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Homemade Trailer Wiring Tester


sclord2002

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This tool has been really useful for me. It is made using the 7 way connector half with the door or cover on it [like the one attached to the truck]. You will need the connector half, 6 18 inch ? pieces of wire [12 or 14 gauge ?], a wiring diagram for the connector half and a small alligator clip. To assemble the tool, connect one end of each wire to the back of the connector half to each of the pins EXCEPT THE GROUND...DO NOT connect a wire to the ground pin, label the wires and connect the alligator clip to the loose end of the 12 volt wire. You can then test your trailer by plugging the tool into the trailer pigtail and clipping the alligator clip to the wire representing the function you want to test...or clip all the wires except the brake actuator wire to the alligator clip and walk around to verify all lights are on. The brake actuator wire will activate the magnets on electric brake trailers and start the pump on electric over hydraulic trailers. You can also check the functionality of your brake magnets by using a DC clamp-on ammeter around the brake actuator wire when it is connected to the 12 volt alligator clipped wire....each magnet draws approximately 3 amps, so a +/- 12 amp reading on the ammeter means all 4 magnets are working, A +/- 9 amp reading means a magnet is not working. A compass held close to the wheel will deflect when the magnet is activated, so you will be able to tell which magnet is inoperative. I hope this makes sense and helps someone. Charlie

Don't ever tell a soldier that he doesn't understand the cost of war.

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Mntom, if you connect a wire to the ground and touch the 12 volt wire to it, it causes problems. The tester works great as I described it, and there is little chance of messing anything up. There is a complete circuit path with the ground wire left off of the tester. Charlie

Don't ever tell a soldier that he doesn't understand the cost of war.

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I think I see what you are doing. Here are a couple things that may clear up some confusion when I first read this. Please correct me if this is not right.

 

  • The "Alligator clip" is actually a12g test lead with alligator clip(s). Attach one end to a 12V source on the truck. Then you clip the other end of that to the desired wire(s) on the test plug.
  • The trailer is physically connected to the truck, which forms the ground path. Having a wire in the ground pin could indeed cause a short if connected to the clip lead.

Sure. Only comment I have is to make sure that the 12V source has a fuse on it. Don't run a heavy wire to the battery with no fuse....!

 

I like the tip about the compass...that really is a handy thought. Would have saved me all KINDS of time troubleshooting a flatbed trailer that the brakes didn't work. Spent a LOT of time trying to find a wiring problem. Finally disassembled the brakes and discovered the actuator arms were all frozen.

 

BTW, I have a ammeter that you just hold against a wire. It is not real accurate, but it gives you a general idea if the circuit is drawing current and was CHEAP.

No camper at present.

Way too many farm machines to maintain.

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I believe he meant using the 12v power from your trailer battery through the connector. If you touch that wire to each circuit, it would power the circuit and find its way back to the battery via each circuits ground. You could use an external power source, like another battery or the truck, but you need to make sure the a ground path exists to that battery.

 

Charlie's tester makes it easy to test without hooking up or lugging another battery around.

2007 Volvo 880

2014 Palomino Sabre 36qbok

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Hi Charlie,

 

This tool has been really useful for me.

What a great, informative post. Thanks for taking the time to write it! This will surely be my first DIY project for my planned 5thWheel, and by just reading it I have learned a lot already. I would like to ask some questions, if you don't mind:

 

 

It is made using the 7 way connector half with the door or cover on it [like the one attached to the truck]. You will need the connector half

You mean, like this one?

 

a wiring diagram for the connector half

This should be standard, no? I googled and found this, is it correct?

 

You can then test your trailer by plugging the tool into the trailer pigtail and clipping the alligator clip to the wire representing the function you want to test...or clip all the wires except the brake actuator wire to the alligator clip and walk around to verify all lights are on.

Brilliant. But this all will depend on the trailer battery being "live", right? What about trailers that have no battery of their own (as, I believe, cargo trailers)?

 

The brake actuator wire will activate the magnets on electric brake trailers and start the pump on electric over hydraulic trailers. You can also check the functionality of your brake magnets by using a DC clamp-on ammeter around the brake actuator wire when it is connected to the 12 volt alligator clipped wire....each magnet draws approximately 3 amps, so a +/- 12 amp reading on the ammeter means all 4 magnets are working, A +/- 9 amp reading means a magnet is not working. A compass held close to the wheel will deflect when the magnet is activated, so you will be able to tell which magnet is inoperative.

Another way to test it which just occurred to me is to lift the wheel to test with a bottle jack and give it a few turns, and then activate the brakes. It would have the added advantage of testing the brake's mechanical actuation.

