Trailer Tow Fault / HVAC Issue

[JediMcMuffin]

So, I've had an issue crop up a few times now. It's happens when the Bronco has been recently washed, when its been raining, or when I hit a water crossing. As long as I stay in motion, no faults will pop up. If I come to a stop, I will eventually get a "Trailer Tow Module Fault" message, followed by my HVAC controls shutting off momentarily. I'm sure they share a fuse or something, but I'm unsure which wiring to go look at.

I have a 2021, Curt Hitch, Ford module and 7-pin harness. I can't imagine it has anything to do with the wiring on the drivers side of the cargo area, but probably the taps at the front. Any other guidance would be some help. Thanks!

Edit: This is the kit that is installed: https://www.ford.com/product/trailer-hitch-wiring-harness-4-7-pin-by-lumen-p2971214188

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[JediMcMuffin]

This is my suspicion, water is running down these wires, and making its way into the harness somehow, maybe the dealer didn't secure it properly or one of those crimps is loose.

 

[Tonka Bronka]

I installed this kit on my 21, very easy to get something wrong, in fact I found a huge discrepancy in the directions and if you don't check your work, you will have to re do it. Pull the left front wheel, pull the fender liner, and see how the harness plugs look and check for loose or bare wires.

 

[JediMcMuffin]

Well, except for when its wet, everything functions pretty perfectly, so I'm just going to check every wire along the path, only real explanation. I agree its probably up front where the crimping and repinning happened, but I think just for good measure I'll make sure that ground at the back is good too.

 

[JediMcMuffin]

@Tonka Bronka : you were spot on. found my issue on the crimps.

all better now:

 

[Tonka Bronka]

@Tonka Bronka : you were spot on. found my issue on the crimps.

all better now:

That's fantastic! Good job! Those look like solder and seal connectors. I have tried them with the same results, the plastic can melt before the solder does. I don't even try them any more.

 

[JediMcMuffin]

Yeah I had the same result, I stopped using them. Maybe the knockoff versions are the issue? Hard to tell. Standard crimp and seal was fine for me. I did secure these down and wrap them in proper tape as well.

Edit: I put ChatGPT in cynical mode, and this is what it thinks:


You’re not crazy, Tommy. Those little “miracle” heat-shrink solder-seal butt connectors are basically the participation-trophy version of proper wiring. They can work, but only if you treat them like a temperamental raccoon: gentle heat, patience, and definitely not a torch.

Here’s how to use them without turning them into molten sadness:

What actually works
1. Use a real heat gun, not a lighter.
A lighter is basically a dragon. The tubing melts at ~120–150°C. The solder ring doesn’t flow until ~180–200°C. A lighter blows right past the tubing’s melt point. A heat gun gives you a wider, gentler heat.

2. Start from the ends, not the solder ring.
Shrink the clear tubing over the wire insulation first. This stabilizes the joint so the wires don’t wiggle and ruin the connection.

3. Keep the gun moving.
Hovering in one spot is how you end up with a sad puddle. Slow circles around the barrel until the tubing snugs down.

4. Then aim at the solder ring.
Once the tube is tight, focus heat on the solder. You’ll see it go from matte to shiny and flow into the strands. If the tubing already shrunk properly, it won’t collapse further.

5. Tug test.
If the wires slide out, pretend it never happened and do it again like an adult with a crimp tool.

Bonus survival notes

• Use higher-quality connectors. Some cheap ones use solder with a weird melting point that might as well require a small star.
• Strip the correct length. Too short and the solder never touches both conductors.
• If the wires are thick, these things simply fail at physics

The honest truth
A good ratcheting crimp tool and proper heat-shrink butt connectors are still stronger and more reliable. These solder-seal jobs are fine for light-duty, non-critical harnesses in dry conditions. If you’re in a vehicle or anything that vibrates or gets wet? Crimp and shrink like a civilized person.

If you want, I can walk you through the right crimper and connector sizes for what you’re wiring.

 

[Brian_B]

like a civilized person.

Ironic advise coming from an AI

 

[JediMcMuffin]

I highly recommend you set the tone of ChatGPT to cynical, its downright hilarious.

 

[JediMcMuffin]

Well, spoke too soon. Did it again this morning. Maybe the tow module literally is just going bad. Still glad I fixed the wiring. All of the pins added to the connectors were pretty firmly seated. The wrench in my logic is that this only happens when it rains, which still inclines me to think there is water getting into something. Perhaps the issue is back at the 4P/7P connector on the hitch.

 

[Tonka Bronka]

Well, spoke too soon. Did it again this morning. Maybe the tow module literally is just going bad. Still glad I fixed the wiring. All of the pins added to the connectors were pretty firmly seated. The wrench in my logic is that this only happens when it rains, which still inclines me to think there is water getting into something. Perhaps the issue is back at the 4P/7P connector on the hitch.

Do you think it would throw the fault if you aren't in motion? If so, have a helper use a garden hose on different locations while you watch the dash.

 

[JediMcMuffin]

Do you think it would throw the fault if you aren't in motion? If so, have a helper use a garden hose on different locations while you watch the dash.

That’s a great suggestion. Well that’s what led me to believe it was water running down a wire. The fault happens when it’s wet out and you come to a stop. Once you’re in motion for a minute or so it clears. I secured that wiring harness pretty good when I reinstalled it. It’s pretty well protected. I should follow the cable along the frame and see if it got nicked

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