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Greetings all, I have created quite the mess for myself and am hoping one of you can laugh at me and tell me what I am overlooking.
My truck is a 98 F150 I got from my client. It was a fleet vehicle for SeaTac Port Authority. It had a commercial/agricultural plug for trailer lights, not your regular RV/Standard type plug. Because I just picked up an enclosed trailer that has the standard plug harness for the lights (7 pin), I went ahead and changed out the plug on the truck to match it. I got it all working except the right turn signal... I thought I must have located that wire in the wrong spot on the new plug. I pulled the trailer plug out of the new receptacle on the truck and used some homemade jumper wires to see where I went wrong. Well, my jumpers managed to slip and shorted out so now on my truck I have no turn signals or hazards. I do have headlights, taillights, brake lights, just no turn signals or hazards, and nothing on the dash, except when I turn the hazaeds on, they just light up and not flash. The truck is disconnected from the trailer, and I removed the receptacle I put on the truck. I checked all fuses, and they are all good. Looked in the manual and it listed relays in the fuse panel. Replaced those. Nothing. Did an internet search and saw that there are 2 relays under the dash to the right of the steering wheel. Replaced those. Thought I must have fried the turn signal, multi controller. Replaced that. Nothing. Went under the hood and checked the fuses there as well. All good. Next to the power distribution box under the hood, there is a smaller box with 5 relays, but can't find any info on what they are or do. There is also a small fuse box with 2 mini fuses, both of which are good.
The 3 boxes under the hood, with a close up of the one with the relays:
I'm at a loss, especially since I have no knowledge of cars/trucks. I'm a carpenter and was trying to get my trailer set up.
The turn signals for the trailer are electrically isolated from the turns signals that go through the multifunction switch. Voltage for each turn signal (right and left) go to two relays. Shorting the trailer turn signal wiring will not affect the multifunction switch.
Look for fuse 2 , 20 amp in the engine fuse box. It provides power for the trailer signals. There are also two relays in the power distribution relay box, one for each trailer turn signals.
If the fuse and relays test OK, then further investigation will be required. Pull the fuse and test continuity (not just visual) or probe the holes in the top of the fuse to verify voltage is on both sides. Testing the relay is more difficult so pull a relay you know is working and swap.
The turn signal flasher unit is powered by fuse 23, 10 amp and fuse 1, 15 amp. Of course the ignition has to be in Run for the signals to flash.
Finally, you need a good ground for the trailer turn relays and the flasher unit.
Last edited by Don Ridley; Feb 18, 2026 at 02:42 PM.
Thanks for the reply! I'm not worried about the trailer as much as I am about the truck. No turn signals or hazards on the truck, not even the green arrows on the dash, except for when I turn the hazards on, but they don't flash, they just stay lit. I'll use my test light on the 2 fuses in the panel as you mentioned, as I only did a visual.I'd really like to know what that cluster of 5 relays under the hood are for. Of course nothing in the owners manual about them, and me not knowing a thing about cars/trucks doesn't lend itself to me figuring it out.
Can't thank you enough for the help. I'll be checking on the truck tomorrow, as we've been having 65 mph winds with snow flurries, and I don't have a proper shop to work in. In that website it shows me the small fuse box and the power distribution box under the hood, (as well as the Mega fuses for the battery), but nothing in regards to that box of 5 relays which makes me think this is something that SeaTac Port Authority added for some reason..... perhaps equipment/electronics?
Thanks again. I'll check in tomorrow, weather permitting, with a fix or more of the same.
I am thinking it's not going to be a trailer tow problem, since the truck's turns are out. They must have tied into the truck's lighting instead of searching out the trailer tow wiring taped up in the harness in the rear. The thread starter should be able to look at the wiring back there and tell, but since the truck's lights are not working, I bet the trailer is tied into the truck lighting.
Since you have no turns, I am betting you have blown fuse #1, 15 amp and or fuse #23, 10 amp. I know you checked them already, but check these particular fuses again. These are in the fuse box under the dash.
Hmmm... comparing it to the photo, the picture would have to be flipped to match the layout. The only two relays that would be missing would be the Fog Light and Red Brake Lamp. It took me a few minutes to make sure I was seeing it correctly.
Once again, you are very appreciated for all the time you're investing in helping me.
We'll get this.... just hopefully not at the expense of taking my truck in to a shop.
@Franklin2 you are correct. It is all in the truck. Rewiring a trailer would be easy, but my shorting out at the plug/receptacle on my truck killed something in the truck. I'm going to take the advice of using a test light, turning the key on, and testing those fuses in particular.
I'm sure my lack of knowledge and terminology doesn't help you all, so I do apologize.
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