 

Cheers,

--

Vall.

Getting ready to join the RV full-time lifestyle in 2017!

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Charlie showed us his trailer wiring tester at ECR and it took about a half a second to convince me of the value . . . my connector is on order. It can also double as an on-hand spare should your truck connector fail. Thanks Charlie.

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Jim & Wilma

2006 Travel Supreme 36RLQSO

2009 Volvo VNL730, D13, I-shift, ET, Herrin Hauler bed, "Ruby"

2017 Smart

Class of 2017

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I'm not Charlie, but I think I can answer your questions. See them embedded below:

 

You mean, like this one?

 

Yes


This should be standard, no? I googled and found this, is it correct?

Yes

Brilliant. But this all will depend on the trailer battery being "live", right? What about trailers that have no battery of their own (as, I believe, cargo trailers)?

Any trailer with electric actuated brakes has to have a battery on board to activate the brakes when the breakaway switch is pulled. This battery should supply enough power to use the tester. Of course this assumes the battery is not discharged or faulty for some reason.

Another way to test it which just occurred to me is to lift the wheel to test with a bottle jack and give it a few turns, and then activate the brakes. It would have the added advantage of testing the brake's mechanical actuation.

 

This would work as well, but obviously requires additional effort.

2000 Kenworth T2000 w/ Cummins N14 and autoshift
2017 DRV Mobile Suite 40KSSB4 with factory mods, dealer mods and personal mods - now in the RV graveyard
2022 DRV Full House MX450 with customized floor plan
2018 Polaris RZR Turbo S (fits in the garage)
2016 Smart Car (fits in the garage or gets flat towed behind the DRV when the RZR is in the garage)
My First Solar Install Thread
My Second Solar Install Thread & Photos and Documents Related to the build
My MX450's solar, battery and inverter system - my biggest system yet!

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Charlie showed us his trailer wiring tester at ECR and it took about a half a second to convince me of the value . . . my connector is on order. It can also double as an on-hand spare should your truck connector fail. Thanks Charlie.

Hmmmm. I already carry a spare connector, why not put it to use? Great ideas, to both of you.

KW T-680, POPEMOBILE
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Lots of old motorcycles, Moto Guzzi Griso and Spyder F3 currently in the front row
Young enough to play in the dirt as a retired farmer.
contact me at rickeieio1@comcast.net

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I think I see what you are doing. Here are a couple things that may clear up some confusion when I first read this. Please correct me if this is not right.

 

  • The "Alligator clip" is actually a12g test lead with alligator clip(s). Attach one end to a 12V source on the truck. Then you clip the other end of that to the desired wire(s) on the test plug.
  • The trailer is physically connected to the truck, which forms the ground path. Having a wire in the ground pin could indeed cause a short if connected to the clip lead.

Sure. Only comment I have is to make sure that the 12V source has a fuse on it. Don't run a heavy wire to the battery with no fuse....!

 

I like the tip about the compass...that really is a handy thought. Would have saved me all KINDS of time troubleshooting a flatbed trailer that the brakes didn't work. Spent a LOT of time trying to find a wiring problem. Finally disassembled the brakes and discovered the actuator arms were all frozen.

 

BTW, I have a ammeter that you just hold against a wire. It is not real accurate, but it gives you a general idea if the circuit is drawing current and was CHEAP.

Jeff, no truck is needed, you are using the trailer battery, however, the fuse is a good idea...just be sure it can handle the brake magnet load. Charlie

Don't ever tell a soldier that he doesn't understand the cost of war.

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Hi Charlie,

 

 

What a great, informative post. Thanks for taking the time to write it! This will surely be my first DIY project for my planned 5thWheel, and by just reading it I have learned a lot already. I would like to ask some questions, if you don't mind:

 

 

 

You mean, like this one?

 

 

This should be standard, no? I googled and found this, is it correct?

 

 

Brilliant. But this all will depend on the trailer battery being "live", right? What about trailers that have no battery of their own (as, I believe, cargo trailers)?

 

 

Another way to test it which just occurred to me is to lift the wheel to test with a bottle jack and give it a few turns, and then activate the brakes. It would have the added advantage of testing the brake's mechanical actuation.

 

Cheers,

--

Vall.

Vall, YEP, that's the correct part and NO, that is not the correct wiring diagram...look up 7 WAY RV CONNECTOR...it is different.. You are correct in that this is for battery equipped trailers. If you don't mind jacking up the wheels, you are correct, you will get a real live brake test. Let me know if you have any more questions. Charlie

Don't ever tell a soldier that he doesn't understand the cost of war.

