We recently embarked on a cross-country move from Syracuse to Dallas, towing a U-Haul 5×8 trailer behind our Subaru Outback. The trip was split into four days, with stops in Mansfield OH, Nashville TN, and Little Rock AR.
Before leaving Syracuse, I checked every item on the checklist, including hitch pin, ball mount, coupler, safety chain, wiring, tires, lock, etc. However, I skipped checking the lights because they were already tested when picking up the trailer, so I assumed that they should be working properly as long as the connection was good.
The first stop was Mansfield OH, about 7 hours drive from Syracuse. We stopped for dinner and gas at a small town near Cleveland OH. It was almost dark and I was a bit upset since I skipped the light check before departure. So I checked the trailer tail lights and found out that the lights were not on.
First of all I double checked all the connections were good, then confirmed with my girlfriend that the light was indeed working when picking up the trailer.
Initially I thought the failure must be on the tailer and was about to call roadside assistance. My girlfriend reminded me that the trailer light tester was in the emergency kit and I could test it first. I bought this tester when I installed the trailer hitch a few months ago, to make sure the wiring was working properly, and it has been in the emergency kit after that.
Turns out the tester didn’t light up either when plugged in — meaning the problem was actually on our car, not the trailer 🤡.
It was getting dark and we were still about 100mi from Mansfield. It was 8pm on a Saturday night and the UHaul store, repair shop, Autozone, etc. were all closed. Meaning if we couldn’t fix the problem ourselves at roadside, we would either need to wait for roadside assistance that no one knows when they will arrive, or risk a 2 hour night driving without tail lights.
First I recalled the process of installing the trailer hitch, trying to brainstorm what could went wrong. The trailer hitch was installed only 2 months ago in April so it was free of corrosion from snow and salt, and I was pretty sure the wiring harness was installed correctly. We discussed and decided to check the trailer wiring to see if we could find any clues.
The trailer wiring harness is located by the spare tire under the trunk, the main components are the plug (white) and two boxes (marked in red and blue), it then extends out of the car through a hole and ends in a 4Pin plug secured next to the receiver. (The photo on the right was taken when installing the hitch.)
At this point I remembered that the installation instruction mentioned the small box on the harness (red box) is a fuse box. I opened it and found a 10A fuse that had already blown.
I replaced it with a spare fuse from the emergency kit, and the tail light worked again. Just in case the problem happens again, I bought another set of spare fuses at the gas station, and continued the trip.
Although the problem was solved for now, I was still upset – there was no way that the small bulbs in the trailer need to use as much power as 12V * 10A or 120W, which meant that there was a good chance that the fuse was blown due to a short circuit somewhere, and the problem would probably happen again. So I drove carefully assuming there were no tail lights. Luckily most of the rest of the trip was within the Cleveland metro area, where there was enough lighting on the road.
When arrived at Mansfield, not surprisingly, the lights stopped working again.
After settling down at the hotel I discussed this with my friend Q. He drives the same Outback as mine, just a different trim. When he installed the dash cam he wired it from the fuse box, while I wired from the power outlet in the glove box, so he was more familiar with the fuse system than I did.
After some research and discussion, I came up with the following plan: Next morning I would go to the local UHaul store and have the staff check out the trailer and fix it, and if it couldn’t be fixed I’ll buy a pair of magnetic external tail light like this one.
Next morning I went to the local UHaul store, I explained the problem to the staff and was told that they didn’t have a mechanic onsite and I had to call for roadside assistance.
Since I don’t trust roadside assistance, I decided to go with plan B. I drove to a nearby Harbor Freight and bought the lights. Once installed I opened the cover under the trunk to replace the fuse.
Surprisingly this time the 10A fuse was good, although I was confused I still replaced it with a new one, and after testing it — the new external lights still didn’t come on.
At this point I remembered that my friend had sent me the fuse box map. One of a 20A fuses was labeled
TRAIL R. FOG. It was confusing, but looks like it’s trailer related.
This fuse is located in the fuse box under the steering wheel, and it was indeed blown.
This fuse is a Low Profile Mini Fuse, and all the spare fuses I had were regular Mini Fuse, so I went to a nearby Walmart and bought the correct fuse. Although the connectors look almost the same, I’m not sure if the Mini Fuse can fit in the Low Profile socket, and I didn’t get a concrete answer on the internet.
After replacing the new fuse, the external lights finally worked.
Then I taped the wire to the trailer.
Although the problem is now solved, there were still two questions remain unanswered:
1, The 20A fuse was labeled
TRAIL R.FOG. What does it mean and why are there two fuses with different size on the same circuit?
After some research, I found that this 20A fuse is used for the trailer light + rear fog light. The 10A fuse on the trailer light harness limits the current of the trailer tail light to 10A. Therefore, by design, the current of the trailer tail light should not exceed 10A, and the trailer tail light and rear fog light together should not exceed 20A. However, the Outbacks in North America don’t have rear fog light, so only the trailer tail light is using this circuit.
2, Why the 20A fuse was blown while the 10A fuse didn’t?
After discussing with friends and some research, I found that fuses can be partially blown. When the 10A fuse is blown, the 20A fuse was probably already half burnt. Therefore, after the 10A fuse is replaced, the 20A fuse will be blown in a much shorter time.
In conclusion, we can derive the following chain of events:
- The UHaul trailer itself already had a short circuit somewhere.
- At pick up, when testing the lights, the problem didn’t happen because it wasn’t long enough to blow the fuse.
- Sometime after the trailer was hooked up, the short circuit generated more than 20A, resulting in a blown 10A fuse in the trailer harness and a partially blown 20A fuse in the fuse box.
- Near Cleveland we found the lights were not working and replaced the 10A fuse.
- Somewhere between Cleveland and Mansfield, the short circuit occurred again and blew the 20A fuse.
- The next morning, after replacing the fuse and using an external magnetic light, the issue no longer occurred.
The rest of trip went smoothly without any further problems. I just had to do additional checks at each stop to make sure the magnetic lights and wires were secured. After 4 days driving we arrived in Dallas.
When I returned the trailer I informed the staff that there was a problem with the trailer and asked them to fix it before handing it to the next customer. I then contacted UHaul customer support and UHaul reimbursed the cost for the lights and refunded part of the rental cost.
If you came across the same trailer (AV1701C, Honolulu), feel free to share your experience.