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'82 720. High beams comes on when I turn on the headlights then shut off.

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Whenever I turn on my headlights the highs will flash for a brief moment, normally not a big deal but as it gets colder the highs will stay on longer, like something is sticking. 

Right now it's -4 celsius so absolutely freezing and the high beams will be on for maybe 10-20 seconds before it "unsticks" and the lights return to normal lowbeams. If I flash the highbeams or turn them on and off they will also stick. The colder it is the longer they stick and they will do that untill the car or relay or switchgear or whatever gets some temperature then they will be all normal and snappy on/off like they should.

Any idea where to start looking? Could it be a worn out relay or switch? 

I also noticed now that the cold set in that as I turn on the lights there is a slight delay from the moment I turn the switch untill the instrument lights turn on, don't know if its connected but thought I should mention it just in case

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I would expose and then look closely at the high beam switch. The contacts may be sticking and need to be cleaned.

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It's not the light switch nor is it the HI/LOW switch as it's not even being used. The headlamp switch sends power to ALL the headlamps and the relay grounds either the HIGH or the LOW beams to turn them on

 

 

OK... the headlamp relay can only select (and ground) HIGH or LOW beams to turn them on.

 

When there is no power energizing it, the relay is OFF and connected to the high beam lamps. So when you are driving around in the day time, the relay is actually in the HIGH beam position but because the headlamps are not turned on nothing happens.

 

Lets assume that when you shut your headlights off last night you were in the LOW beam position. You turn the headlamps ON. The HI/LOW selector is set to LOW beams and grounds the relay energizing it from OFF to ON or from HIGH to LOW beams. This usually happens so fast you will never notice that it was on HIGH beams and switched to LOWs

 

You can try replacing the headlamp relay but it's likely the cold doing this. I've heard of this happening on other 720s in the cold weather, so you are not alone.

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It's the relay, mine does the same thing but worse, it can take mine 5 minutes to go to low beams, if I tap on the relay with a wrench it switches to low beams sooner, but if I hit the high beam even a second it can take over a minute to get back to low beam mode again, I found the longer the lights are on the quicker if functions normally, but when I shut down the engine and the truck cools off it starts all over again, if it sits overnight it is back to 5 minutes again the next day.

I don't hardly drive that truck(720 turbodiesel), so I don't care to fix it, when I do drive it it is during the daylight hours.

I would try replacing the headlight relay, I only have one truck that does this, the 3 other trucks I have that have 720 wiring harnesses in them do not have this issue.

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Thanks for the in depth explanation Mike, that makes it much clearer what's going on.

 

It's somewhat of nuisance now that I'm using my truck as much as I'm doing like I have to just sit and wait outside my garage for the lowbeams to come on before I can drive out in to traffic. Gonna see if I can find a suitable replacement relay and try that

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I've had more temperature sensitive relay issues in 720s than any other vehicle ì have owned.

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Good to know, I've only ever had one other 720 and that never had any issues no matter how cold it was(only glow plug relay)

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Is it possible to switch the low and high headlight wires position on the headlight relay so that the low beams are connected to the normally on contacts on the relay?

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 That would work with the lever in the high beam position. These relays were not known for this when new so I have to assume use and age may be a factor in the cold. Much easier to just replace it.

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My '78 510 has the same issue.  Normally the high beams turn off after a couple seconds but not always.  I just hit the flash to pass part (which energizes all 4 headlights) and usually the high beams shut off.  It's the relay, it gets stuck in High mode.

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How annoying that the relay pins location isn't like the standard type available at the auto parts store... Could have gotten a new one in an instant :P (no I don't wanna modify anything. yet...)

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Find a wrecking yard with any Datsun Nissan that started building in the '80s such as the 720, Maxima. 300zx, S110 and S12, D21 Hardbody truck and WD21 Pathfinder, these will have that style relay. Go by the color there were others Blue Brown Black that did different things. Sometimes they are under the hood on the right fender under cover.

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Actually found one in one of my parts boxes, from a 910 bluebird. But of course as I swapped the relay the weather changed for the warmer so I'll just have to wait until another cold day to see if it worked. New one is almost 60 dollars over here :o

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Approximately half that here..... and why I grab stuff like it from wrecking yards, along with fuses, bulbs and such.

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