Headlight wiring (FG639)

wakked1

New member
Hey all, in the process of upgrading the (pretty awful) stock seal halogens on our rig. I bought a set of drop-in replacement LEDs which are the normal H4 positive-switched wiring pattern (each light supports both a low and high beam). I'm trying to figure out two things:

I can setup the (inner) high beams for now with just the stock wiring, I just don't the benefit of the low beam pattern. I could solve that with a relay and some hackery, so that might be worth it.

The bigger problem is the low beams that have me very confused. In low, both high and low circuits are hot. In high beam, only low is hot but with inverted polarity (which I guess works with halogens, not so much with LEDs). At first I had assumed it perhaps was a negative switched system but then I wouldn't have two hot and one ground in the low beam setting, right? Its almost as though something is mis-wired but yet it all looks stock to me, I doubt the lights have ever been upgraded.

Anyone else run into this? Hoping its something my crap EE "skills" can handle.
 

kerry

Expedition Leader
From the shop manual
 

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SkiFreak

Crazy Person
I cannot speak directly to the wiring on a FG639, but on the FG84/FG140, the majority of the electrical system is negatively switched.
When I added my driving lights, I did it this way, if this is of any help.

Negative_Switching.jpg
 

wakked1

New member
@SkiFreak thanks a lot for your diagram, that's really helpful. Just read your blog entry on this too. For a negatively switched system, would you see polarity reverse though? I thought the positive would stay hot and just the negatives would alternate. But in my case I have two positives (high+low) and one ground in low beam, and just the low with reverse polarity on high (on the low beam harness, the high beam is simply hot or not as you would expect).

@kerry appreciate you posting the stock diagram, though I'm struggling to understand it honestly. From what I can tell though, both high beams (dedicated and high/low) are basically one circuit and the low is triggered separately, which would make sense.

So what doesn't make sense is what I'm actually seeing with the combo HIGH/LOW circuit...
When high beam switch is OFF:
dedicated high beam is OFF
combo both HIGH and LOW beams circuits are ON

When high beam switch is ON:
dedicated high beam is ON
combo HIGH OFF, LOW ON with reversed polarity

Is it possible someone juryrigged the setup to try to get more current through the low beams when high beam switch is off? If so where would I even look.. at the relays maybe? So much for a straightforward upgrade... :)
 

wakked1

New member
I've attached pictures to show the current for the two circuits and switch states for the outer (low/high) lights.

The weird, net result is that both high and low beams on when the high beam switch is OFF and neither are (since one has reverse polarity and the other no current) when the high beam is ON.

P.S. After taking these pictures I realized the fuse on that side was going bad and I replaced it, hence the low voltage.. but it didn't change the net result.

fuso high beam.pngfuso low beam.png
 

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