Headlights low setting not working with headlights switched on.

I am having some trouble with my 1996 K1500 Diesel Suburbans Headlights.
When the light switch is on the marker light setting my headlights are on low setting for daytime running the way they should be. But when I turn the the headlight switch to full on (headlight low light beam)on the headlight low setting headlights will not come on. Marker lights are all on but no low headlight.
If I put the headlights onto high beam setting the high setting works ok.

I have the high beam headlight set up with the modification to bring on the low beam setting at the same time as the high bulb so I know that the low beam light bulbs are good.
The modification to allow the low and high headlight to work at the same time was done many years ago when the truck and me were still living in Scotland across the pond and has never given any trouble (composite headlights are on this truck).

If anyone has had this problem and found the cause I would be grateful if you would let me know what might be causing it before I start chasing my tail :).
Thank you Jim Twaddle, Alberta, Canada

The DRL requirements of Canadian vehicles are different than the U.S., but I don't see how it would create a different situation with this type of failure. I believe the Canadian solution to the requirement simply omits the "override" feature available on U.S. vehicles. Please correct me if I'm misunderstanding this.

The "all 4 on" mod should work independently from the (OEM) low beam circuit. The harness (almost always, unless relays are used, or a shade-tree hack), merely has a diode between them to feed the low beam when high beam is powered, but doesn't allow the low beam circuit to feed the high beam. Likely, the problem you are having is related to the low beam circuit, independently. This could be the dimmer switch, or any connection or wire in the circuit. I would look closely at the mod, how it is wired, and start by chasing that circuit, connection to connection.

I don't think the modification is causing the low beam on only failure and I agree the fault is in the low beam circuit, just mentioned it to say that the low beam light unit was working so it is not the light bulbs that have failed. I was just asking just in case anyone had ever had a low beam failure and maybe there was a common cause of failure. I will need to get an accurate wiring diagram as the only manual I have is a Haynes manual but there is no schematic covering the 96 models headlight circuit. Thank you

I know on the Toyotas the fault is typically in the switch. The low beams fail, but high beams work and bulbs work when checked independently. Sounds similar here - that the low beams will work when they’re fed from sources other than the low beam setting.

Thanks.
I am inclined to agree with you. I messed around for a while today trying to get it sorted out enough to keep the truck mobile. Yesterday I could get the low beam lights to work if the light switch was turned to marker light setting and when driving the truck the low beam head lights would come on but as soon as the switch was turned to the head light on they would not work although hi beam works ok.
Today the lights would behave as they did yesterday to begin with, as time went on by late morning I could not get the headlight low beam to work at all. Bulbs will work if I run a wire from the battery to them so I will try and get a chance to see if can rig something maybe using another switch as a temporary set up just now. I do not want to mess about pulling the dashboard apart right now as I need the truck this week. :) ho hum!!!

I finally got a chance to get the intermittent headlight lights fixed after having one morning run into work where the low beam failed again leaving with the only option of driving into work on a dark morning using high beams to see where I was going.
After doing some more checking now that the low beam headlight failure was a permanent fault I found that there was major voltage drops when circuit load tests were carried out on the circuit. I decided to wire in some heavier cables using the batteries as a main feed directly to the head lights (low and main beams) via relays. I also installed a new dash mounted light switch (just in case). After getting the relay modification finished I took the Suburban a drive down the range road past our property and everything seemed to be ok, lights are staying "ON". just as I got back to the workshop the engine quits just as I am about to stop.
Fuel lift pump is not running. While working on this new test of patience I find that the lift pump is no longer pumping although there is voltage to the pump. I also tried wiring the lift pump directly to a battery, pump still not pumping. Install a new pump, try to bleed air out of the system, more air than fuel bubbling out of the filter head bleeder. Connect up clear plastic hose to the suction pipe feeding the lift pump directly from the tank, still drawing air into the system. Watch this space and I will try and get another chance to figure out where the ingress of air in the fuel system is coming from. I am hoping it is nothing more than a stone damaged pipe and not a problem in the suction side in the fuel tank.
Oh! by the way. The headlights are still working ok and way brighter than they have for ages now they are getting their supply directly from the batteries :) .

Something else you might want to do is set up your relays so when you turn on bights the low beams stay on too. My 94 had plastic housings and running a while in the snow on low beams was fine. But when I got out of town and hit the high beams they went dark. They were so cool they had iced over. At least with low and high working together it provides light while the high beams warm up and melt the ice.

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Originally Posted by a5150nut

Something else you might want to do is set up your relays so when you turn on bights the low beams stay on too. My 94 had plastic housings and running a while in the snow on low beams was fine. But when I got out of town and hit the high beams they went dark. They were so cool they had iced over. At least with low and high working together it provides light while the high beams warm up and melt the ice.

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Good idea. All that's needed is a diode between the high and low switched wires ahead of the relays.