bing tracking image
  1. Electrical Tools
  2. Hopkins
  3. Testers
  4. Tow Function Tester
Hopkins Tow Doctor Trailer End Test Unit

Hopkins Tow Doctor Trailer End Test Unit

Item # HM50928
Our Price: $443.84
You Save: $193.85
Electrical Tools
Shipping Weight: 5 lbs
Add to cart
In Stock
zoom in icon
slider vertical icon
zoom out icon
Comment field cannot be left empty.
Name field cannot be left empty.

Thank you! Your comment has been submitted successfully. You should be able to view your question/comment here within a few days.

Error submitting comment. Please try again momentarily.

Product Images

In Use/Installed

Need help with installation?
Locate installers near me
1-800-940-8924 to order Hopkins electrical tools part number HM50928 or order online at Free expert support on all Hopkins products. Great prices and Fastest Shipping for Hopkins Tow Doctor Trailer End Test Unit. Electrical Tools reviews from real customers.
  • All Info
  • Reviews (29)
  • Q & A (0)
  • Videos (2)
  • Photos

Hopkins Electrical Tools - HM50928

  • Testers
  • Tow Function Tester
  • Hopkins


  • Allows one person to check that trailer wiring and functions are working correctly
  • Tests all trailer functions: right turn, left turn, brake, tail, backup lights; electric brakes and auxiliary power
  • Trailer lights illuminate and LED bulbs light on the tester to show function operation
  • Adapts to 4-Flat, 6-Pole Round, or 7-Way RV Blade
  • Includes battery terminal clips
  • Amperage meter shows actual amp draw
  • For use with any 12-Volt battery
  • For electric brakes, magnets are activated to determine correct operation
  • 1-Year warranty

Note: On trailers with LED lights, amperage will not display.

50928 Hopkins Tow Doctor Trailer End Test Unit

Installation Details HM50928 Installation instructions

California residents: click here

Video of Hopkins Tow Doctor Trailer End Test Unit

Videos are provided as a guide only. Refer to manufacturer installation instructions and specs for complete information.

Video Transcript for Hopkins Tow Doctor Trailer End Test Unit Review

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.

Customer Satisfaction Score:

Customer Reviews

Hopkins Tow Doctor Trailer End Test Unit - HM50928

Average Customer Rating:  4.8 out of 5 stars   (29 Customer Reviews)


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!


I ordered this one an the truck one this one is missing the 7 way plug So just make sure you get your parts

Etrailer Expert

Jenny N.


Oh no! I will have our Customer Service team reach out to you. We will have this taken care of for you.


Easy to use. All the adapters included.


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.


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!


Has worked well in our production setti ng.


I love it. No more hooking up truck to check wiring on trailers. One man job!!!!!


Item is nice to have on hand for a single person to check trailers as they come in, and have them ready to go back out. Fairly easy to use. a magnet on the back would be ideal to hold to trailer tongue while performing tests.


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.


Great unit. Having. 12v battery included would make it a complete package, but it's not difficult to source one either.


Having had the product for a year now it has proven to be very useful. We can do diagnostics with out having the trucks hooked up! Great product!


Excellent investment, has save money and time. Recommend to any shop of trailer owners. Hassle free, and portable.


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.


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.........



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



Still liking it, really good tool


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.




I have used this tool many many times to diagnose trailer lights. It is fantastic.


Improved design from the original design product, easy to use, excellent product


Product received as ordered and very timely. Great service, would highly recommend.


Product was as described and worked as it was designed..


just received my package. Excellent communication throughout the transaction. Item is as described. Will definitely do business again.


Works very good


great product. appreciate the discount compared to direct purchase.
great packaging and fast shipment.
easy to order.


So far so good. Just received the package today.


