Recently discovered that I have an issue I can't seem to find a solution to for my '03 Expedition. The trailer tow relay/parking lamp trailer relay keeps blowing. When I attach the trailer, it blows instantly. When I don't attach the trailer, it blows after just a couple of minutes. After it blew with no trailer attached, I noticed that it got hot.

Although I'm not certain, what my father and I are thinking is that something is drawing juice and causing a minor overload. When the trailer gets attached, it gets overloaded immediately and blows. When there is nothing attached, the overload happens gradually because something is constantly drawing juice that shouldn't be.

On a similar note, lately I noticed that after my headlights are on a while, if I turn on the blower or step on the brake, the lights dim slightly. That is the first issue I have not had time to look into yet. The second issue is that the a/c and heat work intermittently on speed 1-2-3 and work fine on 4. We were planning to solve and fix those issues once I get back from FL early next week with the truck.

Do you think that the headlight dimming and/or the blower issue has something to do with the relay being on? We haven't looked to see if they cross anywhere yet. Once again, a project for next week. I figured I'd reach out here and see if anyone has ever run into this issue before or may be able to provide insight so that our diagnosis time is cut drastically, as I'll only be home for a few days before heading out again.

Thanks for any help in advance!

 

The Trailer tow relay, parking lamp gets the main feed power from 20A fuse 1.10 at all times. The trigger for the relay comes from the Main light switch when the switch is in the Park or Head position. The output of the relay, BN/WH, goes to pin 6 of the connector behind the 7-pin tow connector and to pin #2 of the 4-pin tow connector. There should only be power on those pins when Park or Head is selected. Make sure the 4-pin connector is not mangled. Then I would remove the 7-pin tow connector and disconnect the connector on the back of that connector to see if the problem clears up. The next place to look would be the headlight switch assembly/connector.

 

It's possible your main ground wire to the body from the battery is loose. This would explain the drop in lamp brightness and also current could be backfeeding through the trailer circuits trying to find a path back to ground. The backfeeding would then exceed the ampacity of the wiring and cause it to heat up.

 

What exactly do you mean by "blowing"? Which portion of the relay is "blowing", the coil or the switching contacts?

Check your voltage regulator and battery voltage.

Could be an intermittent anywhere between the 40A fuse under the hood (F105), the relay, the blower motor, the speed selector switch, and ground G200 (behind passenger side kick panel).

Might as well inspect G200 anyway, it's also used as the grounding point for the low side of the trailer running lamp relay's coil as well as a bunch of other circuits.

 

Thank you all for the responses. Sorry for the long delay, but just got back with it a little over a week ago. I drove myself nuts trying to figure this out. I tested everything multiple times, cleaned the grounds, disconnected and then cleaned all connectors related to the headlights and trailer, etc. Ordered a new Motorcraft headlight switch even though the old one appeared fine after the tests. Put it in... still had issue. Since my fathers F250 was at the house, I connected that to the trailer to make sure all was well with the trailer. After cleaning the trailer connectors on his truck, it worked fine, so I put it back on mine. All of a sudden, it started working fine. Not sure if the relay finally reset itself and that made it work in combination with the headlight switch or if it was something else. I'm just hoping I don't think it's fixed when there is a problem somewhere still.

My issue with the relay was that it kept heating up to a point it was hard to touch. I never was able to get it to reset after. No clicking or anything even when I tested it in other vehicles.

Before I take it on my next trip, I'll fix the a/c issue. I plan on starting with the resistor and going from there, as that has been the fix in my Bronco's. Until I get it fixed, I'll just enjoy dropping the top in the Mustang for now... lol. Thank you all again.

 

The resistors for the manual A/C blowers are only about $20 at Rock Auto.

2003 FORD EXPEDITION 5.4L V8 Blower Motor Control Module / Resistor | RockAuto. Easy enough to get to. Might have to pull the glovebox, but that's quick and easy too.


-Mike