the towing package there was no relay (just an empty plug). Put in a relay and it worked just fine.

tobi · 10-29-2006, 05:00 PM

it is only hot when the key is on.

NickKent · 10-29-2006, 05:13 PM

[ QUOTE ]
Hi All,

I thought I'd post here to see if I'm missing something simple before I start digging further into my truck wiring.

I've got a 2001 F250 PSD with the factor towing package. I've been using the 7 pin plug for a while to tow my trailer, but with that trailer I primarily use the lights and trailer brakes.

Yesterday I was towing a box trailer for my kids' marching band. That trailer makes use of the 12V constant pin to run lights on the inside of the trailer. When we went to load the trailer last night, we realized those lights did not work.

I've checked the pin that should be the 12V power with a test light and it appears to be dead. Testing the wire (heavy orange wire) upstream, it appears to be dead as far as I can reach at the back of the truck.

This morning I checked the fuses that appear to be related to the trailer battery charge circuit (what I assume is the 12V line). Both the 30A fuse under the hood and the 10A fuse in the cab are fine.

Looking at a wiring diagram in my Haynes manual, it looks like there's a relay for the battery charge circuit. If that's the case, what activates that relay? Am I incorrect in assuming that line should be hot at all times?

Any info anyone can provice would be appreciated. Worst case, I'm going to run a direct line with a switch, but I figured I'd check here and see if anyone has any tips before I start crawling under and tracing wires.

Thanks,
Jeff

[/ QUOTE ]

I believe you need the relay and the fuse for the pin to work properly. That way a camper with a large battery cannot back feed into the truck.

khokhonutt · 10-29-2006, 05:24 PM

Ok, cool. Thanks for that input. The relay sounds like a good bet. Can anyone tell me where that relay is located? I can't seem to find it in the Haynes manual and my owners manual doesn't seem to help either.

Jeff

NickKent · 10-29-2006, 06:53 PM

I don't know about the newer trucks, but in my truck the relay is in the power distribution box under the hood. Same place the larger fuses are. Should tell you in the manual where the fuse belongs.

D_Dunn · 10-29-2006, 06:54 PM

Jeff,

I replaced the relay with a jumper to get a constant 12 volt feed to the "+" terminal of the 7 pin plug. I made the jumper from a piece of heavy guage wire with spade terminals on each end. I used a test light to determine where to plug it in. I'd have to check to see which two terminals I plugged into. On my 2000, the trailer charge relay and trailer backup light relay are in a small box with a lid, right next to the main power distribution box under the hood. It's the relay toward the front of the truck. I would think your '01 would be the same but don't know for sure [img]/ubbthreads/images/graemlins/shrug.gif[/img]. This is a pretty simple mod to get a constant 12V to the rear of the truck.
Hope this helps...

Dan

khokhonutt · 10-29-2006, 09:17 PM

[ QUOTE ]
Jeff,

I replaced the relay with a jumper to get a constant 12 volt feed to the "+" terminal of the 7 pin plug. I made the jumper from a piece of heavy guage wire with spade terminals on each end. I used a test light to determine where to plug it in. I'd have to check to see which two terminals I plugged into. On my 2000, the trailer charge relay and trailer backup light relay are in a small box with a lid, right next to the main power distribution box under the hood. It's the relay toward the front of the truck. I would think your '01 would be the same but don't know for sure [img]/ubbthreads/images/graemlins/shrug.gif[/img]. This is a pretty simple mod to get a constant 12V to the rear of the truck.
Hope this helps...

Dan

[/ QUOTE ]

If you could check which terminals you jumped, I'd really appreciate it. I checked and the 'relay' is in the same place as where you describe (only the relay is not there). I'd like to get a relay at some point, but for now, jumping the terminals would work for what I need this coming weekend.

Thanks again,
Jeff

Sport · 10-30-2006, 10:16 AM

The relay is not expensive and any good parts store should have it.
Trailer Tow Package Relay
There should be two relays in the box beside the under hood fuse panel.

D_Dunn · 10-30-2006, 11:37 AM

[ QUOTE ]
[ QUOTE ]
Jeff,

I replaced the relay with a jumper to get a constant 12 volt feed to the "+" terminal of the 7 pin plug. I made the jumper from a piece of heavy guage wire with spade terminals on each end. I used a test light to determine where to plug it in. I'd have to check to see which two terminals I plugged into. On my 2000, the trailer charge relay and trailer backup light relay are in a small box with a lid, right next to the main power distribution box under the hood. It's the relay toward the front of the truck. I would think your '01 would be the same but don't know for sure [img]/ubbthreads/images/graemlins/shrug.gif[/img]. This is a pretty simple mod to get a constant 12V to the rear of the truck.
Hope this helps...

Dan

[/ QUOTE ]

If you could check which terminals you jumped, I'd really appreciate it. I checked and the 'relay' is in the same place as where you describe (only the relay is not there). I'd like to get a relay at some point, but for now, jumping the terminals would work for what I need this coming weekend.

Thanks again,
Jeff

[/ QUOTE ]
Jeff,

I have the info you requested. The jumper is between where terminals 87 and 30 of the relay would plug in. One is closest to the brake master cylinder and the other is closest to the Power Distribution Box. Does that make sense? Let me know if you need any more help...

Dan

khokhonutt · 10-30-2006, 03:10 PM

[ QUOTE ]
The relay is not expensive and any good parts store should have it.
Trailer Tow Package Relay
There should be two relays in the box beside the under hood fuse panel.

[/ QUOTE ]

Excellent, thanks.

Dan, thanks for checking on the jumper. If I can get the relay for cheap, I'm gonna go that route. Otherwise, I'll be trying out the jumper solution.

Thanks again guys!

khokhonutt · 10-30-2006, 05:38 PM

Thanks everyone. I used the number from the link Sport provided. Napa was able to cross reference the relay to one they had in stock. I installed the relay and with the key on I've got power to the 12v pin.

The kids in the band will thank you all this weekend!

Jeff

D_Dunn · 10-30-2006, 10:22 PM

Jeff,

Just remember that the reason that I replaced the relay with the jumper was to get 12V to the pin when the key is off. As Tobi mentioned above, the pin will be hot only with the key on if you install the relay. I pull a trailer for our local high school marching band too and it's nice to be able to use the lights without having to have the key on. Just so you know...

Dan