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'82 720 Headlights Won't Turn On


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So my headlights won't turn on, my marker lights all work, my blinkers work, and my brake lights work, only the headlights don't work. I'm pretty sure I have a short somewhere but I'm not sure where or exactly how to go about finding it out.

Below I've put up some pictures and some of the things I've tested to sort this problem out. And could someone explain how the wiring works for the headlight, I've read and reread the wiring diagram but I still can't figure it quite out, a little plain English on the subject would be greatly appreciated.

 

So to settle this problem I tried finding continuity between different points, tried jumping contacts to get the lights working, and all sorts of other weird connections to see what was going on.

 

In reference to the Headlight switch itself I checked continuity to see if it was still good, this is what I got. see photo below

When the switch is in the 'on' position A,B, and D all have continuity.

When the switch is 'on' and the high beams are 'on' A and B have cont, but D doesn't.

C and E have cont in both situations.

To me this says that the switch is working, and operates on the ground side, C and E are for the marker lights, and D is for the high beams.

 

On the Connector to the headlight switch I checked voltage on all the wires and got 12.5 volts on the Red/Blu wire only ( I expected voltage on the Red wire as well)

I then jumped the hot Red/Blu wire to the other wires in the connector to see what happened.

Red/Grn = Headlights on and high beams on (my test wire got quite hot quite quickly)

Red/Yel = Markers came on ( I think this is normal operation)

Red/wht = the relay above the fuse panel clicked on(I didn't check to see if it actually made the connection across the relay, figured if the coil was good the relay was good)

Red = nothing noticeable

 

Red/Grn, Red/Yel, and Red/Wht all go to ground

I also checked continuity between the three pronged headlight plug and found that all three go to ground, I expected at least one to not go to ground.

The two 10A fuses that control the headlights both go to ground( I expected them to be hot, as per the wiring diagram)

I don't have any other electrical issues, the car starts no problem, and the battery doesn't die when parked(as I would expect with a short).

 

I'm not sure where I'm having faulty understanding of the wiring diagram, or a fault in the wiring.

 

 

 

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Headlight%20switch%20connector.JPG

 

Headlight%20Switch%20Recieving%20End.JPG

 

Fuse%20Block.JPG

 

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Electricity always wants a quick and easy way to get to ground without doing any work. A short is a 'shorter' path to ground and will draw more current and will blow a fuse. The term 'short' is mostly misused and it's more likely there is an 'open' or break in the wire preventing power from reaching the lights. Just sayin....

 

Since the headlights are on their own power supply to the fuse box I would start there. Connected to (non California trucks) on the positive terminal/cable of your battery are three fusible links that supply power to the ignition switch, the fuse box and the headlights. (also in the fuse box and powers the two 10 amp headlight fuses) There is a Green and a Black fusible link wire more or less side by side and a separate Green one connected to a Red wire on the main harness. This single Green wire is possibly blown, even though it may look alright on the outside, loose or even disconnected.

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There seems to be a comman problem with the 720 headlight relay, they need to warm up, espiecially in colder climates, sometimes in the winter if I don't touch anything they don't come on for several minutes, but I usually speed things up by pounding on the steering wheel which vibrates the whole dash and then they come on in the brights mode, which I still have to wait till the relay warms up more before they click into normal mode, not all of the 720s I have owned did this, but so far it's been about 20%. If it is the relay, I have found when it clicks into normal mode and I hit the brights right away, they stay on for a while till dropping into normal mode again. Do your headlights mysteriously come on after say ten minutes or more by themselves, or have you even tried that?

 

 

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Since the headlights are on their own power supply to the fuse box I would start there. Connected to (non California trucks) on the positive terminal/cable of your battery are three fusible links that supply power to the ignition switch, the fuse box and the headlights. (also in the fuse box and powers the two 10 amp headlight fuses) There is a Green and a Black fusible link wire more or less side by side and a separate Green one connected to a Red wire on the main harness. This single Green wire is possibly blown, even though it may look alright on the outside, loose or even disconnected.

I'll be sure to check the fusible links, haven't checked those yet. It makes sense to me that it is likely an open that's causing the problem. Do you know, does the power run straight into the two 10A fuses? or does it run into the relay first? I'm confused as to the path of electricity cause the fuses run to ground but the wiring diagram says they should be hot all the time.

 

There seems to be a comman problem with the 720 headlight relay, they need to warm up, espiecially in colder climates, sometimes in the winter if I don't touch anything they don't come on for several minutes, but I usually speed things up by pounding on the steering wheel which vibrates the whole dash and then they come on in the brights mode, which I still have to wait till the relay warms up more before they click into normal mode, not all of the 720s I have owned did this, but so far it's been about 20%. If it is the relay, I have found when it clicks into normal mode and I hit the brights right away, they stay on for a while till dropping into normal mode again. Do your headlights mysteriously come on after say ten minutes or more by themselves, or have you even tried that?

I didn't try letting the relay warm up, I never had the problem before, but at some point I did smell burning inside the car. At the time I figured it was the oil my friend spilt all over the engine, but looking back the smell was definitely of burning plastic and not burning oil.

 

I forgot, Welcome to Ratsun, on the photos,:thumbup: but you forgot the oic. of your truck. :poke:

oic??? is that a picture of my whole truck? if so it will be up soon.

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well I couldn't find a fusible link, (I was looking for it right out of the pos side of the battery, didn't find anything unless they wrapped up in casing and tape), but I did find a red wire separated from its connector, jammed it in and pow!! the lights work, can't believe I ripped off the dash and everything just to find a loose wire at the battery, guess I'm gonna work on all the ducting and try and get the heater to work.

 

Thanks for all your help.

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