Show More Reviews

See what our Experts say about this Hopkins Electrical Tools

  • Availability of 7-Way Circuit Tester that Trailer 7-Way Plugs In To
    We do have the type of trailer wiring tester you are looking for with the Hopkins Tow Doctor part # HM50928. You can plug a trailer 7-way into this and it will simulate a tow vehicle to let you know if all of the wiring is good.
    view full answer...
  • Base Functions of the Hopkins Tow Doctor
    The Hopkins Tow Doctor, part # HM50928 is used to test a trailer's brake and light wiring. It shows if there is a short circuit, overloaded circuit or low battery. You'll need to use a 12 volt or higher powered battery to correctly output a test signal. You can even use a jump box to make the entire kit superbly portable. The kit will test the tail lights, the left and right turn signal and the brake lights, plus any running lights with a simple button press. It also provides a readout...
    view full answer...
  • 7-Way Trailer End Tester Which Operates Off Battery Power
    The Hopkins Tow Doctor Trailer End Test Unit # HM50928 is our only trailer end tester. This will connect to a small 12 Volt battery with the included harness.
    view full answer...
  • How to Test the Trailer Light Functions of a Trailer with a 4-Way
    There is, but I will warn you it's pretty expensive. The Hopkins Tow Doctor Trailer End Test Unit # HM50928 is designed to test the connector on a trailer for function and would be perfect for what you are trying to do. Check out the review video I attached. A much more inexpensive option would be to use a jumper wire from a car battery's positive post to put voltage on all of the pins of your trailer's connector. You would need to test the three male pins next to the female pin. The...
    view full answer...
  • 7-Way Tester for Testing Trailer Side 7-Way
    We have the Tekonsha 7-way tester part # 8010 that can test a trailer's wiring as well as Hopkins Tow Doctor Trailer End Test Unit # HM50928. Due to the nature of testing the trailer side of the wiring these kits cost a bit more than the testers for the vehicle.
    view full answer...
  • Availability of 7-Way Tester for Trailer Without Tow Vehicle.
    We have one tester available that will let you test your trailer lights without the tow vehicle. That is the Hopkins Tow Doctor Trailer End Test Unit # HM50928. This tester can be used with any 12V battery and will illuminate your trailer lights and test all functions. I have attached a video review that shows how the tester works for your reference.
    view full answer...
  • Wiring Tester Kit for Both Vehicle Side and Trailer Side Connectors
    I do have options for you but we do not have an all inclusive tester kit that will test flat and round trailer connectors on both the trailer and vehicle. Instead you will want to use part # HM50928 for the trailer end and part # HM50918 for the vehicle end. Both kits have the ability to test 4-flat, 6-round, and 7-way connectors. I have attached a review video on both the vehicle side and trailer side tester kit for you as well.
    view full answer...
  • Trailer 7-Way Connector Tester Recommendation
    We do have the Hopkins Trailer Connector tester part # HM50928 which will allow you to plug your trailer's 7-way into it and test all of the circuits easily. I attached a review video for this product for you to check out as well.
    view full answer...
  • Does the Tow Doctor Trailer Side Tester Work on a Trailer with LED Lights
    The Tow Doctor for the trailer side is # HM50928. The other one, # HM50918, is the vehicle side. For the trailer side model it does not work if the trailer has LED lights. LED lights draw too little power for the tester to work properly. You could test the trailer side by using a separate 12 volt wire and applying power to each function. If the function does not work doing this test you know there is a wiring issue.
    view full answer...
  • Availability of 7-Way Tester for Trailer Side of Wiring
    We do have a tester for the trailer side of the wiring with the part # HM50928. You'd plug your 7-way into this and it will tell you if all if all of the functions are good.
    view full answer...
  • Hi, I have a 2014 f150 and I have tested the 7 pin
    There are many things it could be, however to really narrow it down you are going to need to use a circuit tester # PTW2993 and go over all your trailer-side wiring since you've already mentioned you went through the vehicle side and everything looked good. You can start with any function when you begin to test and it would help if you had someone else with you to sit in the tow vehicle and engage each function while you test the wires. You're going to also want to check all of your...
    view full answer...
  • How to Determine Functions of Trailer Connectors
    The function of trailer connectors are often similar, as evidenced by the accompanying diagram which shows the functions of 7-ways. It really depends on how the connector is wired; that said, there is an easy way for you to determine the functions of a connector. All you need is the Hopkins Tow Doctor Trailer End Test Unit # HM50928 in order to test the trailer-side 7-way. I have also added a link to a help article on trailer wiring which I strongly recommend checking out.
    view full answer...
    Image 1 for
  • Trailer Tester To Test Trailer Wiring Without Plugging Into Tow Vehicle
