1977 Datsun 620 headlight mystery

[Jeffthe620]

Hey Ratsun

About a week ago my headlights went out. Replaced the fuse, still nothing, so I started messing around with a multimeter. Found out that I get power to the relay and the relay is good to what I know. When I used a test headlight off the power to the relay, it would not turn on the headlight. Any help would be appreciated! 

[banzai510(hainz)]

ground you meter with the black wire and then put the red wire on any of the 3 contacks on the outside lamp(low beam)

if you get 12 then that is correct. pull all the way out then one of the contacks will get zero volts cause it went to ground. If it dont then I say this is the proplem. But always ohm out the lamps just in case they both got blown(this happen it me once, not think what is the chance both are just plain bad)

 

you pull half way out on the switch your parking lights com on?

 

reseat the plugs

Edited February 27 by banzai510(hainz)

[datzenmike]

 

 

Unlikely that all 6 filaments burned out together.

 

Does the relay click when switching between hi and low????? this would (sort of) indicate the relay is working. If so check the one thing all the lamps have in common... the ground connection. It's the black wires and they all collect and are bolted to the radiator support, I think near the battery?

 

If you have power to the relay then only way there is no power out is if it is if it has jammed between hi and low. Probably won't clik.

 

1 hour ago, Jeffthe620 said:

Hey Ratsun

About a week ago my headlights went out. Replaced the fuse, still nothing, so I started messing around with a multimeter. Found out that I get power to the relay and the relay is good to what I know. When I used a test headlight off the power to the relay, it would not turn on the headlight. Any help would be appreciated! 

 

You don't mention if the relay clicks. From this I would say the relay is bad.

[Stoffregen Motorsports]

Check the grounds at the lights.

[Jeffthe620]

8 hours ago, banzai510(hainz) said:

ground you meter with the black wire and then put the red wire on any of the 3 contacks on the outside lamp(low beam)

if you get 12 then that is correct. pull all the way out then one of the contacks will get zero volts cause it went to ground. If it dont then I say this is the proplem. But always ohm out the lamps just in case they both got blown(this happen it me once, not think what is the chance both are just plain bad)

 

you pull half way out on the switch your parking lights com on?

 

reseat the plugs

I’ve tested this I only get about .6 or so volts to all of the contacts, checked resistance on all of the lamps and they aren’t blown. If I pull out halfway on my switch my parking lights come on, two pulls is headlights 

 

7 hours ago, datzenmike said:

 

 

Unlikely that all 6 filaments burned out together.

 

Does the relay click when switching between hi and low????? this would (sort of) indicate the relay is working. If so check the one thing all the lamps have in common... the ground connection. It's the black wires and they all collect and are bolted to the radiator support, I think near the battery?

 

If you have power to the relay then only way there is no power out is if it is if it has jammed between hi and low. Probably won't clik.

 

 

You don't mention if the relay clicks. From this I would say the relay is bad.

 I checked my ground connection, it’s attached firmly with no corrosion. My relay might be the issue or one of the issues, but when i remove it and run power directly to the lights they will still not turn on. 

[datzenmike]

Do you have sealed beam lamps or after market???? H4??? or something???

 

 

The Red/White wire is for the high beams. It should light all four headlamps

 

The Red/Black are for the outer two low beam lamps.

 

 

They can't ALL not work. If true then the wires must be cut.

[banzai510(hainz)]

2 hours ago, Jeffthe620 said:

I’ve tested this I only get about .6 or so volts to all of the contacts

OK on Datsun at least the 510 and I assume the 620 the 3 contacks at the coonnector or lamp will have 12volts. Since the battery +12volts is wired to the lamps via the fuse block.  Now you say you get only .6volts to any of the 3 connection at the lamp or connector behind it.  The switch must be in the OFF mode. to have the 12volt at each connection. when the black wire is grounded on the meter.

 

To me the +side 12volts is not making it to the lamps. As datsuns you can put the lights on anytime. But since the switch is off you should have the +12volts to the lamps. You dont have the 123volts so I dont think its a ground issue

One can use a test light also and if the 3 contacks has good light then you know you have 12volts.

