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Circuit Testing a Vehicle 4-Pole

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Circuit Testing a Vehicle 4-Pole Plug

Today Im going to show you how to test the functionality of a trailer four-way connectors. What youre going to need to do this is a circuit tester and a buddy in the drivers seat to operate the taillight functions. The first thing were going to test is the taillights themselves. Hit the taillights. (beeping) that means thats working. The next thing were going to test is the left blinker.

(beeping) That works. Then were going to test the brake light function on that side as well. So, hit the brakes. (beeping) That works. Then for the right blinker (beeping).

Now the right brake (beeping). Now we know that this four-way connector is functioning.

Daniel S.


Thank You!!! fuse was blown but it made me in the begining clean neg connection then check fuse...wallah....done...

Etrailer Expert

Jon G.


Happy to hear that you got it taken care of!



I just installed the Allen’s tailgate led light bar. I have brake, right/ left signal and hazard lights. The only function not working is the running lamps when the lights are on at night, they are off unless I brake or signal. I checked for blown fuses and all were good. I checked the trailer park fuse in my 2002 f150 and it was fine but I replaced it with a new one anyway and still no luck. Everything works except for the running light mode. What gives? Maybe a loose ground? Where do I find that?

Etrailer Expert

Jon G.


Try testing just the connector on your F-150 with a circuit tester like in the video. This will help you determine if the problem is actually on your truck or if it's on the light bar.



What if the brake lights function but the turn signals dont? 2004 Chevy Blazer. Tail lights and brake lights working properly but turn signals are not.

Patrick B.


If it is a factory original trailer connector, check to see if any fuses or relays have blown on the power block underneath the hood. You can also check the ground connection on the trailer. If there is not a sufficient ground, it may not support the larger, intermittent electrical load of a turn signal. Rust, corrosion or paint can affect a proper ground connection. If it is plug-in aftermarket trailer wiring system on the Blazer, let me know the part number if you can.



@PatrickB how do I find the ground wire? What am I looking for?
Etrailer Expert

Jon G.


@Kenny Typically the ground wire will be white and will be connected to the vehicle/trailer frame using a ring terminal and self-tapping screw. I'm attaching some links to helpful articles about 4-Way and 7-Way connectors that you can check out to help you trace the ground wire.

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