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Topic Review (Newest First)
01-29-2012 03:11 PM
Stale Trooper Like many others, I think you're making a mountain out of a molehill. I have built 2 testers; one for my camper which utilizes the on-board battery of the camper to run the lights via the 7 pin RV plug. I put my back up lights on the center pin. In the attached drawing, I mention a single pole-double throw switch, I used the same type switch, but these are spring loaded momentary switches; either type will work, it's a matter of choice, cost, and availability. I had the switches already, so thats what I used.

The other was a simple light tester with small clearance lights that act as my camper's tail/running lights & turn/brake lights. I mounted the clearance lights on an old metal license type plate, with a pigtail long enough to let it hang on the inside of my tail gate so I can see the lights working as I push the brakes etc. The black wires you see running to each light are the power wires from the 7 pin RV Plug; the lights are grounded through the metal plate. You may notice a bulb for the trailer brakes. This won't work! The signal to the brakes is different, and I don't know how to make that part function.

These tools might have cost me $10-15 a few years ago, but I would guess them at $50-60 for the pair today.

I issue the usual disclaimer, I'm not responsible if you burn your fingers, or torch your vehicle due to your inability to work with DC current, blah blah blah.
12-30-2011 06:56 AM
EOD Guy Thought about it and my tester isn't what you asked I drew up one..... It will allow you to plug the tester into a vehicle with a 4,5,6 or 7 pin connector, and the LEDs (could be swapped out for lights) will let you know if the vehicle is providing juice to all the circuits. Leaving the box connected to the vehicle and then plugging the trailer into the box will allow you trouble shoot the trailer while verifing the vehicle is providing juice.

Hooking the alligator clips (I'd use jumper cables) to a 12v source and Gnd, will allow you to trouble shoot the trailer without the vehicle and the LEDs will verify there is pwr going down the line etc....

Under no circumstances should you hook the alligator clips up while the box is hooked to the vehicle...... or you might invent smoke

hmmmmmmm....... I think I'm going to scrap my old one
12-29-2011 07:36 PM
EOD Guy Here's how I'd do it
12-29-2011 07:13 AM
Originally Posted by 1990chevytruck
Hey I work in the trailer industry as a design Engineer and I can tell you there is a lot easier way to do this than a hand truck. unless you are going to be testing 30 to 40 trailers a day you can build one like the ones Croft trailer supplies make in a tool box and use a break a way battary to run the entire thing. Here is a link to see what Crofts box look like
we use this type of box on almost every trailer we make. this system is set up with a in line fuse for each companant you will test. they are cheaper than the circut breakers if you are only going to be using it every once in a while. they mount a 4 way, 6 way and a 7 way plug in the end of thiers with a ground plate on the backside of them. I hope this helps. If you would like me to go into greater details I can just buzz me on here and let me know!

Mike Edwards
Big Tex Trailers

Sticker shock. Not that i would have much less in my idea though.

The way i see it is that we will be using this very often. And as i stated, its going to be used for testing much more than trailer plugs. Being able to test weather pack connectors will increase its use substantially as we are always dealing with them.

Im ditching the idea of an ammeter for outgoing power, but if its cheap enough would still probably put a voltmeter on just so I know where the battery is at.

Still wondering what to do on the incoming power though. Do i just send every wire to the gauge itself from each circuit? For some reason that doesnt seem like it would work but again i dont know electrical theory very well.

Ive also contemplated the idea of adding leads to use for testing starters.
12-28-2011 12:08 PM
Trailer Tester

Hey I work in the trailer industry as a design Engineer and I can tell you there is a lot easier way to do this than a hand truck. unless you are going to be testing 30 to 40 trailers a day you can build one like the ones Croft trailer supplies make in a tool box and use a break a way battary to run the entire thing. Here is a link to see what Crofts box look like
we use this type of box on almost every trailer we make. this system is set up with a in line fuse for each companant you will test. they are cheaper than the circut breakers if you are only going to be using it every once in a while. they mount a 4 way, 6 way and a 7 way plug in the end of thiers with a ground plate on the backside of them. I hope this helps. If you would like me to go into greater details I can just buzz me on here and let me know!

Mike Edwards
Big Tex Trailers
12-28-2011 08:59 AM
bulletc5 you would want to wire the amp gauge on the incoming power
12-26-2011 06:11 PM
Durallymax Thanks for the advice thus far.

