Low beams don't work - sometimes. Then work again. Then don't. (1 Viewer)

[NewbieMcNewster]

Alright, so electrical gremlins are way above my pay grade. I've been having an issue where occasionally my low beams won't work. The high beams still work, as do a set of auxiliary driving lights which are wired to the low beams (they have their own switch inside, but they only work if the low beams are switched on, even if the low beams themselves aren't lit at the moment).

So far I have tried replacing the headlight relay under the hood, with no success there.

This did happen last year at Christmas, for the last 50 miles of a drive from TX to CO, which sucked... At that time, a local shop replaced the lighting pigtail (I think? It was a pigtail of some sort, though I didn't get too into the details with them), and things worked after that. It has only started doing this again in the last month or two, and it's very occasional. They work most of the time, except when they suddenly don't.

Other things that might be useful to know - they don't go out while the car is running, they tend to simply decide not to work upon a given engine start. But restarting the engine a few times doesn't seem to fix it. The headlight fuses look fine to me. And I just recently installed front LED turn signal lamps, but haven't gotten around to installing the new LED flasher relay for them yet (I don't think that's important, but it's all a system, so who knows...).

Anyone got thoughts? Please note that my limited mechanical skills are even more limited when it comes to wiring stuff - I don't know crap about vehicle wiring or electrical diagrams. I just thought this might be fun and useful before going to see a mechanic, in case there was some easy fix I'm unaware of or this is a common problem that someone has posted a walk-through of before (though I haven't found other threads with this exact issue).

Thanks in advance for the collective wisdom of the forum!

 

[Malleus]

Pull the aftermarket lights off the headlight circuit.

 

[NewbieMcNewster]

Pull the aftermarket lights off the headlight circuit.

Do you mind if I ask for more detail on the theory here? The aux lights are the ones not having problems - is your thought that they are taking some of the electrical draw away from the low beams?

 

[george_tlc]

The stock headlight system is quite redundant. The left side has its own fuse, and own ground versus the right side. The low beam and high beam get the same +12V. For both left/right to fail and only the low beams, implies to me a hacked up wiring system, likely as part of the driving light setup.

It is time for you to look at the wiring diagram (from the EWD) and learn to understand it. The lines are just wires with colour code etc. This is a rather simple system and learning how to use a meter to measure voltages at various points (especially when the low beams are out) will fasttrack your repair.

We can help you understand parts of the attached drawing you don't understand. Time to learn versus a more serious fault occuring when you are far from help.

cheers,
george.

 

[Malleus]

^This

 

[NewbieMcNewster]

Thank you both for the thought and explanation, I appreciate it. It's gonna take a bit of time and work to try to get familiar with all of that to see if I think I could try to diagnose and fix this myself. In the meantime though, do you have any thoughts on why they would only stop working for a while and then be alright again a couple days later? We've established that I don't really know what I'm talking about, but to me that would imply that something is loose and intermittently disconnecting before getting jostled back to connection later...

 

[unclegit]

I just had this occur yesterday and will be digging into it this weekend.

My first though is a relay going bad or potentially a loose plug.

After a quick look at that EWD I don't see a way for low beams to be off and high beams on if the wiring is all as described - which is what's happening on my truck - very odd...

I'll revert to stock wiring and see what happens.

 

[clx16]

Intermittent suggests something is changing. The system is insulated very well by the factory to avoid changes in the path electricity can follow. I am not anti after market but If something was added and care was not taken to keep that system both sealed and also from adding additional flex, it will be part of the failure rate. This is why removing the after market stuff will help you put your hands on the statistically most likely location of fault.

If your aftermarket lights are just using the headlights as a signal for a relay (that is getting separate power on an individual circuit) then i would look at where the relay was spliced in to get the signal. This is what i think you described of your circuit above. If this is your design, then you are getting your signal from either the lowbeam signal wire that triggers your factory headlight relay, or you are getting your signal from the main relayed power to trigger your after market relay for the after market lights. You need to figure that out if you are triggering before your factory relay or after....

If it is before your trigger for lowbeams (from the switch in your vehicle) may be loosing too much TO trigger the relay for your aftermarket lights and not enough to juice the factory relay to turn on your headlights. This would point to general load as an indicator and possible shorts/moisture dragging down the current.

Relays let you use less current to trigger bigger currents, so the less current wires get overwhelmed easier.

If you are running the after market lights without a relay that is forcing the headlights to share the load (this is NOT ideal) then the fault could be at the splice but it could also be too much load for an older wire and you have conductivity issues.

Intermittent activity can be based on humidity in the air, vibrations or the over all load you have on the whole system.

 

[jonheld]

Thank you both for the thought and explanation, I appreciate it. It's gonna take a bit of time and work to try to get familiar with all of that to see if I think I could try to diagnose and fix this myself. In the meantime though, do you have any thoughts on why they would only stop working for a while and then be alright again a couple days later? We've established that I don't really know what I'm talking about, but to me that would imply that something is loose and intermittently disconnecting before getting jostled back to connection later...

As you can see from the schematic that George was kind enough to post, both low beam and high beam bulbs are fed from the same +12 source. If the lows don't work, but the highs do, then there is an interruption in either the ground path from the low beams or the +12 to the low beams. If the aux lamps were installed by "unskilled labor" then that could very well be your issue. I would reverse engineer what the installer did as a very first step and repair whatever damage he/she/it might have caused.

 

[LandLocked93]

Do you mind if I ask for more detail on the theory here? The aux lights are the ones not having problems - is your thought that they are taking some of the electrical draw away from the low beams?

The theory here is that all "user add-ons" are wired and fused separate and independent from any and all factory wiring.
It ensures you get home if any of the 'bright ideas' turn out to be shyte in plastic cladding.

 

[unclegit]

Just look at mine and I'm pretty sure it's a bad relay - I'm using the Slee harness rather than the stock setup andI'm getting power from the main harness into the slee harness. 90% sure it's a dead relay (it's been in there 15+ years now)

Do you know what additional wiring your guy put in - are there 2 relays on the drivers side fender ?

Slee also provides an aux light harness that mates to this on to power the driving lights - all without chopping the existing harness and powering the additional lights the right way - fused and directly from the battery vs forcing more current down the existing wires which were not designed to take the extra amps.

https://sleeoffroad.com/wp-content/uploads/2019/06/PUB-headlight_upgrade_harness.pdf

 

[Heckraiser]

I had a problem like this with my headlights a while back. Intermittent and seemingly random. I bought the truck with a slee headlight harness. I pulled the slee harness and went back to stock, never had an issue again. Like others have said, best to leave the factory wiring alone and wire any accessories separately.

 

[Tedward]

I had a problem like this with my headlights a while back.
I pulled the Slee harness I installed and went back to stock, never had an issue again.

 

[Wrencher93]

I have had similar issues but mine ended up being a relay and then, more recently, a bad connector plug. Check everything for basic connections before you go yanking stuff out of your truck....or don't. Up to you.

 

[unclegit]

I ended up revering to stock harness today - 2 of the relays tested bad - so I'll replace them with potted / waterproof ones and double check the connections and hook it up again next week once the relays arrive.

It's pretty easy to trace with a multimeter.