Arrives before Christmas 
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The Hopkins Tow Doctor Trailer End Test Unit is your all-in-one solution for quickly diagnosing trailer electrical systems. No more hooking up trucks just to check wiring! Simply connect this tester to any 12-volt battery, and it simulates tow vehicle power across a range of connectors including 4-Flat, 6-Pole Round, and 7-Way RV Blade. Whether it's for routine inspections or troubleshooting, this unit saves you time and effort with its comprehensive testing capabilities, enabling single-person operation.
Identifying trailer lighting issues becomes a breeze with the built-in LED bulbs on the tester that illuminate to show which functions are operating. This visual cue not only confirms the status of right turns, left turns, brakes, tails, and backup lights but also demystifies any electric brake or auxiliary power concerns. It's an essential feature for peace of mind and safety, especially if you're managing multiple trailers.
Never worry about electrical overloads or undetected draw issues again. The integrated amperage meter accurately displays the actual amp draw from your trailer's wiring, equipping you with precise information for diagnosing performance anomalies. While amperage won't display for LED-light trailers, the functionality supports any standard 12-Volt battery setup, making it adaptable for varied testing conditions.
The package includes battery terminal clips and a suite of adapters, enhancing the device's portability and ease of use across different trailer types. It doesn't just test the trailer circuits; it does so with seamless switching between connector types. This is ideal for situations where you're handling diverse trailer configurations, delivering adaptability without loss of function.
Convenience meets mobility with this device's ability to operate from a simple car battery setup. With portability at its core, you can perform vital diagnostics wherever your trailer is parked. This design feature frees you from dependency on vehicle power, transforming trailer testing into a straightforward, one-man task, perfect for those who frequently manage fleet operations or travel setups.
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Videos are provided as a guide only. Refer to manufacturer installation instructions and specs for complete information.
Today we're going to be taking a look at the Hopkins Tow Doctor trailer end test unit, part number HM50928. This Hopkins Tow Doctor allows one person to check their trailer wiring and make sure all their functions are working correctly. It's going to come with adapters to work with your seven-way, your four-pole flat, and your six round. It's got your right turn brake, your left turn brake, your tail lights, your backup light, your brakes, and even auxiliary power. If the green light appears, the circuit checks out okay. There are no missed wire connections, burnt out lamps, or trailer shorts to the ground. The LED screen shows your amperage, but it'll also show you your overload, a short circuit, and your low battery.
The included terminal clips allow you to connect it to a power source such as a car battery, and the cable is about eight foot long and is fuse protected. The Tow Doctor module itself features rubber bumpers around the ends here so it can withstand a beating. The whole Tow Doctor system is great for someone who has a lot of trailers that are constantly on the road to where they need to check them quick and easy. Now that we've got all the features, I'll go ahead and show you how to use it. I'm right here next to my trailer, let's go ahead and open up our case. Our trailer here has two different connections, because its owner tows with two different vehicles.
We've got a seven-way here, and then we've got a four-pole flat connection. We could look to our adapters in our case here, we can test either the seven-way or the four-pole, but since our Tow Doctor will test our brakes too, let's go ahead and test the seven-way. After choosing our adapter, we can go ahead and plug it in to our Tow Doctor. That's just going to be at the top here. What we'll need to do is line up the grooves, or the teeth with these slots on top, and then screw it in place. Just like that.
From there we can go ahead and plug in our seven-way. The clip here is going to be for our ground, but we won't be needing it because our trailer wiring is already grounded. But if we're to need it, we would just attach it to a non-painted surface on to our trailer. Now let's hook it up to a power supply. We've got the two alligator clips here, got our positive and negative, so we can hook this up to a 12-volt or greater power supply. You can use a car battery if you'd like.
