Narrowing Down Headlamp Issue - Input Needed

[NBA]

I've been in the process of trying to diagnose some problems with the car. A big one has been the headlight issue. I'll outline the very basic A/B testing I did. Unfortunately my A/B testing does show a problem, I just don't necessarily know if it is a headlamp issue, wiring issue, or both.

Preface: This vehicle has no fog lights (hoping to solve that soon'ish).

Scenario 1: Original headlights that came with the car (2018 manufacture date of car):
- low beams would work while only the passenger high beam would work and the driver side would dim lower than the low beam when high beam is engaged.
- I figured something wrong with the headlamp, and I had new ones already waiting to be intstalled.
- High beam indicator would go on/off with cycling the switch.

Scenario 2: I purchased new headlamps that included fog lights (separate units). Fog lights have remained in the box during this. I think there are some rules about posting non vendor links so we'll have to go with my description. These lights are LED lights that have a round white halo on the outer perimeter. The halo can be a DRL (one wire), and it can be an amber blinker (another wire). Neither of these wires are hooked up, so I'm only utilizing the regular high/low beam H4 3 prong plug (direct plug into my exiting headlamp harness). In this 2nd scenario, all I'm looking for is if the high and low beams work as appropriate with the new headlamps.

2A: New headlamps
- This past Saturday I attempted to install the new headlamps (direct swap, what could go wrong).
- Installed new headlamps. Passenger side kind of worked (not well) while the driver side didn't work. High beam indicator stayed on no matter cycling through the switch. Cycling did not change the intensity of the working passenger headlamp.
- High beam indicator was staying on no matter what.

2B: New headlamps
- On Sunday, I found that the high/low beam switch was partially broken. Replaced that with a new switch from Advanced Auto Parts thinking that was probably the culprit.
- After install, passenger headlamp wouldn't work and I would say the driver side also wouldn't work, except for you could see a tiny glimmer of light inside of it, but barely visible.
- High beam indicator stayed on no matter what.

Scenario 3:
3A:Tonight, swapped back to original headlamps for A/B testing
- same result as scenario 1 upon initial install
- I additionally swapped the driver side and passenger side headlamps to see if it was the headlamps or the wiring
- Result was still the same where the driver side high beam did not work despite that specific headlamp high beam working on the passenger side

3B: swapped to the new headlamps for A/B testing
- Both lights plugged in, no low beam or high beam (not even a glimmer of light in either one), high beam indicator stays on no matter the high/low beam toggle switch
- If driver light is the only light plugged in, it doesn't work, but the high/low beam indicator will cycle on off when pressed.
- If passenger light is the only light plugged in, it doesn't work, and the high beam indicator stays on inside the dash no matter how many times I press the button to cycle between high/low.

Conclusion/Result: Could be new headlamps, but could also be wiring. Fixing what appeared to be a broken high/low beam switch did not change any of the results (so even though I found it broken, it must have still been making enough contact to perform as appropriate).

I also checked the fuses under the steering column area and everything seems fine. Earlier today I ordered a new FF headlamp switch (item #12345) before doing the additional A/B testing, thinking maybe that could also be the culprit. Now, I don't think it is after testing the original headlamps.

Any ideas and opinions would be helpful. It would be great if I can solve this myself (with help) or worse case I can help a more experienced mechanic isolate this issue (along with other issues I'm having) vs. them tearing apart the entire dash or telling me to just get a new front end wiring harness and start over...

Pictures:
1. Original headlamps, low beam:
Screenshot 2025-10-23 at 9.16.21 PM.jpg

2. Original headlamps, high beam:
Screenshot 2025-10-23 at 9.16.30 PM.jpg

3. Original headlamps, low beam, swapped headlamp sides:
Screenshot 2025-10-23 at 9.16.50 PM.jpg

4. Original headlamps, high beam, swapped headlamps sides:
Screenshot 2025-10-23 at 9.16.56 PM.jpg

5. New headlamps, low and high did not turn them on:
Screenshot 2025-10-23 at 9.17.06 PM.jpg

[NBA]

I should add that one of the other electrical issues I'm having is that the driver side (left) blinker switch will not work. Passenger side does, as well as when I activate the emergency lights all front/rear blinkers will work. But nothing for front and rear left blinker when only wanting those to work.

I don't know if that would be related, but worth mentioning because of the driver side high beam issue with the original headlamps...