75gmck25 hit it on the head, easy troubleshooting.

This is going to be more than just testing a trailer plug. We will use it for testing other 12to volt circuits as well. And we work on them a lot so a good easy to use one is what we want. The one time the over complexity helps, it paid for itself.

Thanks for the help thus far.
12-26-2011 05:02 PM
Test box

[IMG][/IMG][IMG]This is a test box I made when I was working on trailers regularly. I made a long set of leads that plug into the power input jacks and have alligator clips on the ends so I can get power anywhere. The fuse is inline with the power switch. The switches send power out to each individual circuit. With the switches in the off position, you can connect an ohmeter at the banana jacks and check the resistance of the circuits to check for opens and shorts.
It may be overkill, but this thing paid for itself many times over.
12-26-2011 12:21 PM
75gmck25 I understand what you want to do. It might be a little bit of overkill, but I can understand the appeal of easy troubleshooting.

Initial setup
- First use one of these and connect the input to 12 volts fused line (I would use about a 50 amp fuse and holder) from a small battery home alarm battery, or motorcycle battery). This will give you six 12 volt fused lines.

- Run a wire from each output terminal on the fuse panel up to a toggle switch, so you can turn the power on and off. Now you have 6 lines of fused, switched 12 volts.

-Then install a 8 line terminal strip like this and use a short wire to connect six of the terminals to the lines coming from each toggle switch.

- Skip one terminal (leave it open), and use the last one to run a ground wire to the battery ground.

Now you have a terminal strip with a set of six fused and switched outputs, and a ground point. Use that terminal strip to connect wires running to each type of trailer connector you want to simulate. You can connect any combination of trailer connectors, and use the toggle switches to put voltage on any of the lines. You could mount the trailer connectors on the side of the test box, or just leave them with short pigtails that run out of the test box.

I would not worry about measuring amperage and voltage unless you think its a problem. You could put an amp gauge inline with any of the lines between toggle switch and the terminal strip, but they would be full-flow amp gauges. I don't think its worth the trouble. You could put one 12 volt meter between the two battery terminals and measure the overall voltage. If you need to see the voltage to any specific output, just measure between the ground on the terminal strip and the output terminal.

12-26-2011 10:57 AM
matts37chev you are going to need a hand truck to move all of that
sounds way too complicated, to have all in one
i would just make multiple testers, one for each kind of system you have
and have a few adapters to inter change a few of them

all of the 7 wire testers i have used were very simple

male plug, on a cord, on one side that went to the truck
wired into a row of labeled lights (so you can see what circuits are doing what)

below that was a row of switches, with 12v power from a small breaker, on one side
(from the other side of the breaker, a small cord with alligators, to hook to any handy batt., supplied the 12v)

the other side of the 12v powered switches, went through another row of lights
(so you see that it was sending out power)
from the lights, they were hooked to a female socket to plug the trailer into

I am sure you could make a more technically advanced one
but i have never needed it

because if you have a problem, you have all your testers out any way, to find and fix the problem
12-26-2011 10:25 AM
DanielC Here is my trailer light test box.

The battery came from an interruptible power supply for a computer. The flat four trailer connector was from an boat trailer I put disk brakes on, and the disk brakes require a back up light connection to disable the brakes, or you could not back the trailer.
I just unplug the spade connector from the battery, to test right turn, left turn, and tail/marker lights.

I also built another version of a test harness, for a boat dealer I used to work for, using a rotary switch. Later, somebody else added a turn signal flasher to it.

To be honest, you are proposing a really complicated solution to an easy problem.

You need two devices. A battery connected to a male socket, to take to the hitch end of the trailer, to check the trailer lights. Use a switch to each function up, or just unplug each lead like I do.

The other device is a hand held trailer simulator. A box with a female plug, and lead, you can plug into the tow vehicle.
Put small lights on it for each function, or a rotary switch, to hook each function individually to a single light.