    The Hopkins Tow Doctor Trailer End Test Unit # HM50928 will allow you to test a trailer with 4-way flat, 6-pin round, or 7-way blade connectors. The test will connect to a 12-volt battery and allow you to test all your lights as well as trailer brake operation and the charge line connection.
    view full answer...
  • What Size Fuse Does The Hopkins Tow Doctor Trailer End Test Unit # HM50928 Use?
    The Hopkins Tow Doctor Trailer End Test Unit # HM50928 uses a 30amp AGC fuse. This fuse is rated for up to 32volts. We currently only carry the AGC Fuses # PT92FR in a multiple package. This will include 10 of the 30-amp fuses.
    view full answer...
  • Equipment for Testing Trailer Wiring and Brake Magnets
    There is not a specific product we carry which tests electric brakes, but if you are looking for something that will test trailer brake magnets you would want to use Multimeter # ALL648349; I have added a help article on testing brake magnets for you to take a look at. You may be interested in the Hopkins Tow Doctor Trailer End Test Unit # HM50928. This kit will allow you to test all trailer wiring functions with adapters that will work with 7-way, 4-flat and 6-round. It will even indicate...
    view full answer...
  • How to Power Trailer Taillights Without Being Connected to a Tow Vehicle.
    The best option I can give you would be to bring with you a 12 volt car battery and run jumper wires from each of the circuits of the trailer connector to the positive side of the battery. Or you could use the # HM50928 in conjunction with the battery to allow you to easily control the power to each of the circuits. This would make things easier, but it is a costly option. To keep the battery charged you would need a battery charger like the part # CTEK56353.
    view full answer...
    Image 1 for
  • Electric Trailer Brake Light Does Not Light Up When Testing with a Hopkins Tow Doctor Tester
    The replacement unit, # 4010509028 is a replacement for Tow Doctor part # HM50928. It looks like the Tow Doctor version you ordered is # HM50918. The difference is the HM50928 has more features. To test the brake controller output someone will need to sit in the truck and apply the manual override so that it is sending power back to the trailer connector. Otherwise, what will happen is what you are seeing, a slight flicker of light. That is the brake controller checking to see if a trailer...
    view full answer...
  • Trailer Wiring Circuit Tester Comparison
    Both the Tekonsha Trailer Wiring Circuit Tester # 8010 and the Hopkins Tow Doctor Trailer End Test Unit # HM50928 are for testing the wiring of your trailer. Both will work with incandescent and LED lights. However, the Hopkins will not display amperage for LED lights. They both connect the same by plugging your trailer's wiring harness into the unit. The Hopkins includes connectors for a 7-blade, a 4-pole, and 6-pole round. The Tekonsha has the 7-blade standard. If you need a different...
    view full answer...
  • Troubleshooting A 7-Way Trailer Connector With A 4-Way Wiring Harness
    In order to help diagnose the problem, I would first need you to check that both trailer connectors are properly wired. You can do this by using a circuit tester like part # PTW2993. You will then need to have a friend sit in the vehicle and perform each of the trailer connectors functions while testing the corresponding wires for each circuit on the back of the vehicle's 7-way connector both before and after the plug. If you have power to all wires before the connector and not after,...
    view full answer...
  • What to Use to Test 7-Way Trailer Connector
    To test a 7-way trailer connector on the rear of your vehicle you can use the Pollak Tester # PK12741. This tester is designed to work with all 7-way connectors. Another option is to use a circuit tester like # PTW2993 and go come pin to pin testing each individually. If you're trying to test the trailer side connector it's a little different. A disconnected trailer doesn't have a power source so you will need something to provide it. The Hopkins Tow Doctor # HM50928 and Tekonsha Circuit...
    view full answer...
  • Does the Hopkins Tow Doctor Trailer End Test Unit Test Both the Trailer and Vehicle Side
    No, the Hopkins Tow Doctor Trailer End Test Unit # HM50928 does not work for both the vehicle and trailer end. The test unit is basically simulating the power from the vehicle and the adapters included allow for the trailer plug to hook up, but not the vehicle as well since it is simulating the vehicle itself. To test the vehicle end, you can use the Pollak 7-Way to RV Socket LED Circuit Tester # PK12741 for a vehicle 7-way or the Brophy Circuit Tester for 4-Pole Flat Trailer Connector...
    view full answer...

Do you have a question about this Electrical Tool?

Info for this part was:

Employee Jameson C
Expert Research:
Jameson C
Employee Joshua S
Video Edited:
Joshua S
Employee Dustin K
Video Edited:
Dustin K
Employee Rick A
Installed by:
Rick A
Employee Hayden R
Installed by:
Hayden R
Employee Patrick B
Video by:
Patrick B
Employee Chris R
Video by:
Chris R
Employee Wilson B
Updated by:
Wilson B
Employee Alexander C
Updated by:
Alexander C

At we provide the best information available about the products we sell. We take the quality of our information seriously so that you can get the right part the first time. Let us know if anything is missing or if you have any questions.