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Jeff, no truck is needed, you are using the trailer battery, however, the fuse is a good idea...just be sure it can handle the brake magnet load. Charlie

 

Ah--that makes sense now...guess I got to many old trailers/farm implements with just lights to think about the battery on board. Yeah that is a great idea.

No camper at present.

Way too many farm machines to maintain.

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You are my hero Charlie...

Paul & Paula + Daisy the amazing wiggle worm dog...

2001 Volvo 770 Autoshift, Singled, w/ Aluminum Bed - Toy Draggin

2013 395AMP XLR Thunderbolt Toy Hauler

2013 Smart Passion

2012 CanAm Spyder RT

2013 Harley Davidson Street Glide

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This thread has me thinking about how most of us run switched power back to the trailer. It would be a little more expensive, but what about using a battery separator somewhere in the charging circuit (like this) to combine them? That way, when you're hooked up, the trailer's charging (whether solar, shore, or generator) would keep the trucks batteries charged, and with the truck running, the trailer is charged.

 

Just a thought...

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NUke: I thought about something like that, and it sure could be done. But the size of wire it is practical to run from thru the trailer plug and all the way between the two battery units limits the charging ability. 20' of 10g wire is only good for around 15A before the voltage drops off. To me, its just easier to run a drop cord from the trailer 120V outlet and plug in the truck trickle charger that is already in the battery compartment.

No camper at present.

Way too many farm machines to maintain.

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Charlie: Can I order that with the automatic trailer wiring repair option?

Thanks again for the presentation, great ideas and useful information. Dave

Yamaha V-Star
2007 Kenworth T2000 tandem aircutter
ThermoKing TriPack
2003 Holiday Rambler 36' Presidential
short trips-88 Allegro on Astro

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Yeah, Roger but that is "store bought". You know we would rather spend $250 on something home grown. :)

2006 Volvo 780 "Hoss" Volvo D12, 465hp, 1650 ft/lbs tq., ultrashift

Bed Build by "JW Morgan's Custom Welding"

2017 DRV 39DBRS3

2013 Smart Passion Coupe "Itty Bitty"

 

"Don't go around saying the world owes you a living. The world owes you nothing. It was here first!"

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I'm not Charlie, but I think I can answer your questions. See them embedded below:

 

I took too quick of a look at the trailer wiring diagram you linked. That is a commercial 7 pin with round pins. You want an RV 7 pin with flat pins like this one. Here is the trailer side.

 

You mean, like this one?

 

Yes

 

 

This should be standard, no? I googled and found this, is it correct?

 

Yes

Brilliant. But this all will depend on the trailer battery being "live", right? What about trailers that have no battery of their own (as, I believe, cargo trailers)?

 

Any trailer with electric actuated brakes has to have a battery on board to activate the brakes when the breakaway switch is pulled. This battery should supply enough power to use the tester. Of course this assumes the battery is not discharged or faulty for some reason.

 

Another way to test it which just occurred to me is to lift the wheel to test with a bottle jack and give it a few turns, and then activate the brakes. It would have the added advantage of testing the brake's mechanical actuation.

 

This would work as well, but obviously requires additional effort.

2000 Kenworth T2000 w/ Cummins N14 and autoshift
2017 DRV Mobile Suite 40KSSB4 with factory mods, dealer mods and personal mods - now in the RV graveyard
2022 DRV Full House MX450 with customized floor plan
2018 Polaris RZR Turbo S (fits in the garage)
2016 Smart Car (fits in the garage or gets flat towed behind the DRV when the RZR is in the garage)
My First Solar Install Thread
My Second Solar Install Thread & Photos and Documents Related to the build
My MX450's solar, battery and inverter system - my biggest system yet!

chadheiser.com      West Coast HDT Rally Website

event.png    

AZCACOIDIAKSMNMOMTNENVNMNDOKSDTNTXUTWYxlg.jpg

 

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Another way to test it which just occurred to me is to lift the wheel to test with a bottle jack and give it a few turns, and then activate the brakes. It would have the added advantage of testing the brake's mechanical actuation.

 

Cheers,

--

Vall.

 

 

That's how I do it. At start of each camping season, I inspect hubs, shoes and my NevRLubes and plug trailer into truck and have wife step on brakes and use slider on Maxbrake. Yeah, more work but makes me feel good about my setup.

2006 Volvo 780 "Hoss" Volvo D12, 465hp, 1650 ft/lbs tq., ultrashift

Bed Build by "JW Morgan's Custom Welding"

2017 DRV 39DBRS3

2013 Smart Passion Coupe "Itty Bitty"

 

"Don't go around saying the world owes you a living. The world owes you nothing. It was here first!"

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