Truck battery drain when 5ver connected?

SOLVED. Truck battery drain when 5ver connected?

Something is going on with my trailer charging connection... A little background might help:

I have a 2011 F350 6.7 with Ford's OEM in-bed 5th wheel receptacle wiring and hitch. I recently started my truck after being parked beside the house for 10 days with my new Bighorn plugged into the in-box 7-pin receptacle. My truck seemed to struggle to turn over for a second or two, but then started. However, the truck threw some codes and the service engine soon light came on. I had my truck checked a couple of hours later and the dealer confirmed that all of the codes thrown were low-voltage related. They tested my two batteries and said they checked out fine. They suggested I might want to consider replacing the batteries, but keep an eye on things as something didn't add up.

Hmmm...

During this 10 day period the 5th wheel's shore power was plugged in a few of days while I worked on a couple things. There were no loads left on in the 5th wheel except the usual parasitic.

Well I get home from the dealer and used my cheapie harbor-freight load tester on my truck's batteries -- they both passed in the green. I had not previously noticed any battery drain or sluggish starts on this truck so I became suspicious of the Ford OEM receptacle wiring and my 5th wheel connection. I decided to run a test.

First, with the truck shut down I tested volts across the ground and 12v+ pins on my receptacle. Nada. Just what I expected, the trailer charging circuit should be ignition relay controlled. No juice unless the truck is running.

Next I backed the truck up to my Bighorn, shut it off and came back out 10 minutes later to plug the RV bed plug in. Five seconds after plugging in I hear my truck's on-board air compressor kick in to top off my trucks air bags!? Not unusual to hear the compressor do this when I first start the truck, but my truck was locked and not running at the time. I wired the compressor in myself with an ignition controlled relay so now I'm really confused.

Tine to double check things. I disconnected the RV plug and fully deflated my truck's air bags. I unlocked my truck and turned the key to ACC and waited. Nothing... just as expected. I turned the key to run and waited. Five seconds later my compressor turns on to re-inflate the air bags. Just as expected.... so I am positive the compressor is operating properly off the run/ignition circuit relay.

It appears that Ford's OEM RV receptacle when plugged into a trailer charging cord, is allowing the ignition circuit to become energized. I also suspect the 12v+ feed on the trailer plug receptacle is also being energized (I should have tested this at the pin junction box). That might explain how my parked truck's batteries were pulled down over the course of a week by the trailer connection.

I'm dealing with a factory wired OEM Ford receptacle and a factory Bighorn RV plug here.. nothing custom or modified. My Bighorn has two new 12v 100ah AGM batteries that also appear just fine.

Obviously, I do not want my parked tow vehicle's batteries at risk being drawn down by the trailer. I'm thrown for a loop on this. I'm off on vacation for a week so I'll have to figure this out when I return.

Your thoughts on what might be going on?

Kurt

I wonder if maybe the control circuit is breaking the ground connection rather than the B+. If so, the trailer might be providing an alternate ground connection that completes the circuit.

Interesting idea. When I get back I'll try grounding the RV receptacle ground pin back to the truck frame... would that be a proper test?

danemayer said:

I wonder if maybe the control circuit is breaking the ground connection rather than the B+. If so, the trailer might be providing an alternate ground connection that completes the circuit.

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I'd put the meter across the 12V pin and the truck frame to see if the pin is hot. The truck frame is connected to the battery and should complete the circuit.

Be gentle with me... How is that different than when I tested accross the 12v pin and the ground pin within the receptacle and determined there was no voltage when the the ignition was turned off? I assume the receptacal ground pin is tied into the frame/negative ground.

danemayer said:

I'd put the meter across the 12V pin and the truck frame to see if the pin is hot. The truck frame is connected to the battery and should complete the circuit.

Click to expand...

kdubinwa said:

Be gentle with me... How is that different than when I tested accross the 12v pin and the ground pin within the receptacle and determined there was no voltage when the the ignition was turned off? I assume the receptacal ground pin is tied into the frame/negative ground.

Click to expand...

If the engine circuit that controls power to the trailer connector is opening and closing the ground side (admittedly a big if), when the engine is off, the ground pin isn't connected to the frame/negative ground. We always assume the control interrupts the hot wire, but since you have this odd situation, we have to consider other possibilities.

If the control is on the ground side, then an alternate ground connection, through another pin on the connector, or through physical connection at the hitch, might be completing the circuit, allowing the trailer to suck the life out of your batteries.

There's another possibility - if you're checking the wrong pin(s), you won't read 12V. You didn't say that you verified the 12V with engine on. Maybe try that just to make sure.

Btw, the 7 pin connector on my 2014 RAM is hot all the time. I use the connector to power my air compressor and don't need to start the engine to get power.

Got it.

It's possible I checked the wrong pin, however, I printed out the wiring standard for an RV 7 pole receptacle as a reference for my check. I did not check the pin with the truck running so I can't really be sure. Good input. I'll verify when I get back.

Curious, does Dodge wire the receptacle hot like that and would you be at risk for draining the truck battery via the trailer connection?

danemayer said:

If the engine circuit that controls power to the trailer connector is opening and closing the ground side (admittedly a big if), when the engine is off, the ground pin isn't connected to the frame/negative ground. We always assume the control interrupts the hot wire, but since you have this odd situation, we have to consider other possibilities.

If the control is on the ground side, then an alternate ground connection, through another pin on the connector, or through physical connection at the hitch, might be completing the circuit, allowing the trailer to suck the life out of your batteries.

There's another possibility - if you're checking the wrong pin(s), you won't read 12V. You didn't say that you verified the 12V with engine on. Maybe try that just to make sure.

Btw, the 7 pin connector on my 2014 RAM is hot all the time. I use the connector to power my air compressor and don't need to start the engine to get power.

