One headlight works OK and three are about 5%

[matrophy]

I sure could use some insight on what might cause this. I have taken apart and cleaned the combo switch and the flasher switch twice in the past two weeks so I'm pretty sure for once that's not the cause. Fuses are good and the ground in the inner fender well passenger side is cleaned and retightened. Except for the license plate light all of the other lights are decently bright making me think that corrosion in one of the other lights (for once) isn't the cause.

[SamL]

I hear you from someone who've been dealing with lighting problems since I've bought my truck. Switch worked initially, but strange behaviors pop up after putting it back to factory. But I know my problem is more functional than anything else. I had it working properly until I switched the in-line fuse to positive side (factory) from negative side.

 

What I would do from easiest to hardest:

1. Check battery.

2. Check alternator. 

3. Using a DMM (digital multimeter) check continuity across last two fuses in fuse block in drivers side. Should be 10A each. Then check continuity from switch to lower end of fuse and check switch to top end of fuse.

4. Check voltages across low beams. On drivers side its red/yellow and red/blue wire. On passengers side its red/yellow and red/brown.

5. Turn engine on and check how much voltage drops.

6. Check voltage across high beams.

7. Check all grounds.

8. Check in-line fuse to battery.

 

If voltages are okay, I would replace bulbs first. If not, then:

9. Replace lighting relay above fuse block (black).

10. Combo switch. The lighting switch is located on the drivers side on the back of the  combo switch. There are 4 (or 5?) copper strips which makes various contacts depending on if switch is in hi/low/pass. Do NOT take it off. Mine had built up grime between contacts. Take contact cleaner and spray and spray it again.

 

11. Check continuity between lighting switch and the gauge cluster (high beam indicator lamp). Make sure cluster has no broken contacts.

 

If problems still exist, you probably have a short somewhere. Clean all contacts with appropriate cleaner, check parking lights or all lights that comes on when beams are engaged and lastly check wire/wire bundle integrity. 

[bottomwatcher]

Mine was doing this and it was dirty relay receptacle. Swap the relays around and see if anything improves.

[datzenmike]

When the headlights are turned on, power is supplied to left and right side hi and low lamps from those two fuses. Power to the right side (hi and low) is on the Red/Blue wire and power to the left side (hi and low) is on the Red/Blue wires. Hi and low is selected by the hi beam relay which provides the ground to all hi or all low beams.

 

 

 

1/  First is the lighting switch. Try pulling the headlight stalk towards you to the 'flash to pass' position. If all headlights work, then fuses and grounds are obviously working in this position and there is something wrong inside the headlamp switch. Contacts with dirty grease... try flushing with electrical contact cleaner. The 720 is problematic for dirty contacts in the headlamp switch

 

2/  Next, check for power on the Red/Black and the Red/Blue at the headlamps, with headlamps on. If missing power on the right then check the far right 10 amp fuse in the box is functional.

[Thomas Perkins]

I replaced all 4 of my head light connectors years ago.All 4 were dry rotted and corroded.I posted here.Here are some pictures.I got mine from Auto Zone.2 come in a pack.See the dry rotted ones.See the date on my pictures when I did it.

Edited Thursday at 03:23 PM by Thomas Perkins

[matrophy]

One thing I noticed while checking the fuses is that with 1 of the 2 headlight fuses removed from the fuse box (I don't remember which one), the headlights were dark. If the other fuse was removed and the original fuse was put back, the lights shone like in the pictures. Is that normal?

Edited Thursday at 05:09 PM by matrophy

[Thomas Perkins]

why is one of my headlight working good but the other three are dim on my 85 nissan 720 +8 When one headlight works perfectly but the other three are dim, it indicates a voltage drop in the circuit rather than a completely blown bulb. Your 1985 Nissan 720 uses a dual rectangular, 4-lamp headlight system.Diagnose this specific issue by checking the following components:Headlight Connectors: The 3-prong (H4) harness plugs are prone to corrosion and melting due to age. If the connector for the dim headlights has high resistance, it won't pass enough current, starving the bulbs of power. Unplug the lights and inspect for green corrosion or burnt pins.The Ground Circuit: Unlike most vehicles, the Nissan 720 headlight circuit routes grounds through a shared relay system rather than body grounds at the radiator support. If a ground wire is corroded or loose at the relay or near the steering column, it causes the other lights to glow dimly.Headlight Switch: The factory multi-function switch under the dashboard builds up resistance over 4 decades of use. Power for each side of the truck or each beam type may be dropping across pitted contacts inside the switch.Fusible Links: Check the fusible links located on the positive battery terminal. The green or black links protect the lighting circuits; if one is partially burned or corroded, it can cause the exact low-voltage, dimming issue you are experiencing.If you would like, let me know:Is the dimming happening only on low beams, only on high beams, or both?Are the bulbs halogen or have they been converted to LED?I can help you pinpoint exactly where the voltage is being lost.

