Short to power?? Headlights always on, truck won't start

[jaronpham]

They are not related except that they are both in the same vehicle.

ok thanks! I appreciate the photo, i'll go check.

so strange, seeing as both issues (the fuel issue and eternal headlights) both arose at the same moment.

 

[Curious Hound]

I just learned that the DRL module is also disabled by turning on the headlights with the switch.

 

[RobbieD]

It sounds like the DRL module is stuck on. The DRL pulses power to the hi beams, so they won't look fully bright. It looks like pulling fuse 21 will kill the power to the DRL, and still let the headlights operate manually.

 

[Curious Hound]

hi, thanks for your interest in my fiasco.

I have been playing w that 15amp fuse throughout the day, when I pull it the lights turn off.

It seems to be the lowbeams that are always on. Which is even more strange, if what youre saying is true about the DRL energizing the high beams.

Is that 15 amp fuse specifically for DRL? would be be bad practice to just pull that fuse permanently and call it a day?

That fuse also controls the light under the hood. Not a big deal.

Are you SURE it's the low beams that are on? Or does it just seem that way because they seem more dim than normal high beams? See the explanation I just posted.

 

[jaronpham]

That fuse also controls the light under the hood. Not a big deal.

Are you SURE it's the low beams that are on? Or does it just seem that way because they seem more dim than normal high beams? See the explanation I just posted.

I was under the impression that they the same bulb? How would I tell the difference? All I can see is the lights that are always one seem less bright than the lowbeams that I switch on manually.

 

[jaronpham]

It sounds like the DRL module is stuck on. The DRL pulses power to the hi beams, so they won't look fully bright. It looks like pulling fuse 21 will kill the power to the DRL, and still let the headlights operate manually.

You are correct, when I remove the 15 amp fuse 21 in the engine box the lights still operate manually. I'm leaning towards the DRL module acting up. I was totally unaware of the DRL until the replies on this forum, which now is a relief. I was so confused as to why the headlights seemed to be on two different circuits!

 

[RobbieD]

so strange, seeing as both issues (the fuel issue and eternal headlights) both arose at the same moment.

That's simply one of the greater joys that comes with owning a 30 year old classic truck.

Anyway, pulling the fuse will let you operate the truck, then if you want to keep the DRLs you can score another module later (either new or from the boneyard).

 

[Curious Hound]

I was under the impression that they the same bulb? How would I tell the difference? All I can see is the lights that are always one seem less bright than the lowbeams that I switch on manually.

There are 2 different filaments in a headlight bulb. Think of it as 2 bulbs built into one. The DRL module uses electronic magic to send less energy (still 12 volts) to the high beam filament. So it's in but dimmer than normal.

 

[Curious Hound]

Now I'm wondering if the DRL module is bad. Or did you just not know that you had the DRL function and just now noticed it.

 

[jaronpham]

Now I'm wondering if the DRL module is bad. Or did you just not know that you had the DRL function and just now noticed it.

I think the DRL module is toast. Because the lights wont turn off even with the key out and engine off.

Thinking I'll just pull fuse 21 in the engine box and call it day, that seems to be a safe bet? Unless the cause is not a faulty DRL and actually a short to power somewhere along the line?

I suppose if the DRL was scooping power from somewhere else then fuse 21 wouldn't turn the lights off when pulled though. I'm unsure

Thoughts?

 

[RonD]

Head lights have 2 different fuses

Fuse 12 in the Cab Fuse Box, Flash to pass always on
Fuse 2 in Engine Bay fuse box, powers main Light switch

Both headlights high/low go thru the Multi-function switch(turn signal stalk)

Check that the turn signal stalk is not push forward, high beams on

Yes, DRL uses fuse 21 in engine bay fuse box

1993/4 are the same wiring

 

[Curious Hound]

I think the DRL module is toast. Because the lights wont turn off even with the key out and engine off.

Thinking I'll just pull fuse 21 in the engine box and call it day, that seems to be a safe bet? Unless the cause is not a faulty DRL and actually a short to power somewhere along the line?

I suppose if the DRL was scooping power from somewhere else then fuse 21 wouldn't turn the lights off when pulled though. I'm unsure

Thoughts?

It's an electronic module. They eventually go bad. If it were mine, I would remove that fuse or unplug the module and tape over the open plug so nothing gets in it.

 

[jaronpham]

do you guys think there's any possibility the DRL module is gaining power through a short? or does the fact I can shut it off through the removal of the DRL fuse in the engine compartment proves that it isn't? I suppose there could be a short in between the battery and engine fuse box but that seems unlikely

or... is my electrical knowledge completely BS

 

[RobbieD]

do you guys think there's any possibility the DRL module is gaining power through a short? or does the fact I can shut it off through the removal of the DRL fuse in the engine compartment proves that it isn't? I suppose there could be a short in between the battery and engine fuse box but that seems unlikely

or... is my electrical knowledge completely BS

No, it's not "gaining power through a short"; and yes, it shutting off by your pulling the fuse proves that.

The DRL module takes power from the battery (through the fuse that you pulled) and it pulses that power very rapidly (I suspect it's just a control circuit and a big power transistor) and sends that pulsed power to the headlight hi beam filaments. This causes the headlights to light, but at a reduced level whenever the ignition switch is on. Turn the ignition off, or turn the headlights on manually, the DRL turns off.

Your DRL module has failed, because the transistor is stuck "on" all of the time. Pull the fuse, you're removing the power to it.

I think that somebody posted the schematic for the headlights; look it over, follow the circuit paths, and hopefully things will make better sense.

or... is my electrical knowledge completely BS

Don't sweat it, man. Look at the bright side; the more your truck breaks, the smarter you'll get.

 

[jaronpham]

No, it's not "gaining power through a short"; and yes, it shutting off by your pulling the fuse proves that.

The DRL module takes power from the battery (through the fuse that you pulled) and it pulses that power very rapidly (I suspect it's just a control circuit and a big power transistor) and sends that pulsed power to the headlight hi beam filaments. This causes the headlights to light, but at a reduced level whenever the ignition switch is on. Turn the ignition off, or turn the headlights on manually, the DRL turns off.

Your DRL module has failed, because the transistor is stuck "on" all of the time. Pull the fuse, you're removing the power to it.

I think that somebody posted the schematic for the headlights; look it over, follow the circuit paths, and hopefully things will make better sense.



Don't sweat it, man. Look at the bright side; the more your truck breaks, the smarter you'll get.

Thanks guys. you are heroes. The truck is running well, and indeed I have learned a lot haha.

The only thing is that the battery light is on in the dash. Perhaps it just means the battery needs some charging since the lights have been on quite a lot during the troubleshooting?

EDIT: I've tested the alternator, the reading at the battery terminals is showing 11.5 volts. Is my alternator suddenly toast too??? I'm at a loss