Good Afternoon All,

Long time reader - first time poster. I scoured the threads and did not find a solution. If it has already been addressed, I apologize and accept the wrath of the community.

I have an 02 F150 supercrew I have owned for about 10 years. A few years ago I inherited a 1987 model basstracker and trailer. Up until now, the lights have worked. When they didn't, cleaning the plugs took care of the problem. This time; however, it didn't fix the issue. The entire right side would not work.

I'm a pretty good part changer but terrible at diagnosing. Because the trailer lights were old and showed considerable corrosion, I decided to completely re-wire the trailer and replace all lights. Upon completion, plugged it in to the truck and.....nothing. I mean nothing.

Because I'm smart (usually after the fact) I checked the fuses. I found 5 and 20 blown under the hood. I replaced the fuses and tried it again. Both fuses blew again.

Because I'm not smart, instead of further troubleshooting I plugged the trailer into a 1991 Ford Taurus we have. Guess what.......lights work fine.

This is what I found when I tested the truck plug.

Engine off - keys not in ignition - Ohm:

Ground: Plug ground to truck frame 0
Tail: Tail terminal to truck frame 1
Lt: Lt terminal to truck frame 0
Rt: Rt terminal to truck frame 0

Engine off - key in acc position - Volt:

Brake applied: Plug ground to Tail terminal -0
Running lights: Plug ground to tail terminal - 11.94
Left turn signal: Plug ground to Lt terminal - pulses but voltage varies
Right turn signal: Plug ground to Rt terminal - pulses but voltage varies

Without plugging in the trailer fuse 5 & 20 do not blow. As far as I can tell, 5, 19, 20 are the fuses that control the trailer wiring. 19 has never blown.

There is no visible damage to the truck plug. This seems like a short to me?

Any thoughts? Prayers?

 

Well the logic of it as you report this is;
Trailer lights work ok plugged into another truck but blows fuses 5 and 20 on yours tells me you have a problem at your trucks connector.
Sounds like it's been worked on one to many times.
What do ya think?
Good luck.

 

More likely it's a problem with the trailer re-wire or a defective plug on the trailer, or some other fault that hasn't been checked.

First off, are you using the four pin plug on the truck directly or are you using the 7-wire plug with a 7-4 adapter?

Your voltage readings at the truck connector are as expected but the resistance readings are wrong and the posted readings are mutually exclusive. I suspect multimeter misuse or misinterpretation.

Engine off - key in acc position - Volt:

Brake applied: Plug ground to Tail terminal -0 [NORMAL]
Running lights: Plug ground to tail terminal - 11.94 [NORMAL]
Left turn signal: Plug ground to Lt terminal - pulses but voltage varies [NORMAL, SEE BELOW]
Right turn signal: Plug ground to Rt terminal - pulses but voltage varies [NORMAL, SEE BELOW]

[You should see the voltage pulse from 0 to 12 volts and back. A digital meter may make this difficult to discern, so be patient and "average" a number of readings. A simple test lamp or an analog meter would be more useful.]


It might still be the vehicle connector. Corrosion, if present, can cause some weird faults.


Time to test the trailer if you haven't found a problem so far.

Take your multimeter to the trailer. Set it on the lowest ohm reading. All measurements are made from the four wire plug.

1) Measure the white wire to the trailer chassis (bare metal). You should read zero ohms (direct short) since this is the trailer ground connection.

2) Measure from the white wire at the connector to the brown wire at the connector (running lamps). You should read a very low resistance but not zero as you're testing through the running lamp filaments plus any marker lamps.

3) Repeat measurement from white wire to the yellow wire (left turn/brake lamp). You should read a low, non-zero value.

4) Repeat measurement from white wire to the dark green wire (right turn/brake lamp). You should read a low, non-zero value.

For tests 2, 3, or 4, remove bulbs in that circuit and the reading should go to an open circuit or infinite reading. If it does not, the socket is shorted or the wiring insulation is chafed and is causing a short-to-chassis. Drilled holes where the wire feeds through a frame rail or where the wire passes under a fixture are the most common areas where wires may be shorted.

 

Operator error is always a possibility.
My multi-tester has the following Ohm settings: 2000k, 200k, 20k, 2000, 200. Per your instructions I used the 200 setting. However, when testing the unit by touching the leads together, I got .5 reading.
Trailer test:
Ground (plug to frame): .5
Brown: 1.5
Yellow: 1.1
Green: 1.1

With both bulbs removed from the rear lights (I left the bulb in place for the yellow marker light mid trailer):
Brown: 1 (no reading or change)
Yellow: 1 (no reading or change)
Green: 1 (no reading or change)

I next tested the truck plug again. This time with a simple circuit tester. The alligator clip was placed on the ground terminal of the plug. Test conducted with engine off and ignition on. Results:

Brown (Brake): Nothing
Brown (Running lights): Circuit good
Yellow (turn signal): Pulses
Green (turn signal): Pulses

Next thoughts?

 

The readings for the trailer are wrong. With the bulbs pulled, the meter should read open, the same as if you were just holding the meter leads without them touching anything. What does the display read then? With the bulbs removed, the readings should have gone up to infinite (except the one with the remaining bulb in-circuit).

Your truck plug test indicates a "Pass".

 

I guess I'm not understanding or explaining myself correctly. The meter displays the number 1 at the extreme far left of the display when the leads are not touching. When I touch them together, the 1 disappears and I get a reading of 0.5 on the extreme right of the display. With the bulbs pulled, I get the same readings (a 1 at the extreme left of the display) as when the leads are not touching.

 

Okay, that's just the way that your particular meter represents an open circuit.

Based on all those readings, the truck and the trailer individually test good. Therefore, there has to be a problem like Bluegrass suggested, at the connectors only when mated. I'd probably replace the connector on the truck.

 

Electrical issues really confuse me. It seems strange it can test correctly at both connectors but still have a problem when the connectors are mated. I will replace the connector at the truck and report back.

 

Okay, changed out the plug at the truck today. I also bought a tester that plugs into the plug and has LED lights for the tail, rt & lt terminals. The only problem resulted when the truck park lights were turned on. The tail terminal showed power; however, when I tried the turn signals they would not work. They do when the park lights are not on. It would appear to me that the problem may be in a connector somewhere else?

Unless anyone has other ideas, I'll probably go to my Ford dealer and see about getting a replacement wiring harness.

 

Have you added any outher light's to the truck that are tyed in to the tail lights. or backup cammera?

 

It's factory all of the way. I've not added anything to it.

 

Is the plug on the truck spliced into the tail light wire ? Or does your truck have the seprate harness for the trailer circet.

 

It looks to me like it is a separate wiring harness. I believe it is factory. I have not been able to go to my local Ford dealership to talk to them about the harness - work has been in the way. I will update when I do.

 

Okay, I finally had time to really look into the problem again and solved the problem. Ultimately, I'm an idiot - but here's what happened. Hope it helps someone else.

I have a pigtail that I use to complete the connection between the trailer and the truck as the trailer wiring is too short. When the problem originally presented itself, I checked the pigtail but found no problems. Part of checking it involved taking it apart (I made it from two different plugs). I wound up re-wiring the trailer but it still didn't work. I found I was blowing fuses....

I couldn't see the forest for the trees. I looked at the pigtail again and realized I had a short due to me putting it back together incorrectly..... That explains why the trailer wiring tested okay and my truck wiring tested okay.

The answer was so simple I couldn't see it.

Thank you to all of you who posted and tried to help me. I did learn a lot from your troubleshooting tips.

 

Glad you found it. Always the simple things that get over looked.