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Headlight oddities


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Hey all, I've got a weird headlight problem. When I put the beams on low I get side markers, but no lights. When I put the switch to high I get low beams, but no high beams, and the side markers stay on. The bulbs all work, and the relay seems to work. Is the switch my next move? What's the service on that like, can I take it apart and clean it to make it work?

Thank you for your time.

Edit: 1985 4x4 KC

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I'm assuming it works normally in the parking light position?


Does the high beam indicator in the dash come on when you push the lever forward (towards the dash) or in the "flash to pass" position (pulled towards you)?.


The switch has a lot of settings, and the most common one of course is the switch turned all the way on but the lever in the center position, and that's the position that turns on the main beams.  Nissan had a habit of NOT using a relay for the main headlights properly- only to switch between high and low, with full power going through the headlight switch.  That tended to burn out the contacts.  I just replaced the one in my Pathfinder- new switches are available, depending of course on year.



If I recall, this is the way things SHOULD work:



Switch in off position:  ALL lights off.  But pulling the lever towards the wheel (flash to pass) will turn on the high beams (and indicator light) only.  Pushing towards dash does nothing.


Switch in PARK position:  Only parking lights on, but pulling the lever towards the wheel will turn on the high beams (and indicator light), but pushing forward will not.


Switch in ON position:  Parking lights and normal headlights ON.  Pulling lever towards wheel will turn on the high beams in ADDITION to the main headlights.  Pushing towards dash will turn OFF the main beam main element and turn on the high beam bulbs, as well as the high beam element in the main bulbs.  Both pushing/pulling will turn on the high beam indicator.


If the main beams only work when pushing (or pulling) the lever, and the high beam bulbs don't work at all, I would suspect someone unplugged the high beam bulbs, and the main beams are only working (albeit in the wrong mode) in the high beam position because the switch is worn out.

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Have you added any gauges, fog lights, changed to round headlights, put a new radio system in, added any wire for something in the dash harness?

Do not add any grounds to the 720 headlight circuit, it is a stand alone mini harness that has no grounds, the relay itself has the ground for the headlight circuit, there are no other grounds in that circuit.

First I would check/test the headlight relay with another relay as that is easy, second I would cycle the headlight switch rapidly, move it around into all the positions several times rapidly, if that doesn't change a thing then I would pull the combination switch and clean the headlight switch section, it is put together with a lot of small screws, remember how it comes apart so that you can get it back together.

I repeat, you cannot try to grab power for any type of aftermarket gauge light or fog light off the headlight/gauge light circuit, it will screw things up, as when you do that you are adding a ground where there isn't supposed to be one, the 720 wiring harness is a weird harness, even the heater blower motor doesn't have a ground, it's mounted in plastic with no ground wire, the heater relay has the ground for that circuit.

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Funny, I had not thought to try pulling the stick. I'll try that and get back with the results. It could be a bad ground too. By parking lights you mean the first position, the one before low beams but above off?




As for grounds, the headlights don't ground to the chassis directly; they ground through the headlight relay.  So what would normally be a common ground for the main bulbs is actually a common +12V, and the relay switches which filaments get ground.  Hence why the switch gets full power through it.  It turns the power on to the lights and then signals the relay as well.  It's not a very well thought out system.  But if the headlight relay wasn't grounded, the lights wouldn't work at all.


I should note that this is how a '83 truck is wired.  You don't say what year your truck is.  1980-81 are wired very differently, with NO RELAY at all, and the lights +12 are controlled directly by the switch and the commons are grounded to the chassis.  I don't have an '82 service manual, so can't tell you how they work.  '83-84 use the ground relay, and I don't know about 85-86 since I don't have a manual for them either.  But if it has a relay, it's a ground relay.

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The first position of the headlamp switch turns on dash side markers, tail lamps and front marker lamps.


The second position continues the above description of lamps on and they are all on their own fuse and have no connection to the headlamps.

The second position, in addition, provides two separate power supplies to all the left side, and all the right side head lamps.


The headlamp relay selects HI or LOW beams by grounding one or the other. The relay is in the HI position when at rest with all headlamps off. When the headlamps are turned on, and if set in the HI position, nothing changes as the relay is at rest in the HI position. Only if the switch is set in the LOW position is power sent to the relay which quickly turns the relay on which removes the ground from the HI beams and connects the ground to the LOW beams. Very simple, hard to describe.



You should have parking lamps in the two positions of the headlamp switch


LOW beams, no lamps... could be the outer LOW beams are burned out or loose wiring to the LOW beams..

HI beams .... could be both center lamps burned out or disconnected and only the HI beams in the outer lamps working.

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When I turn on the headlights I get both on, when I turn on the brights passenger side works normally and drivers side has no brights and the regular light dims out. The drivers side bright comes on just barely when I turn on the headlights normal. Any thoughts? I have tried replacing all headlights with known working spares from my parts horde

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6 hours ago, captain720 said:

When I turn on the headlights I get both on, when I turn on the brights passenger side works normally and drivers side has no brights and the regular light dims out. The drivers side bright comes on just barely when I turn on the headlights normal. Any thoughts? I have tried replacing all headlights with known working spares from my parts horde


 Power is sent to ALL headlamps when the headlight switch is turned ON. To turn them on the dimmer switch selects HI or LO and grounds them. There are only two choices HI or LO.


The fuse box has 2 fuses on the far left for headlights. The far right powers both lamps on the right side at the front. The fuse to the left of it powers the head lamps on the left front.


Because both LO beams work we know that both fuses are OK and there is power and ground through the dimmer switch is working.

Because HI works on the right side we know that the ground through the dimmer switch dimmer switch is OK. (at least on the right side)


Go to the plugs in the left side headlamps. With headlamps on and dimmer on HI beams, ground the Red wire on the back of either plug and the HI beam should come on. This should prove that there is something wrong with the ground on the left side but not as far back as the right side which seems to work OK.




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