[Thomas Perkins]

Replace all 4 of your head light connectors like me big guy.Probalbly are corroded.See what mine looked like in the pictures.The rubber on them were dry rotted so bad that they were getting wet.Take them off your headlights and see.Also you may need the big round pieces that keeps water from getting on them.2 of mine were bad and I found some good used ones from Ebay.See my big round pieces in my pictures.Time for a upgrade.I may have a extra good one.

Edited Thursday at 07:49 PM by Thomas Perkins

[Thomas Perkins]

Their are some on Ebay right now.

Edited Thursday at 07:49 PM by Thomas Perkins

[matrophy]

Thanks Thomas, It wouldn't surprise me if they were corroded. It's one of the few things I haven't replaced.

[datzenmike]

10 hours ago, matrophy said:

One thing I noticed while checking the fuses is that with 1 of the 2 headlight fuses removed from the fuse box (I don't remember which one), the headlights were dark. If the other fuse was removed and the original fuse was put back, the lights shone like in the pictures. Is that normal?

 

Remember ALL headlamps are powered when the headlamps are turned on. It's the ground that completes the circuit and turns them on. Ground is selected by the headlamp relay, either hi or low. 

 

Replacing the headlamp plugs is all well and good but if it's a bad fuse or dirty headlight switch that's a lot of work to not fix the real problem. Ask yourself: what are the chances that 2 or more plugs decide to go bad at the same time???? Identify the problem... fix the problem once.

[matrophy]

Like every good 720 owner, I have  bag of guts from various combo switches in my collection and I swapped the contact section tonight and will see if that was the problem. I have a theory that I have roughed up the contacts so much on the original that they might not be making good contact. I didn't see any corrosion on the contacts of the relay like was suggested. I looked at my headlight connectors and they are intact, not really deteriorated and not corroded so I'm going to say that they are OK. The fusible link didn't feel crunchy but I'll do a continuity check tomorrow. Hoping it's the combo switch.  

 

 

 

 

 

[bottomwatcher]

12V 90% of problems=Bad grounds. 

[datzenmike]

Ground for all lamps is through the headlight relay. One headlight works so the ground is there.

 

 

On 6/4/2026 at 5:12 AM, datzenmike said:

When the headlights are turned on, power is supplied to left and right side hi and low lamps from those two fuses. Power to the right side (hi and low) is on the Red/Blue wire and power to the left side (hi and low) is on the Red/Blue wires. Hi and low is selected by the hi beam relay which provides the ground to all hi or all low beams.

 

 

 

1/  First is the lighting switch. Try pulling the headlight stalk towards you to the 'flash to pass' position. If all headlights work, then fuses and grounds are obviously working in this position and there is something wrong inside the headlamp switch. Contacts with dirty grease... try flushing with electrical contact cleaner. The 720 is problematic for dirty contacts in the headlamp switch

 

2/  Next, check for power on the Red/Black and the Red/Blue at the headlamps, with headlamps on. If missing power on the right then check the far right 10 amp fuse in the box is functional.

 

Do the work to identify where the problem is. 

[matrophy]

I had a little time today to do some troubleshooting.

 

With low beams on, the low beam on the driver's side is lit (not extremely brightly) and the remaining 3 bulbs glow at around 5%. Activating the high beams shows no change except that the high beam on driver's side is lit and the remaing 3 are glowing at 5%.

 

The high beam indicator on the cluster comes on with the combo switch stalk switch.

 

No sure if it is significant but unplugging any of the glowing bulbs causes all 3 to stop glowing and the driver's side low beam to shine more brightly.

 

I'll check the voltage to the headlight bulbs tomorrow with a multimeter.

 

The headlight relay clicks when the combo stalk switch is activated.

 

I reinstalled my combo switch that I took apart and cleaned for the 3rd time and the marker lights and associated lights on that circuit, and cluster illumination worked for a few minutes and then stopped working.

 

I spent what limited time I had today checking tha fuses -  all of which are good. I cleaned and retightened the ground in the engine bay and took apart the tail light assemblies and checked for corrosion of which there was none. 