 

Do you have 12volts at the fuse? try both sides of the fuse. Clean them. Maybe try to reseat the connectors going in to the fuse box.

Usually there are 2 fuses one for the left and one for the right. try to find where there is a common point where it feeds.

 

As a last resort I seen the back side of the fuse box rotted out where its corrcded from water underneath. But weird how it bothe sides.

 

[Jeffthe620]

4 hours ago, datzenmike said:

Do you have sealed beam lamps or after market???? H4??? or something???

 

 

The Red/White wire is for the high beams. It should light all four headlamps

 

The Red/Black are for the outer two low beam lamps.

 

 

They can't ALL not work. If true then the wires must be cut.

I’m thinking the rad melted some wires in the harness leading to the headlights causing some to ground out. Their stock lamps and harness as far as I know. 

 

3 hours ago, banzai510(hainz) said:

OK on Datsun at least the 510 and I assume the 620 the 3 contacks at the coonnector or lamp will have 12volts. Since the battery +12volts is wired to the lamps via the fuse block.  Now you say you get only .6volts to any of the 3 connection at the lamp or connector behind it.  The switch must be in the OFF mode. to have the 12volt at each connection. when the black wire is grounded on the meter.

 

To me the +side 12volts is not making it to the lamps. As datsuns you can put the lights on anytime. But since the switch is off you should have the +12volts to the lamps. You dont have the 123volts so I dont think its a ground issue

One can use a test light also and if the 3 contacks has good light then you know you have 12volts.

 

Do you have 12volts at the fuse? try both sides of the fuse. Clean them. Maybe try to reseat the connectors going in to the fuse box.

Usually there are 2 fuses one for the left and one for the right. try to find where there is a common point where it feeds.

 

As a last resort I seen the back side of the fuse box rotted out where its corrcded from water underneath. But weird how it bothe sides.

 

I have full power at the fuse and switch, and the fuse box is clean and in good condition. 

[datzenmike]

Rad at most only gets 2/3 of the way to melting plastic. You'd have to have a fire. If grounded out on the power side the fuse would blow. If grounded out on the ground side the lights would be on with the headlight switch but wouldn't switch with the relay.

[banzai510(hainz)]

15 hours ago, Jeffthe620 said:

I have full power at the fuse and switch

OK if you have 12volts at the fuse now trace to the lamps where the proplem is.  Thats is you say at the lamps you get .6volts

[datzenmike]

Make sure the fuse is seated and making good contact with the clips that hold it. My lights would randomly go out while driving which made me quickly run my fingers over the fuses to get them back on a fast as possible. The clips were so hot it took the temper (springiness) out of the holder clips. Had to replace it.

[Stoffregen Motorsports]

My dad was working on a similar issue in his Sunbeam the other day and traced it back to the fuse panel. On top, everything looked good, but the underside of the fuse block had loose connections. Loose from years of vibrations.

[Jeffthe620]

I’ll check the fuse box again make sure everything’s connected well, and check that my wires are good up to the headlights. Thanks your all the help I’ll inform if I have any more info!

[banzai510(hainz)]

this dont have a alarm system mixed in with the lights?  I assume this is stock wiring on the truck

 

you have 2 LIGHT fuses?  one for left and right? If yes and you say both lights dont work there might be a copper bus bar under the fuse box that feeds both sides

 

 

PS I aways change out the old fuses couse soem can crack at the ends cause the vibrate and crack but still touch at the end caps

[datzenmike]

Two fuses. One for parking lights only. One for both headlamps. The headlamp fuse is 15 amp, it's located farthest from the driver on the right side of fuse box. It's hot or powered at all times. A RED wire takes the power to the headlight switch and a RED wire takes the power to the headlamps when headlight switch pulled out to second position.

 

This has taken too much time to find out what's wrong. It'll be the fuse or fuse holder. Much less likely is the headlamp switch. Even more unlikely is a bad ground wire. Least likely is the relay at fault.