What I've got is a 1998 F-150 Kingcab that I use as my daily contractor truck. I sometimes have to pull one of my two trailers, a 4 x 8 flatbed or a 4 x 8 open top box trailer. Both trailers have worked just fine for over 10 years and so has the truck. UNTIL last week. All the trailer lights stopped working on the flatbed.
NO turn/stop/running ANYTHING.

I twiddled with all the fuses (that I can find) Replace a single blown 20amp one under the hood. Have running lights ONLY

I have rechecked every single fuse that I can find. I've been through the very unspecific owners manual and the local ford dealer isn't much more help.
I just know that there has got to be another fuse block or a relay panel that I don't know where it's located. Everything that I've read in the forums seems to indicate that there are fuses or relays for these non functioning trailer lights. I just have to find them.

I have two polarity oriented relays in the engine compartment that I do not have a listing for in my manual and there is also another relay panel behind and partially below the big power fuseblock that I also do not know what they do (My manual doesn't have it listed-and the dealership has no clue)

Anybody else run across anything like this before and know exactly where I need to look?

BTW, both trailers work just fine on my neighbors toyota and one of my customers trucks

If nobody has a clue then I guess I'll just have to try and find a schematic and start opening up those lovely brittle plastic connectors/boxes and trying to track the issue down the hard way.

Thanks

 

First check your truck plug wiring. 4 way 6 way 7way? Take a test light and clamp to good ground probe with test light and see what works on the socket and what doesn't I.E when you turn on right turn signal on a 7way socket the contact on the right in the middle should blink with the turn signal and so on. Always check for a good ground on the ground wire. rv code is white wire but could be black or green. See where the wiring is tied into on the truck and check those connections. I don't think the 98's had factory towing sockets I'm not sure but hope that helps.

 

The test plug idea is the right first step. You have to know what's really happening. If the parking lights work, then at that moment you have a sufficient ground. Are the connections clean?

 

The white wire is indeed the ground, it is soldered to a ring connector which is in turn bolted to a section of the frame which was wire brushed to bare metal. After the white wire was attached the whole thing was rust-resistified with a layer of paint. I have a GREAT ground to the ground prong of both the four prong and the 7 blade RV connector.

As per my initial post, the only lights I have on the trailer or the only voltage I have at the trailer connector is for the RUNNING LIGHTS only. All lights on the truck work perfectly. I have a solid 13.68 to 14.15 volts between the ground and the running light wire. NO voltage whatsoever on the L or R turnsignals or stoplight wires. There is also NO continuity between the ground and the L/R turn/stop wiring circuit. IE NO SHORTS

Once again, I have only running lights, I have a great ground, and I have checked all the fuses that I can find. Indeed, just for giggles I changed them all out for new ones. Still only running lights.

The dealership doesn't have a clue, my manual doesn't tell me what one entire relay block with four relays in it even does, it doesn't tell me what two relays in the main engine compartment fuseblock do, and I can't even find a schematic for it or a wire layout chart to even try and figure out where the twisted mind of the designer hid what I'm trying to find.

The only thing that I can think of is that there is another fuseblock or relay that controls the lights that don't work right now. Now I just have to find it! If somebody knows what that extra relay block controls or what the two relay/fuses/circuit breakers???? are in the engine compartment fuseblock on the far RH side under the big blade fuses are????


BTW, my first degree is in Computer Hardware Engineering, (IE, I can read a schematic, etc, and I do know about wiring, grounds, continuity, ohms and all that stuff) and, I've owned Ford Trucks/Vans/Cars for 30 Years. I work as a contractor cause I like setting my own hours and I make LOTS more!
Computer geekies pay me big money to fix their houses!!!!!!!