Click to expand...

Absolutely can drain the truck batteries by leaving the trailer hooked up. For example, if you stop overnight at a Walmart in the winter, the drain from the residential refrigerator, furnace, and water heater could suck the life out of your coach and truck batteries. Much less likely in warm weather though.

I always try to not have the truck hooked up to the trailer when the trailer is plugged in to shore power.

Perhaps this will solve your problem.

Makes sense, however, I noticed that the trailer connection appears to complete my ignition circuit even when the trailer is not plugged in to shore power.

JohnD said:

I always try to not have the truck hooked up to the trailer when the trailer is plugged in to shore power.

Perhaps this will solve your problem.

Click to expand...

danemayer said:

If the engine circuit that controls power to the trailer connector is opening and closing the ground side (admittedly a big if), when the engine is off, the ground pin isn't connected to the frame/negative ground. We always assume the control interrupts the hot wire, but since you have this odd situation, we have to consider other possibilities.

If the control is on the ground side, then an alternate ground connection, through another pin on the connector, or through physical connection at the hitch, might be completing the circuit, allowing the trailer to suck the life out of your batteries.

There's another possibility - if you're checking the wrong pin(s), you won't read 12V. You didn't say that you verified the 12V with engine on. Maybe try that just to make sure.

Btw, the 7 pin connector on my 2014 RAM is hot all the time. I use the connector to power my air compressor and don't need to start the engine to get power.

Click to expand...

I just checked on my truck,13-F450, the relay must break the positive wire no voltage to ground when truck if off in run position have voltage. OP did you isolate batteries when testing? Had similar issue on an 06 one battery was starting to go.

kdubinwa said:

Makes sense, however, I noticed that the trailer connection appears to complete my ignition circuit even when the trailer is not plugged in to shore power.

Click to expand...

When the truck is turned off with the trailer plugged in to the truck the trailer still draws power (not battery charging, though) from the truck battery when the trailer is not plugged in to shore power.

I think, anywayz . . .

That has been my experience.

As previously noted, not on a Ford.

I did not isolate the trailer batteries when plugging in the trailer. I need to test that scenario as well.

All I know for sure at this point is that my parked truck batteries experienced drain over several days and that my ignition circuit seems to be energized when I plug my trailer (w/ charged batteries) into my bed receptacle when the truck is not running.

bdb2047 said:

I just checked on my truck,13-F450, the relay must break the positive wire no voltage to ground when truck if off in run position have voltage. OP did you isolate batteries when testing? Had similar issue on an 06 one battery was starting to go.

Click to expand...

For my truck the relay and fuse for charge circuit is under hood perhaps relay is bad and staying closed .Could check by pulling relay or isolating charge circuit at trailer junction box or circuit breaker. Fuse box under hood is pain to work with so opening charge circuit at trailer may be easier.This should tell if problem is in that circuit

kdubinwa said:

I did not isolate the trailer batteries when plugging in the trailer. I need to test that scenario as well.

All I know for sure at this point is that my parked truck batteries experienced drain over several days and that my ignition circuit seems to be energized when I plug my trailer (w/ charged batteries) into my bed receptacle when the truck is not running.

Click to expand...

OK, you said that your have added air bags. I installed Air Lift auto on mine. Too lazy to bore a hole in the firewall to run a wire to "ignition on only" for the height sensor. Truck sat for 2 weeks and batteries stone dead. Took truck to dealer to find out why. Found out that the height sensor draws a .5 amp every 45 seconds. Moral of the story, don't be lazy and read the instructions. Did not have to bore a hole , just had to find a "key on only" fuse in the under hood fuse center and install a "fuse tap" and problem solved.

My thoughts exactly. I plan to open the pin junction box and test the circuit from the junction with the the truck running and off.

bdb2047 said:

For my truck the relay and fuse for charge circuit is under hood perhaps relay is bad and staying closed .Could check by pulling relay or isolating charge circuit at trailer junction box or circuit breaker. Fuse box under hood is pain to work with so opening charge circuit at trailer may be easier.This should tell if problem is in that circuit

Click to expand...

Thanks Bob. I have the same airlift system. It could very well be that the drain is caused by the air lift system operating and not trailer parasitic loads. That actually makes more sense to me as the trailer had no loads on.

Now I need to track down why that circuit operates when the truck is not running and the trailer is plugged in. Perhaps I tapped the wrong wire when I installed my relay for the compressor. One hypothesis is that I tapped a run only circuit related to the trailer 12v and just plugging in to my trailer battery circuit is triggering my relay. The irony.

Bob&Patty said:

OK, you said that your have added air bags. I installed Air Lift auto on mine. Too lazy to bore a hole in the firewall to run a wire to "ignition on only" for the height sensor. Truck sat for 2 weeks and batteries stone dead. Took truck to dealer to find out why. Found out that the height sensor draws a .5 amp every 45 seconds. Moral of the story, don't be lazy and read the instructions. Did not have to bore a hole , just had to find a "key on only" fuse in the under hood fuse center and install a "fuse tap" and problem solved.

Click to expand...

kdubinwa said:

Now I need to track down why that circuit operates when the truck is not running and the trailer is plugged in. Perhaps I tapped the wrong wire when I installed my relay for the compressor. One hypothesis is that I tapped a run only circuit related to the trailer 12v and just plugging in to my trailer battery circuit is triggering my relay. The irony.

Click to expand...

Here is what I know about my trailer setup . . .

If I leave my trailer plugged in to my truck . . . the trailer will draw juice off of my truck battery until it is dead.

Ie. - inside trailer lights, and anything else 12-volts, will run off of the truck's batteries first, then trailer batteries second . . .

Moral of the story:

Don't leave your trailer plugged in to your truck when boondocking unless you have a back-up plan for a jump start!