We're going to be using a jump box, so our red is positive, goes to the red connection, and our black to the black connection. That's going to be negative. Now we can run through the functions. I'll press the right brake button and without even checking my light, I know that it's wired correctly and that it's working correctly. For one, it's green, and it's showing the correct amps. It's not showing an OL for overload, or SC for short circuit or LB for a low battery. Now I can just run through and check them all. As you can see with the right brake engaged, the right brake light is on. Then we could do left, that comes on, and then we click on tail light, the whole rear of the trailer lights up. If all of your functions are working properly and you don't need to repair anything, you're all set. That's going to complete our look at the Hopkins Tow Doctor trailer end test unit, part number HM50928.
Worked as expected and has been very us eful.
Nice tool for testing trailer wiring.
Good tool. Fast shipping.
Works well to let you know if you've wired or are wired correctly. Big time saver. No more hours tracking down what the problem is. This device tells you when a circuit has no connection.
I have 4 trucks that tow trailers everyday, we have been trouble shooting the wiring for months. This tool has made solving all my electrical problems so easy!
Great tool
Bruce
Just what i needed!
Thank you!
Bought this item and also the trailer circuit tester. Both work great and save tons of time checking lights and brakes for the company fleet.
Was delivered last night. It was well-packaged. Will be trying the tester on a 1999 F250 when I get a chance.
What a great product. After having multiple trailers and electric problems pop up, I found this test unit to test them quickly. It’s also very well designed and made.
Great product for exactly what I needed and ready to use.
Wish I was able to buy individual connectors. We are constantly switching between the seven round and the flat four this is our second tow doctor trailer end test unit that we have purchased. The only reason I had to purchase a new one is the wires broke too close to the plug where they could not be fixed. Wish I could just buy a new plug instead of a whole new unit
Though I don't do a whole lot of trailer work, every time that I've hooked this up to test a trailer's wiring, it worked very well. I never had it malfunction. The fact that it gives the amperage draw of each circuit and it's energized is very useful for diagnosis. Seems solidly made. I recommend this tester.
This is an essential tool for anyone working with large numbers of trailers. As a trailer dealer, we often encounter situations where we need to test trailer lights, or demonstrate the lights are working correctly. This tool has saved us countless headaches. We highly recommend!
Excellent product!! I own a new vehicle and had problems with the 7 pin trailer wiring from the vehicle. The car manufacturer blamed my trailer for the problems. After purchasing this machine and running thru the series of test's I was able to prove that the car dealer was incorrect in the diagnosis (which i was suspicious of to begin with) and that it was a bad electrical system in the vehicle... Not my trailer. Thank you etrailer for the excellent product, insanely fast shipping. The you tube video's of this product showing all the in's and out of how it works and it's effective use were also greatly appreciated. I am Canadian, and will be purchasing more great priced, products from your site. Thank you very much!
I love it. No more hooking up truck to check wiring on trailers. One man job!!!!!
Bought this item to check trailers and the vehicle tester too. Both work great and save me tons of time checking trailer circuits on our fleet.
It seemed pricy but my son and I have a lot of trailers to deal with so figured it was worth it. Tested one trailer...worked well. Agree with other reviewer.....nice if it had a battery.
Excellent investment, has save money and time. Recommend to any shop of trailer owners. Hassle free, and portable.
The service was superb I did an exchange and the folks there made it so easy and I received the correct one (i miss ordered ) very promptly I will continue to do business with etrailer
Thank You
Kevin D
E trailer is great got my order very quickly and the tow doc makes my life doing inspections a lot easier.
Great unit. Having. 12v battery included would make it a complete package, but it's not difficult to source one either.
What a nice tool....just what I was hoping for. I tried it out it worked great. It is built well, with its own nice case and is simple to use.........
Lynn
2/22/2020
Still like the product, the only improvement would be to have vehicle specific connectors so that you could have a no wire cut install..... I have had some wire breakages on the vehicle side with the small led wiring of the pickup, If I had had a spilt connector with the vehicle specific connectors it would eliminate this problem
Lynn
2/23/2020
Still liking it, really good tool
Easy to use. All the adapters included.
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Nate
5/13/2025
I own an equipment rental company, and the individuals that I hire dont always have the training off the bat to troubleshoot wiring issues. This product is still going strong and has made troubleshooting wiring issues a breeze. Its easy enough to use for most of the newbies to pick it up pretty quick.