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I wanted a power feed that was only active when the engine was actually running, so the batteries wouldn't get drained by accident. In my case, I wish to recharge my trailer battery while driving, without having to worry about killing the batteries.

For this purpose, the red wire on the rear 7way plug and in the 5th wheel harness are the 12v battery wires. These wires are connected together and get their feed from "stud 1" in the under hood fuse box. This wire is protected by a 40A fuse in the box. You have to remove the fuse box cover to access this stud, then it will be at the upper left corner. It is referenced in the diagram attached to the underside of the fuse box lid. There may be other wires attached to the stud, if so, make sure you select the proper one. I cut this wire, and extended it so both ends were long enough to reach the relay mounting area I chose. The power wire from the stud attached to terminal #30 and #86 on the relay, the other half of the wire from the harness attaches to terminal #87.

This setup could be used for many other applications, like air compressors, inverters, etc. You can also use the output from terminal 87 to control one or more other relays, which also have switches that go to ground to activate. This way you can use a second relay to control, say, an air compressor. The air compressor pressure switch can be attached to terminal 87 on the 2nd relay, the other end of the switch would go to ground. When the air pressure gets low enough, the compressor would turn on IF the engine were also running, otherwise the relay would not have control power and would not energize.

The switch I used was a GM/Delco part D1811 or 10002798. I believe it was used to control a fuel pump originally. Cole Hersee also makes suitable switches. The key is, the switch should be OFF normally, and turn ON with rising pressure.

The relay I used was a Bosch unit, part number 0 332 014 157. This relay is normally OPEN, and closes when current is applied to the control coil.
Some similar relays have both NO and NC contacts, these will work fine also, just choose the NO contacts for this setup.

The normal place to attach this switch would be to T into the oil pressure sender. However, the sender is in a tight area, and you would probably have to space out the two switches quite a bit to install a nipple and T fitting. I didn't like this idea, with the vibration of the diesel, it is possible for the pipe nipple to snap off over time. So I attached the new switch to the engine oil filter adapter/cooler housing. See photos for the switch location, as there are several different plugs. I think some of them control a bypass function, and I didn't want to mess with them.

The one I used has some sort of metal down inside the port, but it didn't come out, and didn't see to be affected by the plug I took out. At any rate, it works. The plug I removed had a recessed allen bore, and was 1/4 NPT (pipe thread). The oil pressure switch is 1/8 NPT.
So you need to pick up a 1/8 female by 1/4 male adapter from the auto parts store. I had a brass 1/4 NPT hex plug handy, so I drilled it and tapped it for the 1/8 NPT so the switch would fit. There are reducer bushings that you can buy that are almost exactly what I used.

At any rate, remove the plug from the oil filter adapter as shown. Then install the adapter Use plenty of teflon tape to seal the adapter, but make sure not to get any into the hole.

Next, install the oil pressure switch. The one I used had pre-applied sealant, so I just cranked it into the adapter. Make sure both pieces are tight, but don't get medieval.

The switch has two terminals that close when the switch senses pressure. One terminal should be grounded with a wire. You can attach this wire to the chassis or body, as long as it has a good connection. The other wire will be routed to your relay location and attach to terminal #85.

I located my relay right next to the underhood fuse box, so that's where I ran the wire to. I would suggest sleeving the wire with plastic loom cover, and secure it with zip ties.

Notice that the power feed supplies two terminals, #30 which is the power feed for the relay, and #86, which provides a very small amount of power for the relay coil. Although the relay has power to the coil all the time, the relay will not operate until the terminal #85 becomes a ground.

Hope this info is helpful.
Thanks,
Bryan
 

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Discussion Starter #2
Have a few trips with this setup, works great. It charges the trailer batteries as I drive. I think the charging amperage is limited due to the wire size, but that's OK. You could also install a heavier wire to the socket if you wanted to.
 

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GM engineering put some thought into this. What you have done may actually create a problem by have no regulation for your trailer batteries and cooking them. The ECM takes care of these problems, including the charge rate by sensing on that power feed. The system voltage is also raised to accommodate the additional trailer/camper batteries when you are in tow/haul mode. I don't mean to be rude but, IMO, take out your mod and use the trailering electrical system the way it was designed.
 

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Discussion Starter #4 (Edited)
Not sure I agree

I'm not sure I agree. I have not affected the power lead except to insert a relay into the existing power wire. When the engine is running, the circuit should behave exactly as normal. When the engine is off, power to the circuit is cut. I would also suspect that any load sensing is done prior to the relay, in the fuse box or front harness. The only thing this mod does is cut off power to the trailer when the engine isn't running.

See copy of wiring diagram, attached. The diagram shows a wire directly from stud 1 to the 2 trailer harnesses. GM should have installed a relay to prevent trailer use from discharging the starting batteries by accident when the engine is not running.

If I am in error, please be SO kind as to point out how.
Bryan
 

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Discussion Starter #7
Man, everyone's getting cranky:(
The benefit of this, is that if you have an RV attached to your truck, you can charge the batteries as you drive, without having to worry about the truck not starting. As you use the battery power on the RV, you can kill the truck batteries, if you don't disconnect the trailer connector.
The 12v feed to the trailer is on all the time in the stock configuration.
The relay just cuts the power if the engine isn't running.
Bryan
 

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I didnt think you could kill the trucks batteries by leaving the trailer pluged in.
My trailer has been almost dead of battery power and the truck was fine.
Anyone know for sure?
 

