Trailer Electrical Tester

**IntheGroove** · Feb 8, 2021, 09:51 PM

I needed a tester for working on electric parts on trailers. I didn't care for the commercially available units so I made one with parts I had on hand. Inside the box are two automatic-reset circuit breakers, one for the lights and one for the brakes. A 30 amp relay for the brakes. A solid state flasher. I mounted a voltmeter and a amp meter. The voltmeter will check the test battery or the battery on the trailer. The amp meter is for checking the brake coils.

It will test all electric functions of a trailer, Lights, Brakes and Battery
I made a 25 foot extension for the trailer that can also be used on the tow vehicle with the plug-in tester so I can test from the driver's seat or under the hood where the fuses and relays generally are.

Most trailers don't have backup lights put some trailers with surge brakes use a solenoid in reverse to stop the brakes from engaging, also will power-up backup cameras.

**DIYer** · Feb 9, 2021, 04:26 AM

Thanks IntheGroove! We've added your Trailer Circuit Tester to our Electrical category,
as well as to your builder page: IntheGroove's Homemade Tools. Your receipt:

Trailer Circuit Tester by IntheGroove

tags: tester, electrical

**hemmjo** · Feb 10, 2021, 03:31 AM

It is nice to have all the testing required in one package.

**jott** · Feb 10, 2021, 08:01 AM

Would you share the schematic?

**IntheGroove** · Feb 10, 2021, 10:50 AM

I'll try to draw a schematic but it might take longer to do than it did to build the thing...

**odd one** · Feb 11, 2021, 03:20 PM

Very nice. There are commercial units out there like that, but they are big bucks.

**mccwho** · Jan 8, 2026, 10:49 AM

Originally Posted by odd one

Very nice. There are commercial units out there like that, but they are big bucks.

That's a fantastic job. All that work is a lot of time spent figuring all that out. We would be greatly appreciative of schematics when you have time to do it. That would be a big help!