E'99 F-350 SD Trailer Tow Charge Relay Location - Ford Powerstroke Diesel Forum
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Old 10-22-2010, 05:08 PM
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E'99 F-350 SD Trailer Tow Charge Relay Location

Been lurking for a while and know I need some help from those of you who know much more than myself.



As stated above, I have a 1999 (manuf date 6/98) F-350 SD 4x4 7.3 with tow package, etc. While I have towed plenty since the truck was new, I've never used or connected the Trailer Tow Battery Charge circuit....until today.



Here's the problem:

1) There is no power at this wire at the factory Hoppie Plug near the rear bumper. All other circuits at this Hoppie Plug work fine.



2) The owners manual has a chart and diagram for each fuse box. both indicate that the fuse for the Trailer Tow Battery Charge circuit is in position #16 inside the High-current power distribution center (fuse box) under the hood.



3) All fuses at both the under hood fuse box and the interior fuse box are installed correctly, have clean contacts, and have been tested with a trouble light.



4) The owners manual chart correctly indicates that the fuse box under the hood has 37 positions, including relays and fuses.



5) The owners manual diagram shows that there are a total of 40 fuses and relays in the under hood fuse box, but the diagram does not show a position/location for items 38, 39, 40.



As luck has it, item #39 is the Trailer Tow Battery Charge relay, but as mentioned, it does not show a location for position #39 and despite much effort, I have been unable to locate this relay.



As mentioned, the fuse is good and there are no empty relay sockets that I have found in either distribution box or anywhere else I have searched under the hood and dash. Since the fuse is good, I suspect the relay is defective, but I cannot find it ANYWHERE!!!



I did read a few articles indicating that this relay is mounted next to the under hood fuse box, but I have definitely looked and looked and looked and looked and looked again, but do not see any other relays nearby.



In summary, I trust the circuit exists in the truck and is pre wired because there is a a wire for this circuit at the rear Hoppie plug, and further, there is a fuse for this circuit in the fuse panel. The only missing link is the relay, which seems to be MIA.



Does anyone know where the F this relay and/or blank socket could be hiding?!?!?



Thanks in advance for your help!



Rob
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Old 10-22-2010, 08:14 PM
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Hmm-not sure about the relay,but mine's a 6-98 too.
Mine didn't come with a factory hitch plug-just all the wires bundled up by the hitch.Only got the 4-pin cheapy plug
I had to wire the 7 pin all in myself-and there is a charge wire underneath-at least on mine-I believe it was orange-but don't quote me-truck's not inside right now-
Take a look under there and see if there isn't a wire tied up in the harness-it should be there and have a factory label on it
If not-let me know,I'll look under mine tomorrow
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Old 10-22-2010, 08:38 PM
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Quote:
Originally Posted by icysmooth View Post
Hmm-not sure about the relay,but mine's a 6-98 too.
Mine didn't come with a factory hitch plug-just all the wires bundled up by the hitch.Only got the 4-pin cheapy plug
I had to wire the 7 pin all in myself-and there is a charge wire underneath-at least on mine-I believe it was orange-but don't quote me-truck's not inside right now-
Take a look under there and see if there isn't a wire tied up in the harness-it should be there and have a factory label on it
If not-let me know,I'll look under mine tomorrow
I have the same harness your truck came with and also had to wire in the 7 pin myself. The charge wire is in the loom, but has no power so I'm hoping the relay solves the problem....

Does your truck has the mystical relay box next to the main distribution box under the hood? That's what seems to be the missing link for me.
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Old 10-22-2010, 09:51 PM
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i had to wire my 7way into my 4way. the stock 4way was broken and the other 3 wires were just capped so i upgraded to the 7. but my trailers dont have brakes so i dunno about that.
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Old 10-23-2010, 06:53 PM
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Quote:
Originally Posted by PESCAHOLIC View Post
I have the same harness your truck came with and also had to wire in the 7 pin myself. The charge wire is in the loom, but has no power so I'm hoping the relay solves the problem....

Does your truck has the mystical relay box next to the main distribution box under the hood? That's what seems to be the missing link for me.

It was already dark when I got online-I did take a pic-tell me if this helps
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E'99 F-350 SD Trailer Tow Charge Relay Location-img00426-20101023-1841.jpg  
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Old 10-23-2010, 09:40 PM
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Quote:
Originally Posted by icysmooth View Post
It was already dark when I got online-I did take a pic-tell me if this helps
Thanks for taking the time to post the picture. It helps to confirm that my truck definately does not have either of those relay boxes.

While my truck does have the Orange or Red Trailer Tow Charge wire in the loom at the rear of the bed, and does have a fuse in the Trailer Tow Charge position of the high current distribution center, it does not have either of the relay boxes which led me to posting this thread.
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Old 10-23-2010, 09:45 PM
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I took the pic from the front,with main box on right-now I didn't take covers off of left 2 boxes.
Do you have these?
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Old 10-23-2010, 10:51 PM
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The Hoppie plug and wires for the Electric Trailer Brakes are installed under my dash and were actived at the Hoppie plug by the rear bumper by simply tying them into my brake controller.

However, the wires for the Trailer Tow Charge circuit at the same rear Hoppie plug are not active and I cannot find a relay they should tie-in to, and have not yet traced the wire back from the rear of the truck. There are unterminated wires on the loom at the firewall just behind the high current fuse panel, but those simply pass through the firewall and are unterminated under the dash as well-likely for the SOF circuit if I had one.



I will take some time and try to trace or tone the wire from the rear of the truck tomorrow, but if cannot find an existing relay I may end up bypassing the wire for a much larger wire with an isolator or combiner.

Thanks again for the help.

Rob
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Old 10-24-2010, 05:12 AM
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on my truck i had to go to ford and buy the actual relay and it was listed in the manual as the trailer battery charge circuit I think. Well when i opened the small fuse bos there was space for 2 fuses so i put it in both and finally got a 12 volt charge to the 7 pin connector. I was happy i could finally charge the dump trailer battery. hope this helps.
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Old 10-24-2010, 11:00 AM
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those loose wires in the wire loom are also loose inside the cab, they are there for you to get through the firewall with upfitter switches etc. I will see if I can get pics of numbered relays later.
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