Tekjonsha P3 brake controller

I've had a Tekonsha P3 controller for about 4 years. All has been good but now all the lighhts on the trailer go out when I touch the brakes. Have checked all the cables and all is good except theres power coming back from the controller to the negative battery terminal (via the cable direct from the controller).

I've met one friend who had their tekonsha controller replaced because it developed an earth. Anyone else had similaar probs? Any suggestion on proving this problem to be the controller? what's the best controller - Red arc?



Posts: 452

Date Joined: 24/04/11


Fri, 2018-09-14 12:47

 do your indicator lights work?


i don't understand squat you mean  you say power coming back from the controller to the negative battery terminal...

Posts: 272

Date Joined: 10/12/07

Re wire your trailer

Fri, 2018-09-14 13:37

There has been similar threads on here with controller problems. Usually turns out to be a short in the trailer wiring. Probably through corrosion. I spent hours trying to sort out why my controller fuse kept tripping. My mate said just run new wires, it will be in the trailer somewhere. Turns out he was correct. No sure how you test for that. Hook someone elses boat up.

Rob H's picture

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 Tekonsha, from memory has

Fri, 2018-09-14 14:27

 Tekonsha, from memory has something like a 5 yr or lifetime warranty.
My original one started playing up, got onto the local importer "sorry only 12 months warranty".

Contacted Tekonsha who got onto the local bloke and put a rocket up his clacker and got a new one a week later.

P3 is pretty good I find, so not keen to change to anything else except maybe a brake commander


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The older you get the more you realize that no one has a f++king clue what they're doing.

Everyone's just winging it.


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Date Joined: 03/06/09

Thanks for the replies so far.

Fri, 2018-09-14 16:17

BSIR - If I disconnect the negative wire from the battery and put a volt meter from negative battery terminal to the negative wire I get 10 volts! Indicators and tail lights work until I touch the brake - then all lights go out.

West Coast - I'll try another search for controller probs. Thanks as I hadn't seen them

Rob H - Thanks I never expected more than a year so I better go check the documents

Aas I was alone when I did the testing I wasn't able to test the output at the tow vehicle plug. I had to check the trailer lights although I think I tested 10 volts in the negative wire with the car in the garage (trailer not connected).


gruntre69's picture

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Date Joined: 15/10/16

Mate that test you are doing

Sat, 2018-09-15 09:25

Mate that test you are doing only proves there is a potential on the brake controller and not a fault. I doubt it is your brake controller and as others have said it's most likely a bad earth or short on the trailer wiring. The P3 has a plug on the back and is only really connected to the trailer by 2 wires.

By the way these have a lifetime warranty.

If you disconnect the P3 plug do you have the same fault?

Does the circuit breaker for the P3 trip when it happens?

If no, then disconnect just the blue wire and try again. If the problem returns you have shown the problem to be with the service brake (Blue pin 5)wiring on the trailer (possibly an overload condition) or a bad trailer earth or both.

My money would be on a bad earth on either the trailer plug on the vehicle plug or the trailer plug or if the earths are grouped down stream of the trailer plug.

I just installed a P3 on my 200 series and it works very well but there were some problems which ended up being failed connections on the trailer plug installed on the rear bar... rewired these and all good...



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Gav475's picture

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Date Joined: 16/11/11


Fri, 2018-09-14 18:35

 100% wiring on the trailer. You will have multiple shorts causing the problem. Went through it myself, to the point where they even looked at the controller on my new 200 series cruiser. Rewired the trailer and now good as gold.

Vinesh87's picture

Posts: 2747

Date Joined: 02/04/11

Yep, similar story to a mate.

Fri, 2018-09-14 18:39

Yep, similar story to a mate. Due to shorts ended up needed new brakes, bearings, hydrastar all because of a short internal to a sealed LED light!

Posts: 562

Date Joined: 27/08/09

 Could be any number of

Sat, 2018-09-15 13:05

 Could be any number of faults, but the most likely is a bad earth to the trailer. Check your plugs and sockets on the trailer and vehicle and the wiring that supplies them. 

Posts: 130

Date Joined: 03/06/09

Thanks for comments - Update

Sat, 2018-09-15 14:27


Yes, I'm now considering it's a bad earth somewhere. I'm uncertain to re-connect the controller as when I put the earth cable from the controller onto the neg battery terminal it sparks (only if the controller is connected). I don't want to damage the controller. This is without the trailer connected. So I've got power in the negative side of the controller when there is no trailer connected. Where's my bad earth? Is it the blue wire earthing in the vehicle?

I've never heard the circcuit breaker trip.


With P3 disconnected the problem still occurs - if tail lights are on and the brake is applied then tail lights go out and no brake lights. Applying the brakes does not illuminate the brake lights. This suggests an earth on the trailer?

Gruntre69, u've offerred a lot of suggestions. I'll now disconnect the blue wire and see what happens. maybe the earth is in the vehicle plug. Could I be so lucky? Next I'll check the trailer for earths. Multiple earths sounds like the only connection I have under water, or internally in LED lights.

Thanks for your comments everyone.

Posts: 452

Date Joined: 24/04/11


Sat, 2018-09-15 14:37

 Still sounds like an earth to me.

Do the tail light glow a little duller than what they normally would?

Id download the pinout for your trailer plug, check which in is the earth, then reconnect it on both the car side and trailer side.

Posts: 562

Date Joined: 27/08/09

 OK if its happening without

Sun, 2018-09-16 15:23

 OK if its happening without the controller connected it is a bad earth. One of the pins of your trailer plug is an earth. On the vehicle side this will run to a solid earthing point. On the trailer side all your earths for lights and brakes will connect to it.

Posts: 130

Date Joined: 03/06/09

It's not the blue wire

Sat, 2018-09-15 16:08

I disconnected the blue under the dash and still same result.

No, the tail lights go out totally.

Appears to be an earth in the trailer plug. Just noticed a piece broken out of it and bit of a sloppy fit. Replaced it and all seems fine although I'll check it over agian when I've got a second pair of eyes with me.

Thanks again for all your help - truely a wealth of knowledge on this site.