Trailer wiring issue when headlamps are on

[r4ndll Avid PP Poster! Find all posts View Profile Close r4ndll]

Trying to track down a fault on a recently purchased a CT which has had electric brakes retro fitted. And had redarc towpro fitted today by someone else, so don't know where the fault lies. As I haven't hooked up the trailer since brakes/tow pro install I don't know what's causing the issue. (Trailer place dropped it off for me).

Is it as simple as a bad earth? And if it is, where the heck do I start looking? Its a Toyota Factory Towbar and wiring harness

Symptoms:

• Headlamps off - Trailer lights off.
• Prado Headlamps on, trailer lights on but look dim. Repeater lights on trailer rear bar look fine.
• Headlamps off - indicators work ok
• Headlamps on - indicators don't work, trailer lights flicker on and off in time with indicator and both indicator lamps come on, same as if you had hazards going
• Brake lights don't work in either scenario or when brake pedal is depressed.

Prior to this work things worked normally prior to the brakes apart from an intermittent fault with left indicator - appeared on my box trailer and neighbour's box trailer. This cleared recently - hence my confusion. Is it tug? Trailer? Brake Controller? Brakes?

Took the Prado socket apart and all looks wired correctly. Cleaned the socket terminals as best I could as they're fairly dirty. Took the plug on the trailer apart and there's no blue wire for the brakes (two reds) and have a black and white wire are both in the same terminal. Is that normal? I'm assuming the new brake harness is just using red/black instead of blue/white
Also going to see if I can hook it up to my neighbour's boat trailer tomorrow to isolate the issue to trailer/prado.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Have you got a test light as opposed to a multimeter? The tow pro gives spurious voltage readings on a multimeter but that is an aside. Test each pin on the Prado plug. If ok disconnect the trailer brake cable and see if the wiring behaves normally. With a test light connected to the Prado plug switch the towpro to manual mode and see if you have the same issue. It sounds a bit like a bad connection and looking at the plug that ain't pretty but you know that. If the towpro won't change settings then there is an issue there. LED lights on the trailer? Ballasts?

Don't really have anything and sadly trial and error. If you don't have the towpro instruction manual it is on Redarcs site or I can scan and email it to you.

[r4ndll Avid PP Poster! Find all posts View Profile Close r4ndll]

Cheers for the advice MJ. Didn't have a test light so got hold of one. 5 year old has enjoyed helping me test the Prado plug. Bonus!

No LED's on the trailer. Definitely think it is more the trailer end now.
Prado tested all OK. Plugged in the trailer.
Head lamp off/Right indicator on, everything ok.
Headlamps on/Right indicator on = The whole system is energised ie the test light was pulsing in sync with the indicator on all wires. Funnily enough so was the Tow Pro LED.
Headlamp off/Left indicator On= not working. Jiggled the wires and the trailer plug and fault cleared. eg HL/RT test was fine

Can't test any more now as the Prado main battery has gone flat - thinking its related.

I'll get back to testing with brakes disconnected once I've charged it. The plot thickens...

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Reckon there is a cock up with the towpro wiring or the brake wiring trailer side. Like the wrong wire tapped for brake signal. Disconnect the brakes at both plugs and see if normal service has resumed.

[r4ndll Avid PP Poster! Find all posts View Profile Close r4ndll]

Definitely trailer side.
Kindly neighbour plugged his Pathfinder in (no brake controller) and had same issue but did have brake lights & no left indicator. Neighbour had to go out so put the Prado back on.
Removed brake signal cable from trailer plug & issue went away. Cable reconnected & problem back. Gotcha I thought.
But because neighbour had brake lights and I didn't, I removed the Stop light signal from trailer but reconnected brake signal. No issue
Reconnected Stop light signal - No issue and it all seems to be working normally now. Blooming frustrating.

Which got me thinking that the issue is somewhere in the trailer cable run.
So I had a good look over the cable and there looks to be a nick in outer insulation 10-15cm back from the plug. So I'm now wondering if its been crushed/damaged at some point up near the plug? There's a heap of slack so I've got room to play with if I need to take it back.

And the outer insulation is missing further back by the wheel and been taped up. Looks like someone was going to splice in there and thought the better of it. No visible damage to the inner.

Its a shame really as the trailer is a tough as nails aussie original. Its been frustrating but fun. I'll see if I can spend some more time on it but we want to use it in two weeks so might bite the bullet and get the experts in. Can't really blame the trailer place as Electric brakes look to be on their own circuit completely. Not tapped in anywhere.

Flat Prado battery appears to be my fault... hasn't been run in a while and I accidentally left the air-con on whilst diagnosing. oops.

[mjrandom Out of control poster! Find all posts View Profile Close mjrandom]

Start from scratch. Cut 100mm or so of trailer cable and rewire I reckon and then redo at the other end. We run an Anderson plug for brakes mainly because you can run bigger cable. But it leaves the plug to do low current stuff. At least you now know which side is the problem