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Discussion Starter #9
Is your trailer battery charged by the truck? The trailer power lead is attached to stud 1 in the fuse box, which is always on. If you have one battery on the trailer, you might be able to drain it faster than the truck batteries can supplement it (due to the wire size). However, over time, the system should attain equilibrium, with the 2 truck batteries at the same voltage as the trailer battery. Over a weekend, I'm sure you could kill the truck batteries, I've done it with other trucks.
I like the convenience of charging up the batteries as I travel, without necessarily having to disconnect the coupler as soon as I stop.
The system would also be useful for controlling air compressors, inverters, etc. Otherwise, they'd be on all the time, or as soon as you turn on the ignition. That might be alot of load while trying to start the truck, it all depends on what power feed you use for your device and/or relay. Some circuits, like the headlights are off during cranking, others, like the computer are on.
Bryan
 

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In my opinion you have over engineered the system-- all you had to do is use the ignition to energize the relay.

That would close the relay when the switch is on-- so the engine would likely be running and it would work the same way.

The mods would keep from running the truck batteries down. :D
 

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Discussion Starter #11
The problem with the ignition on system, is that some items, like my onboard air compressor, would kick on the second you turned on the key. Meanwhile, the glow plugs may be operating at 110A, and you're getting ready to start the truck, possibly in cold weather.
The ignition on system would prevent you from killing the batteries, but could add substantially to the drain during starting. Other high drain items, like the headlights, turn off during cranking. But they are probably controlled by the computer, the pressure switch is simple, and works. This mod is not new, its been done by trailering types for years, for the reasons listed at the beginning. A related item is the aux charging relay or diode, which allows an extra battery to charge, but not kill the starting batteries when discharged. It looks like GM has an optional relay for that, but in my truck, the dual batteries seem to be directly connected for starting use.
Peace.
 

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No offence but this is just dumb. It does work and do its purpose, but what a huge waste of time and money. Like the previous guy said, use ignition power to run the relay. It would be a more reliable and trouble free setup in the long run. What is this onboard air compressor your talking about? One that runs off the engine belts or some kind of 12v compressor in a trailer? I cant see for the life of me why you would want it to work only when the engine is running. What if your trailer needed a constant power supply to run the interior lamps, would you go start your truck just to turn on a dome lamp?
 

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Well, I know EVERYBODY is giving you a hard time, but I congratulate you. You saw something you wanted to improve and you designed a method to do it.

Obviously it's not for everybody, but I appreciate you sharing the info. I don't personally have a use for it or plan to use it, but I'm sure somebody does.

That said, I also don't see how you would have a problem with "regulation". Voltage regulation should be out of your alternator regulating to around 13.9v, if the vehicle is not running the battery voltage is around 12v. I'm not understanding the concern.

You should figure out the amperage draw and ensure your wire is sized appropriately though, that could create a potential problem.
 

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Looks like a great place to put an Engine oil temp sender. This was a question on another thread.
 

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I have left my trailer plugged into my truck for a week w/ the lights on and never had any dead batteries in the truck??????? Just the battery on the trailer runs dead after a while.
 

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To the original poster !! I

I too appreciate that you did this !! It works for you and opens our minds up for more thinking and learning.

I know many times I make something or do something to my truck to make MY life easier, I sometimes share on the forum. Sometimes I get what you have gotten!! A Big WTF For response..

So to all of you nay sayers, F*ck off, and have a bit of respect and an open mind..

HmmMMmm
 

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So to all of you nay sayers, F*ck off, and have a bit of respect and an open mind..
Well he never did answer any of my questions. I dont like to open my mind up to too many useless ideas. I will say to the original thread starter that he did do a good job on showing his idea, but too bad its of no use to 99% of us.
 

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So to all of you nay sayers, F*ck off, and have a bit of respect and an open mind..
Touche

I may not be that familiar with how the computer can supposedly adjust voltage by sensing a trailer in tow/haul mode. Because an alternator does this automatically anyways. If it senses increased load it increases output regardless of it sensing a trailer.

Also I cant possibly see how it can cook a trailer battery any different then the stock way, with the one exception of having a marine battery in the trailer that charges slower then a cranking battery does. Which is of no fault of the truck or the OP, it always happens when you mix 2 different types of batteries together on the same charging circuit.

I personally see it as a very cool idea, and not just for the trailer plug application.

On the semi there is a big use for this very setup. You have 2 banks of batteries. 4 to start the truck and a separate bank of at least 2 for your battery reserve for your bunk items. When you dont want to run down your starting batteries you run off your reserves only. You use a ford solenoid and a oil pressure switch to charge them and have them only connected to the other 4 batteries when the truck is running, so you dont run down your starting batteries ever.
 

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I have left my trailer plugged into my truck for a week w/ the lights on and never had any dead batteries in the truck??????? Just the battery on the trailer runs dead after a while.
im thinking that there may be circuitry that acts as an isolator in the the trailer feed then. Much like when you leave your cargo light on and it has a timer to kill the power after a certain amount of time to prevent drainage.
 

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I would agree with some of the concerns you all have but the guy who started the thread only posted it to possibly help out someone else. No, not everyone has a use for it but I dont see a reason to knock a guy down for posting something to help other people out.
 
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