Everything is 12 volts, isn't it? You do not really need to check the voltage. It is there, or it is not.
I would suggest you put some sort of two pin plug, that has a wire jumper, that you could unplug, and then hook an Ammeter to the two pin plug. A problem I see is that an Ammeter that could read the largest load, like all the marker and taillights on a 18 wheel trailer, would be reading a very small number if you are only testing a single LED light. leave the ammeter in the testing small load position, and you damage the ammeter, or blow its fuse.
12-26-2011 10:24 AM
OneMoreTime Sounds to me like this has been over complicated for my ole brain..All i did was to use a standard fuse panel with the flasher circuits in it and wired it to a trailer plug and had a couple of leads that went to the battery with alligator clips like the ones on jumper cables..Worked for me. I also had a four pin connector on it so I could check lites on little trailers as well. this all went in a small toolbox.

12-26-2011 09:33 AM
T-bucket23 I would have a main circuit breaker and an individual one on each circuit you are testing. You will almost be setting up the same way as a household circuit panel is connected. You need to be real careful on the battery side, especially if you are using a full size automotive battery. They are capable of producing a lot of current. If it were me I would be more apt to use a smaller battery like you would find in an alarm box. They are still 12V but only produce about 8-10 amps. They are a lot smaller and a lot safer and will allow the uses of smaller wire and lower current rated parts.
12-26-2011 08:58 AM
Building Trailer Wire Test Box Couple Questions.

We have been wanting to build a trailer wire test box for awhile for the shop here on the farm.

This winter we are a little more caught up and finally have some time to get some of these projects done.

As of now we are going to be building it off of a hand truck so that it is easily portable.

The basic concept is that there will be two female 7 round pin (SAE wiring) connectors mounted in the front of the electrical box we will be making. We will then have two pig tails hanging on each side that can get plugged into each of the two connects or plugged together with an adapter for more reach.

The pig tails are one idea because we have some extras. I dont like the idea of fighting pig tails though. It would be ****** to have two reels on each side with about 10-20' of 7 wire wrapped in them. if anybody knows where i can get cheap reels let me know.

But anyways each cord will have the round pin 7 pin plug on the end of it.

We will then be making adapters for every style of plug both male and female that will be only the length of the plugs plus a couple inches to allow a little flex if needed. These will all be stored in cubbies below the switch box and labeled to suit. At first we will just make all of the different trailer plugs but may add some weatherpack adapters as well later on.

We will have all of the charts and diagrams laminated and on a ring sitting in a slot between the cubbies and the switch box.

A decent size 12 volt battery will be sitting on the base of the hand truck with a battery tender mounted to it so we can plug it in when not in use.

The reason for the two plugs in the switch box is for incoming power and outgoing power. We want to be able to power up trailers, and also just check the other equipment (semis tractors pickups etc.) And we also would like the ability to quickly check pigtail cords.

Now heres where my basic wiring questions come into play.

We want to be able to have a switch for each wire that will be sending power out. We will be labeling what each switch is powering and we were going to have a light for each switch going out but now I have decided it would be better to add a voltmeter and ammeter so you truly know whats going out which would mean we would not need the lights anymore really.

Now on the power coming in side we would still use lights which would be labeled just like the switches, but i would like to also run them through a volt meter and ammeter.

Furthermore I am sure i will need to have this system wired with circuit breakers as well to protect it, the vehicle, trailer and also help with diagnostics.

Essentially what I am looking for is how to wire everything inside the box. I can do good wiring and make everything but the concepts of it still confuse my brain as i havent really worked to try and understand it too much.

Starting with the power going out side.

Here is what I am thinking, the power goes to the switch to the voltmeter to the ammeter then out through the plug.

Now here is my question, where do I put the circuit breaker in this system? I know i need to protect the lowest rated items but if im thinking correctly i want to be testing the trailer system at its full capacity. The Ammeter should be fine im assuming, i cant seem to find what some of the volt meters are rated for though. And If i use a toggle switch rated higher than the trailer i should be fine there too correct? So then should I be putting the circuit breaker before the switch or just before it goes into the plug? Or do i need it just before the plug and then a fuse between the battery and switch?

Now on the incoming power side, I want to power the light and both gauges. So Should the circuit breaker be the first thing the power goes through?

Now as for the gauges. How do I splice them into each of the 7 wires while still keeping each circuit isolated from each other?

Now for the supplies.

I would like to use digital gauges. Doesnt matter the style or type really just would like digital readouts. Anybody know which ones are good but still affordable?

Circuit breakers? I want ones that are each to reset like the little white push button ones. Am i on the right track or what else should I be doing?

If you can think of anything else im missing let me know.

Thank you much.

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