 

I'm not sure how to test the combo switch but that seems like a logical next step.

 

[Thomas Perkins]

Don't want to buy new headlight connectors Did you check them?Or 3 bad lights.Take all three 5% lights and plug into the good working socket.Take good light and plug into 3 bad connections.Or cut the good electrical connector and tie into 3 bad connectors wires and see it they work.Take 3 bad connectors and tie in with good wiring.Process of elimination.If one works and 3 dont.Guess what?Corroded.Fudible link is good as you say and both fuses in fuse box.

[datzenmike]

The headlight ground is at the relay in the cab not under the hood.

 

Yes, could be the plug in connectors but what's the chance that all 2 or 3 go bad at exactly the same time???????

 

The headlight switch is known to cause headlight problems. The lubricant used gets hard and collects dirt.

[matrophy]

10 hours ago, Thomas Perkins said:

Don't want to buy new headlight connectors Did you check them?Or 3 bad lights.Take all three 5% lights and plug into the good working socket.Take good light and plug into 3 bad connections.Or cut the good electrical connector and tie into 3 bad connectors wires and see it they work.Take 3 bad connectors and tie in with good wiring.Process of elimination.If one works and 3 dont.Guess what?Corroded.Fudible link is good as you say and both fuses in fuse box.

The bulbs on the driver's side are bright when either low beam or high beam are selected on the lighting switch. The other bulb on that side is dim whyen the other bulb is lit. The passenger bulbs are always dim. The connectors on all 4 bulbs are clean and relatively shiny with no corrosion or deterioration of the plugs or the rubber protector things. It just seems like the connectors are OK.

[matrophy]

I'm trying to figure out how to troubleshoot the light switch and I could use a tutorial on how to use the diagram on the right with the circles and lines. Is there supposed to be continuity across the pins when the 3 possible combinations of the switch are activated? What does 'A', 'B' and 'C' indicate?

 

 

For tracing voltage, which pins should I test? I'm not exactly sure how current flows in this circuit.

Edited yesterday at 04:34 PM by matrophy
ADD SWITCH DIAGRAM

[toshts]

Had this issue when I first got my truck as well. Was intermittent for me, if they were dim and I kicked the fuse box they'd go bright again. Ended up being corrosion in the fuse box itself, windshield had been leaking directly onto it. Half the pins in it either didn't grip the fuses tightly or were broken off. I'd check the fuse box itself both where the fuses go in and the backside along with the headlight relay and it's connector if you haven't already. 

 

Might be wrong but I think abc are the switch positions and off12 are key positions. Pin 35 activates the relay which sends power to the headlights only when keys in position 1 or 2. When the dots have a line connecting them they have continuity in that switch/key position. 

It looks like power goes from bat > fusible link > fuse box > headlight switch (pins 31 and 33) > relay (rw) > low beam (r) or high beam (ry) > relay ground (b). 

[banzai510(hainz)]

do ez stuff first.

re seat the fuses

reseat the connecter to fuse block  reseated might be enought to clean the corrossion off the contacks

 

right  side fuse block for lights.

[datzenmike]

A: headlamp switch is fully towards the dash or hi beam position.

B: headlamp switch is in the neutral position and lo beams are selected.

C headlamp switch is fully towards the driver and overrides all other hi/low off position and turns hi beams on regardless of whether the headlamps are on or not. When released the stalk returns to the neutral or 1ST position.

 

 

OFF: The C position is when you pull the headlamp stalk fully towards you. This forces all the hi beam lamps to light. This is a 'flash to pass' signal and is a potential warning to oncoming traffic.

 

1ST: Parking, (clearance, running and license lights) are on in A, B position with lo beams on and C position (flash to pass)

 

2ND: Hi beams and parking lights are on in ABC

[Thomas Perkins]

10 years to fix basic head lights.Gee Wally.

[matrophy]

Problem solved. Thanks for everyone's help. One of the brass contacts in the headlight switch was not making contact by probably 1/32" of an inch. It looked like it was making contact until I shone a brigh light on it. I tweaked it a little so it made no contact when switch was off off but full contact when switch was on.

 

I'm not sure how this happened all by itself.

 

Because of this one little problem, the dash illumination, tail lights, license plate lights, side marker lights and 3 of 4 headlights stopped working. I took apart and cleaned the hazard switch twice and the turn signal switch 3 times. 

What an ordeal. 

[wayno]

Happy endings are good, you figured it out and if it ever happens again, you know where to look and so do we.