Rear fog wiring confusion...

hillstrubl

Founding Member
Ever since I got my 110 about a year ago, the rear fog hasn't worked. I never looked into it until today when I assumed it was just a bad bulb. When I pulled the lens cover the bulb was fine and the connection surfaces were good enough. Here's where I'm confused.
A test light illuminates if:
a) the running lights are on
b) I connect the black wire to a known good ground (pin 3 on my 7-pin euro trailer plug)

The rear fog switch's position doesn't seem to matter and if it is switched on (along with running lights) If I bridge the red/orange + black wires, no test light.

Is it possible that the running light is providing current via black, and the red/orange is actually the ground (and is bad)? Maybe that means the rear fog switch is bad too?

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JimC

Super Moderator
Staff member
I‘m having a Little trouble interpreting everything here, but looking at the age of your wiring, it probably runs up to bullet connectors in the rear corner where anyone could have jammed those connectors in the wrong sockets at any time during the last 30 years. I’d do the tests at the bullets before worrying about the light fixture.
 

hillstrubl

Founding Member
I‘m having a Little trouble interpreting everything here, but looking at the age of your wiring, it probably runs up to bullet connectors in the rear corner where anyone could have jammed those connectors in the wrong sockets at any time during the last 30 years. I’d do the tests at the bullets before worrying about the light fixture.
Thanks Jim, I'll take that apart next and report back.

I'm going to check connectivity at the switch as well.
 

Napalm00

Technical Excellence Contributor
What year is your truck number one. With that information we can figure out where your power feed is for your fog lamp switch. Always start at the source of the power and then work your way towards the end of the circuit.
 

jymmiejamz

Founding Member
Callsign: KN4JHI
“Is it possible that the running light is providing current”

Anything is possible when you have trailer wiring that is almost always badly installed.

do you have a volt meter?
 

hillstrubl

Founding Member
“Is it possible that the running light is providing current”

Anything is possible when you have trailer wiring that is almost always badly installed.

do you have a volt meter?
I do, the voltage when it was working (running lights on, black wire on rear fog housing, using trailer's ground) was just shy of 12v (its analog)
 

hillstrubl

Founding Member
What year is your truck number one. With that information we can figure out where your power feed is for your fog lamp switch. Always start at the source of the power and then work your way towards the end of the circuit.
Its a 95, UK market truck
 

Napalm00

Technical Excellence Contributor
I do, the voltage when it was working (running lights on, black wire on rear fog housing, using trailer's ground) was just shy of 12v (its analog)
If you're getting voltage on the black wire something is definitely fucked up. I would work your way back from that wire in that case. If you do have trailer wiring agreed that it is usually suspect
 

John Z

Well-known member
Check the wiring from the inside of the car. If you need I'll take a pic of the wires on my 95 300tdi
 

jymmiejamz

Founding Member
Callsign: KN4JHI
If you are measuring voltage from the ground wire to ground and seeing 12V, you have 12V of voltage drop which is an open circuit
 
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