1956 Mercury Medalist (1st car) (312 T-bird motor-dual teapot quads)
1957 Crown Vic (have always regretted selling this one)
1960 F100
1963 F100 (shoulda kept this indestructible roadbeast)
1967 F200
1967 Falcon (blame exwife #3 for me getting rid of this one too)
1969 F100
1969 E300 (owned this one for 22 years, still would if it wasn't for exwife #3)
1972 F150
1971 Crown Vic (police)
1973 Mustang (I know, I'm so sorry about that, but it was the right size and weight to reframe and shove a big block in for the 1/4 mile)
1975 F150
1989 F150
and a couple other miscelleneous Mercury's and I had a 50% interest in a Lincoln Hearse for a while

I have also owned 1 dodge, 1 nissan, and 1 mazda (the exwives cars, but I bought em, fixed em, and kept em running; tried talking them into Fords, but they didn't like what was offered those years)

 

It sounds like you have covered the right first steps. My guess is that the relays don't have anything to do with your trailer wiring, but the official schematic would seem to be the best starting point for that.

But, given the circumstances:
- blown fuse
- partially working 7-pin connector after changing fuse
I'd suspect damage to wiring between the fuse block and the 7-pin connector itself, or possibly something whacked the connector itself.

Do your 4-pin connector signals and brake lights work? If so, time to trace the wiring, or use the working 4-pin wires in the 7-pin connector if it comes down to it. One can buy an inexpensive adapter if you really still need both.

Of course, without seeing the truck itself, this may be pointless advice. Good luck,

Hank Murphy
currently truckless - previously 2005 F150 XL RCab, 1992 F150 XLT Scab, 1980 F150 RCab, 1964 F100

 

HankMurphy is right. You have done everything right. Now it's a schematic and painful inspection

 

Only the ground and the running lights work on both the four pin and seven blade RV terminals. The four pin and the seven blade RV terminals are NEW

 

Is there a tech manual available for the vehicle?

 

I do not have a tech manual, The Ford dealership would be glad to work on it at something like $70 an hour. I'd like to work on it and get $70 an hour too!

 

One or two hours of labor would pay for the manual

 

Go here for your manual:

http://www.bedlib.org/ebsco.html

Type in the word "library" for both the user name AND the password. All vehicles are listed.

 

alldatadiy.com is, in my opinion, a decent online service manual. For my truck is shows where all the fuses and relays are for the trailer plug. It costs $25 per year for the first vehicle and $15 per year for each additional vehicle. I mention it because it's immediately available. No order and wait. And it's continually updated with all TSB's and such.

My truck is newer than yours, but on mine, there is a fuse and relay for every function of the 7 way plug.

It probably doesn't help you any, but the battery charge (power), and the parking light relay's are in the central junction box and the left/right signal relays are either in auxiliary relay box 4 or 5, the manual is confused on this point. These are on the drivers side firewall near the rear.

Each of these circuits are individually fused as well, but it looks like you've already figured this. Make sure you check all the under hood fuses and circuit breakers as well as the fuses in the central junction box.

 

Have you checked the connections where the 4 and/or 7 pin connectors join the main wiring harness? Ie: get their power from the truck itself? Could be that the tee from the brake lights to the 4 pin connector is only giving you the tail and ground, but no signals. Check that connection.

 

Excellent Suggestions

 

As it turns out, there ARE fuses for the brake/turn trailer lights as well as relays.
If this information helps anybody else, here it is.

Just behind the main engine compartment fuse block there is an additional relay box. It's about two inches wide and six inches long running parallel to main fuse block. It contains five relays. The two closest to the engine are the LH and RH turn and stop Trailer light relays. about six inches closer to the firewall and two inches closer to the tire up under the hood lip there is a fuse block about 1/2 an inch wide and 3 inches long that really looks like a big wiring connector. When the covers finally come off. Guess what, there's an additional 20 amp minifuse for the stop/turn lights in it!!!!!

This information may be 98 specific, but I doubt it, as Ford tends to reuse concepts, parts, and styles for decades at a time. My owners manual did NOT have either of these relay block or fuse blocks listed. Another dealership was just as baffled and really confused when I showed the parts/service manager the relay/fuse blocks. I guess the F150 when ordered with the towing package it's added on and there's no supporting documentation provided.