Maybe with the new headlamps I took the advice too literally that “any 7” headlamp will work” and these LED one’s require additional components to work? Regardless, I still clearly have an issue with the original headlamps on the driver side when going to high beams.

[edwardb]

First of all, I'm assuming you have the Ron Francis harness (always important to state when asking electrical questions) and the Factory Five Delco knock-off headlight switch. Maybe not your problem, but if that's the switch you have, I've found them to be somewhat suspect quality and typically replace with a genuine Delco D1588. About $25 on Amazon. Since you've swapped headlights without fixing the problems, I'd rule that out as the solution. LED's don't normally require anything different and should be compatible with the kit standard Halogen's which I suspect was on the car.

My recommended troubleshooting would be:

1. Battery condition. It's pretty amazing how many questions and problems are posted and it's stated the battery is "fine" when on further testing was found to be the issue. Have you tried a different battery or maybe just jumped what you have to see if anything changes?

2. Bad grounds. This also is one of the most typical problems. You should have a solid ground to the chassis from your battery. Check it. Then another ground from the wiring harness to the chassis. I typically add additional grounds at each corner. But if the other two are OK shouldn't be necessary.

3. How are the low and high beams wired? Just with a simple toggle switch? Some builds of that vintage used a somewhat complicated (my opinion) relay setup so they could be switched by pushing a button versus a toggle switch. Could be something going on there.

4. Beyond that and no problems found, time to start tracing wires, measuring voltages, buzzing out continuity, etc. The usual electrical troubleshooting.

Your turn signals and hazards are different circuits than the headlights, so in theory should not be related. But all the points above could affect those circuits too. Good luck.

[Jeff Kleiner]

Grounds, grounds, grounds. Wonky behavior like this virtually always comes back to grounding problems.

Jeff

[AC Bill]

I agree, bad grounds are often the issue, and fortunately, usually very simple to fix.
I doubt the headlights themselves are the issue, wiring and connections need close checking.

[NBA]

@Edwardb thank you for the response.

How can I tell if I have the Ron Francis harness? I have a mostly complete packing list from factory five (no invoice tho) that could help unravel some of this mystery. I'll go through the packing list again to see if I have the page regarding the wiring harnesses.

Your comment about swapping headlights and not solving the problem, so unlikely it is the headlight switch; did you mean swapping driver and passenger lights around or swapping the old lights for the new lights? Right now, I have zero headlights plugged in while I continue to troubleshoot. I was able to cancel my headlight switch order with factory five.

1. Battery is an Optima battery and I recall it having a 2021 date on it. I checked with a multimeter using the terminal posts that showed I had 12.5v with the car off and 14 or 14.5v with the car on (I'll run the test again and double check). That is about the max for my skill at the moment with a multimeter unfortunately. I have not tried a different battery (or jumped this one) as this battery is under the coolant tank and I would have to remove that, and possibly the accusump that is bolted below the coolant tank, to get access to remove the battery...unless I'm missing an easier way...

2. I'll check for these grounds. I should be able to spot the one from the battery easily, but where would/should I find the one for the wiring harness?

3. High/low beams are toggled on/off via the switch in the picture. I found that the black top of this particular switch was partially broken and so I replaced the entire switch. Unfortunately it did not solve the headlamp issue like I was hoping.
Screenshot 2025-10-24 at 1.05.48 PM.jpg

4. I've been having nightmares if I get to this point of problem solving.

@Jeff, fully agree. I've read horror stories of people taking about almost their entire car simply to find it was a random ground.

[Indy Shu]

The picture of your dimmer switch shows a ground wire connected to each mounting screw. Not sure where those grounds go, but there could be others along the tubes behind the dash or in the engine bay. For any ground connections, the powder coat should be removed to bare metal under the terminal ring otherwise your only getting ground through the screw and those aren't very reliable.

[Jeff Kleiner]

Dimmer switch doesn't require a ground; all it does is shift the blue incoming power between the red low beam and black high beam wires. You have ground wires at both headlights as well as the turn signal/park lamps. Check resistance through their plugs back to battery ground at all of those. If you find high resistance work backwards through all of the connections. By the way, the labels on the wires in your photo identify it as a Ron Francis harness. Here's info and the schematic:

https://s3.amazonaws.com/cdn.ronfran...INST-REV-U.pdf

Jeff