I have a 2002 4.6L Lariet 4x2 estended cab with +270,000 mi

My headlights stopped working altogether (no Hi, Low or “Flash to Pass”)

Parking lamps work and wont go out until the fuse is pulled

Read a couple of forums that said the Trailer/tow relay may be the problem, but pulling those made no difference.

I replaced the Multifunction switch thinking that might be the problem, but no change



Current symptoms:

· With Key in “Run” (engine off) – Main light switch (MLS) can be moved to any position, and when returned to “Off” everything goes out (normal except headlights don’t come on)

· With engine running – MLS moved to “Park” and back to “Off” parking lights go out, MLS moved to “Head” or “Auto” and then returned to “Off” parking lights stay on until fuse #6 in Power Distribution Box (PDB) [located under the hood] is pulled.

· With engine running – Fuse #18 (Hi beam) in the Central Junction Box (CJB) under dash pulled, MLS can be moved to any position and returned to “Off” all lights go out

· 12v applied via a Power Probe to CJB Fuses #18 (hi beams), #26 & #28 (L & R lo beams) all lights work (not a bulb problem)

· Connector C205b (Main Light Switch) pins 1-6 not in use (I don’t have Fog lamps) pin 8 (from PDB fuse #3) & 10 (from PDB fuse #6) show 12v. 12v applied to Pins 11 & 12 park lamps come on, pin 9 headlamps come on. Pin 7 goes to Autolamp sensor

· Checked MLS with multi meter for continuity (got “tone” through each circuit) seems fine (not load tested)

Checked wire from MLS to Multifunction by disconnecting C202b (multifunction) and C205b (MLS) used ect3000 to check for STG….no short detected

Figured I would do the same for wire running back from C205b pin 10 to PDB fuse #6, and found that I had 12v to both sides with fuse pulled. Pulled fuse #3 and the “out” side of fuse #6 lost voltage.

So it seems I have the wire from Fuse #6 TAN/WH (parking lamps) in contact with the wire from fuse #3 DK BLU/OG (headlights) between the PDB and MLS.

I was wondering if anyone had any good “tricks” to find where they may be in contact so I can concentrate my efforts to a single area instead of having to physically trace and pull out the wire for inspection along the entire harness.

Any help/guidance would be greatly appreciated

 

The lights, as you have found out are operated by the gem, autolamps and security system .

I have had a problem for 10 years where I can't turn my lights off once they're on at night..... Occasionally it will work right...... Most of the time not. If it does work right for a couple of times switching them on and off then it stops and they just stay on. If the lights are off once I turn them on they won't turn off again, but they won't turn on by themselves unless in auto. The security system just turns them off when the vehicles turned off..... The switch won't do it. I replaced relays thought to be sticking, replace autolamp module , etc .. .nope Looking at the schematics there is NO possible way for it to be energized.....but it is. I gave up ... Only time it's a problem is it the engines are running and my lights are shining on something and I want to turn them off, in a window, etc.

Its faulty bad circuitry somewhere in the gem. I really don't think it's pinched wires or such. The schematics. .. are simplistic representations of how things like the electronic modules are SUPPOSED to work... They are not real circuit diagrams when involve these modules...a cracked solder connection produces different result, etc.

 

Update

So with the fact that it doesn't act the same when running vs not running I focused in on where power is coming from....I saw that the Autolamp sensor Fuse #29 (CJB) is "hot in Run/Start" so I decided to see if anything would change if I pulled it..
Sure enough, with the CJB fuse #29 pulled everything worked normally (except the autolamps). So my primary assumption is it will be either the sensor (Ill have to figure out how to test it), or if not that, then the headlamp relay will be my next suspect.
I have spent a week trying to figure it out, so I will have to catch up everything I put off and live with no autolamps for now.

Will send another update when I have time to investigate further.

 

Is the truck equipped with autolamps? DRL?

Sounds like you have the schematics. Correct?

 

Yeah, I have autolamps, no DRL

 

If I unplugged my auto lamp sensor I could switch the lights off./on .... But there was nothing wrong with the auto lamp sensor or module ... because I tried replacing it as a last resort even though it didn't make much sense....
that was not what was keeping my lights on..... Some other bad circuitry in gem
I'd rather have the auto lamps and live with the problem of not being able to turn them off, than have manual headlights. I actually considered wiring in a hard switch that would let me cut them off manually and positively.... But never did. It's really not a big deal.. until I'm parked with engin running and the lights shining in somebody's window or something like an ahole

 

I looked at the pin designations for (C201a) the GEM, it only shows 1 wire (pin 11) associated with the head/park lamps it is a white/red wire which is indicated in the "Warning system" schematic.
It is for the warning bell when you have left the lights on. Near as I can tell the GEM isn't otherwise part of the Head/Park lamp circut?

 

The GEM is not supposed to have anything to do with the functioning of the headlights.

Have you pulled your kick panels on both sides and checked the grounding connections? Faulty grounds can cause weird symptoms.

 

Going through your comments and the circuit:

1) CJB F18 is for the instrument illumination circuits via the PWM module in the MLS, it doesn't run any of the headlights. CJB F16 supplies the high beams AFTER the MFS.
2) BJB F03 should be feeding both pins 7 & 8 on C205b.
3) Your observation regarding measuring voltage on both F03 and F06 (both legs) is normal. You're basically measuring the open circuit voltage through the two relay (K52 & K53) coils since neither possible source of a ground signal is present.
4) Switched power from MLS C205b-7 is supposed to go to the autolamp sensor to expressly disable it (and the autolamp function) when the headlight switch is in the OFF position.