2005 Excursion: No Trailer Lights

[98GP]

I need help figuring out what these relays are:
Under the hood next to the master cylinder:

Same location but up close:


My problem is I have no running lights on the trailer with the head lights on. Turn, stop, and hazards work. I have checked the fuses in the dash under the stirring wheel and all are intact with resistance verified with a meter. Can someone tell me what these relays do under the hood? Are the three bigger ones the same and are the 2 smaller ones the same?

Thanks.

 

[bjm206]

The three relays closest to the firewall makeup "auxiliary relay box 4"
4.1 - big relay - A/C clutch relay
4.2 - small relay closest firewall - trailer tow relay, right turn
4.3 - small relay furthest from firewall - trailer tow relay, left turn

The two relays furthest to the firewall makeup "auxiliary relay box 3"
3.1 - rightmost (PS) - transfer case low to high relay
3.2 - leftmost (DS) - transfer case high to low relay

This is only valid for 2005 US market 4x4 with gas engine.

Relays 3.1 and 3.2 are the same components as are relays 4.2 and 4.3.

The trailer running (parking) lights are controlled by a relay that is an internal not separately serviceable part of the central junction box. The 20 amp fuse labeled F2.38 can cause the trailer running lights to fail with no other lighting issues.

 

[98GP]

Thanks for the superb information!
You said "central junction box"... This is the where all the fuses are located in the dash under the steering wheel?
Sounds like I need to start chasing wires since the fuse is intact and the relay is not serviceable.

Thanks for the help!

 

[bjm206]

Yes, the "central junction box" for a 2005 Ex is where all the fuses are located in the dash under the steering wheel.

 

[98GP]

I had a chance to look at the fuses closer. The 20A fuse labeled as 38 on the control panel under the steering wheel is intact. I swapped with another 20A from the same panel with no change. A meter set to ohms confirmed the fuse is still intact. The fuse socket does not have any voltage across it. I should be getting 12v with the head / running lights on, correct? If this is true, then the internal to the control panel trailer light relay would be toast, right? It would be a terrible design by Ford if this relay is not serviceable. Where does this relay and fuse get its power? Maybe this needs repaired?

 

[bftank]

check your wiring for corrosion in the connections in the back, i had an 03 f350 that hauled a boat before i got that happened to. water had corroded the connection for the trailer lights.

 

[bjm206]

You should have 12V from one side of fuse 38 to ground (an unpainted bolt or piece of metal attached to vehicle body). Going from one side of the fuse 38 to the other will have no voltage drop on this circuit.

If you do not have 12V between one of the sides of the fuse 38 and ground then there is something wrong inside the Central Junction Box (fuse box). Fuse 38 according to the schematic is connected to the CJB bus bar on one side and the internal CJB trailer relay on the other. The bus bar is a common hot at all times 12V source for the CJB that supplies other critical circuits.

It is worth checking the CJB for corrosion but if you need to replace it I would try a junk yard as these are not known for being particularly unreliable.

 

[98GP]

I didnt know that about reading the voltage had to be from the port to ground and not across the port. Although, this seems strange.

I will double check my voltage reading.

Next step will be pulling the bumper and checking the connector behind it for power. I am looking forward to getting this resolved.

 

[98GP]

OK, good news. I do have power at the fuse port reading from the port to ground. It seems that all is well in the cab at the CJB.
I guess next will be pulling the bumper and checking the connector back there for corrosion and power. This is going to be a pain...

I wish I had more then 10-15 minutes at a time to troubleshoot. Why does work always get in the way of things I need to do?

 

[bjm206]

That is good to hear. There is also non-serviceable relay in the CJB after the fuse 38 that should be checked for operation.

This is the schematic for the circuit from fuse to trailer connector. You are looking for the middle relay with fuse 38 above labeled as F2.38.



Autozone repair guides (where the image above came from) has a good collection of these schematics.

 

[98GP]

Thanks for the link