Bizarre trailer wiring problem...(NOT ground related!)

ElAviator72

Original poster
Member
Jan 11, 2012
118
About a month ago, I was towing my utility trailer at night, and by the time we got home, I had no running lights on the trailer (but the brake lamps still worked on both sides). I checked the fuse box in the truck, and fuse #42 was blown (trailer parking lights).

The trailer has a 4 pin flat connector on it (converted to that by me).

An ohmmeter was revealing zero resistance from ground to any of the other three pins (brown, green, and yellow).

So I jacked up the trailer, and started inspecting wires. The left tail lamp (yellow) and left running light (brown) wires were both chafed through where they go through a hole drilled in the frame. Then I snipped that wire where it connects to the central harness (going down the middle of the trailer, and back at the brake lights, it is running along the wood floor via staples, so no place for it to ground out). I checked out all places where the wiring harness was running inside the metal channel for the tongue, and it is well insulated.

With that wire pair snipped, the ohmmeter was reading a more believable 4.7 kOhms between ground and brown, or ground and green, and infinite between ground and yellow.

Then I replaced the wire pair that I snipped, and put a rubber grommet in both sides of the hole through the frame where the brake/tail light wires pass to the left light. Now I'm back to ground to any other pin reading zero ohms. (assuming it is a short), A short inside the left hand light assembly shouldn't have any effect on the restistance from the right hand brake light wire to ground, but it does(!). The trailer only has two brake light assemblies (with the running light sub-function), no other lights (not even the required DOT amber forward markers-I was going to add those later).

Anyone have any idea what's going on here? I'm at whits end trying to think like a lazy electron...:redface: Haven't tried hooking it up to the truck again yet.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Most home voltmeters can't resolve the low resistance of a cold incandescent lamp. Tungsten's resistance rises as it gets hot - the inrush current of a lamp due to the initial low resistance is why lamps most often fail when turned on, not while they're running.

Take a spare lamp and try to measure its resistance with a meter - you might find it's reading the same as or very close to what it reads when you short out your leads.

Better is to use a battery (with a fuse) to test the wiring. You can also use a battery without a fuse - the trailer wiring will arc and spark, but it will lead you to the fault very quickly. :biggrin:
 

Black LT

Member
Dec 4, 2011
211
Probably a ground issue. (I couldn't resist!!) :biggrin:

OK, I read your post and admit I got lost once or twice. But, are you sure your light assemblies are grounded - or is the ground backfeeding (or whatever you would call it)?

Take the bulbs out and check the voltage back there - make sure the correct wire (and only that wire) has voltage for the function being tested.

:undecided:
 

ElAviator72

Original poster
Member
Jan 11, 2012
118
Black LT said:
OK, I read your post and admit I got lost once or twice. But, are you sure your light assemblies are grounded - or is the ground backfeeding (or whatever you would call it)?

:undecided:

Well, the white wire on both light assemblies bolts to the frame of the trailer...I understand that that is grounded on trailer wiring :thumbsup: I have to admit, though, I haven't taken the light assemblies apart yet to examine the innards...a little afraid to. Seems the deeper I dig on this trailer, the more I find wrong with it :sadcry: I've easily doubled what I paid for it just trying to keep it on the road.
 

ElAviator72

Original poster
Member
Jan 11, 2012
118
kenmyfam said:
OK, 3 questions.
Did it used to be OK ???
Have you tried the trailer into a different (known good) tow vehicle ??
Have you tried the tow vehicle into a different (known good) trailer ??

Yup, it used to be okay...
Haven't let anyone else borrow the trailer :wink:
The TB was fine towing a U-haul trailer with the same 4 pin adapter that I use for my trailer (well, once I replaced the blown running light fuse :crazy: ).

Haven't been brave enough to plug the trailer into the truck again yet. But, now I need to. I just loaded it up with yard debris this weekend and I need to run it to my buddy's compost pile at his farm. Worst case, I blow the running light fuse again...
 

RayVoy

Member
Nov 20, 2011
939
ElAviator72 said:
Worst case, I blow the running light fuse again...
It probably will, these things seldom fix themselves. Usually a fuse blows when there is a short to ground, inspect the running light wire along the length of the trailer.
If nothing, remove the lens and remove the bulbs, there may be internal corrosion inside the lamp socket.
 

bore_pig

Member
Nov 25, 2011
113
Do yourself a favor and buy one of these. Four-Way Trailer Light Tester Makes testing the tow vehicle very fast and easy.

You could very well have fixed the problem already when you repaired the two wires that were worn through. One more thing, a problem on the left side can most definitely affect the right side if the tail light circuit is involved.
 

ElAviator72

Original poster
Member
Jan 11, 2012
118
bore_pig said:
Do yourself a favor and buy one of these. Four-Way Trailer Light Tester Makes testing the tow vehicle very fast and easy.

I actually have one. That's how I discovered the fuse was blown :wink:

Seems to be better now, I used the trailer last weekend, and no problems. Think I solved the problem. Just wish my $89 Fluke could actually register some resistance going through a 12 volt bulb...but maybe you have to heat the filament up before it would.
:crazy:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
ElAviator72 said:
Just wish my $89 Fluke could actually register some resistance going through a 12 volt bulb...but maybe you have to heat the filament up before it would.
:crazy:
A typical 1157 old lamp I learned was around 2A, or 6 Ohms, while running. Rule of thumb is cold tungsten filaments are 15 times less resistive than while running, so that would imply about 400 milliOhms (0.4 Ohms) while cold. Very difficult for hand held meters under about $200 to resolve that low a resistance. At work I have a few 4-wire meters that can resolve 10 microOhms, so let me try some lamps on one of those meters. You have me curious now about our modern lamps.
 

meerschm

Member
Aug 26, 2012
1,079
the roadie said:
A typical 1157 old lamp I learned was around 2A, or 6 Ohms, while running. Rule of thumb is cold tungsten filaments are 15 times less resistive than while running, so that would imply about 400 milliOhms (0.4 Ohms) while cold. Very difficult for hand held meters under about $200 to resolve that low a resistance. At work I have a few 4-wire meters that can resolve 10 microOhms, so let me try some lamps on one of those meters. You have me curious now about our modern lamps.

would be fun to run this lab (or find an A+ report from a student)

http://www.colorado.edu/physics/phys1140/phys1140_sp05/Experiments/E1_Spring_2005.pdf
 

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