View Full Version : Trailer Keeps locking up in reverse

06-11-2011, 07:54 AM
Well 3 season with boat.. it stays at my house. I pull it out get to boat ramp and brakes lock up in reverse.. I quickly thought oh I forgot to plug trailer or it came loose. No such luck.
One of the plugs had a little road rash.. so thought ok will replace. Well new male plug didn't fix it. So what the freak is the deal??? Oh FYI no problems last season with it.
2001 MC trailer.. according to my electrician buddy the solenoid In Tongue is working.

06-11-2011, 08:33 AM
What year trailer?

06-11-2011, 08:53 AM
Sorry 2001 MC trailer w brakes..

06-11-2011, 09:02 AM
Gotta be your reverse lights or an issue with the connection to the master in the tongue.

Table Rocker
06-11-2011, 09:10 AM
Do you have a test light? Make sure you are getting 12V on the reverse light pin on the vehicle half of your plug. Put the plug together and see if you are getting good voltage where the wire attaches to the solenoid. If you are getting 12V to the reverse lockout solenoid and it isn't working, you probably need a new solenoid.

06-11-2011, 09:22 AM
Got to be reverse lights define that statement??? You referring to my truck side or boat trailer side? I pull w a 2002 Duramax not that this matters but FYI.

06-11-2011, 11:05 AM
most boat trailers have a flat 5 pin connection... exact same wiring as flat 4 pin except the 5th additional pin is connected to the back up light in the tail light (the white light portion of the tail light that comes on when the vehicle is in reverse). This pin allows 12 VDC to the solenoid to prevent the brakes from locking up. get a test light, ground it to your vehicle, have someone in the truck, engine running (foot on brake) and put in reverse... test the 5th pin to see if it's hot.

if you are getting power to the 5th pin, more than likely the solenoid or wiring between truck connection & the solenoid.

if your truck has the larger 7 pole RV style connection and you are using an adapter, check to insure there is power to the 5th pin... most of these adapters will NOT work on a Dodge!!! I am not sure about a Chevy/GMC. (but you said it worked last year)

06-11-2011, 12:54 PM
I've got an '08 MC trailer, with the big 7 pin round plug. Last summer I started having intermittent trouble with the brakes locking up in reverse. Luckily it first happened at home so I had a 1/4" bolt around to override the brakes. I now keep one in my truck at all times.

I tested the vehicle recepticle and it was fine. I recommend just turning the ignition on, park brake set, put in reverse. No need for the engine to be running. Never did solve the problem. So far this year the solenoid has been working. (knock on wood)

06-11-2011, 01:28 PM
Also be sure check the reverse lights on the Trailer as that will make sure the plug and wires on your trailer are good.

06-11-2011, 05:41 PM
Plug and wires good.. but may have a bad reverse bulb.. surely that's not it.

06-11-2011, 06:25 PM
I have the same trailer I'm curious how do you override the brakes with the 1/4 in
bolt I've never had a problem but would like to be prepared Thanks

Jeff d
06-11-2011, 09:18 PM
There's a solenoid in the actuator that when activated by the 5th pin (Blue wire) redirects the pressure generated by the master cylinder into the reservoir instead of to the brake calipers as it would under normal circumstances.

If you put your ear near the tongue of the trailer and have someone put the truck in reverse with the key in the "On" position you should hear the solenoid click. If you don't hear a click you either have a wiring issue with the solenoid or the solenoid itself has failed.

I think you probably have a UFP actuator and the solenoid is replaceable.

Checkout the 2nd video from the left on this page to see the solenoid in action:

BTW you can usually pop a nickel in the slot on the actuator to lock out the brakes if necessary. It will fall out as soon as you pull forward unless you tape it in place or something though.

If it's a UFP this is probably the solenoid you'd need:

Jeff d
06-11-2011, 09:20 PM
Oh and even if the solenoid is clicking there could be a problem with the valving of it that prevents it from working properly.

06-11-2011, 11:15 PM
I had the same issue taking the boat out this year for the first time. I checked the solenoid and it was bad. Pretty simple replacement.