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Old April 14th, 2009, 12:00 AM   #1
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Default Headlights blowing fuses...

Alright, help me out here...

The way I have my headlights now wired on the jeep is apparently causing problems. Previously, I had a standard 30a automotive relay from Autozone wired in to power the headlights. A powered switch in the dash triggered the coil and the lights did work for a bit. However they blew within a few minutes and since I hadn't wired in a fuse before the relay, the relay fried also taking out one headlight bulb. I thought at the time it was due to a short in the plug on the back of the lights because I could see wisps of smoke there after it fried.

So fast forward to today. I grabbed new bulbs (well new headlights really since they are the sealed beam disposable ones) and a new 30a relay like before and went to work. I have the high power lead from the batt fused with a 30a before the relay. Power is coming from the dash switch as it should, and the relay is grounded well. The high power output is running into the passenger light and then over to the driver. All wiring is brand new 12 gauge.

However, tonight I drove the jeep out to test the lights and sure enough, after 5-10 minutes, the headlights went out. The inline high power lead fuse popped so I'm hoping that saved everything else. But the questions is, what the hell is my problem?? All the diagrams I've seen online seem to match that way I've wired this, so I'm a bit stumped. Fwiw, this time the fuse blew the jeep was running, however the previous time it was sitting off with only the accessories on.


I drew a quick diagram of exactly how I have it wired.




And here is the relay. Well, the top one (the bottom is for my HEI distributer).






Any thoughts?
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Old April 14th, 2009, 01:59 AM   #2
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Is it all new wires? cuz i had a small rip in a wire that had a similar effect.
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Old April 14th, 2009, 02:09 AM   #3
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Is it all new wires? cuz i had a small rip in a wire that had a similar effect.


Yes, all wiring is brand new. I'll double check for worn sheathing tomorrow if by some chance I snagged something during routing though.
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Old April 14th, 2009, 02:28 AM   #4
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if you have a multi meter, check every end of the wires for resistance. If there is any in one of the wires, replace it. Also test the resistance from all of your grounds to the battery and see if they are good.
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Old April 14th, 2009, 05:03 AM   #5
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they need to be wired in parallel. there is too much resistance with both headlights sharing the same power source. Each headlight needs its own independent power feed from the relay.


Last edited by rustcharger; April 14th, 2009 at 06:08 AM. Reason: added diagram
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Old April 14th, 2009, 07:11 AM   #6
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they need to be wired in parallel. there is too much resistance with both headlights sharing the same power source. Each headlight needs its own independent power feed from the relay.


I wired each headlight with its own relay on my last rig... I also ran the grounds to the frame. I had a ground strap from the batt to the frame as well. Nuzz good luck with it
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Old April 14th, 2009, 10:29 AM   #7
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they need to be wired in parallel. there is too much resistance with both headlights sharing the same power source. Each headlight needs its own independent power feed from the relay.

They are wired in parallel.

55w x 2 = 110w

110/12 = 9.1 amps...

Not really a huge draw.

I'd say you have a frayed wire, or a bad ground. Check your tub grounds as well.
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Old April 14th, 2009, 01:42 PM   #8
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Something is odd there. I would check all of the wiring to make sure nothing is shorting. That would be the most likely explaination.

Are you, by chance, powering both the high and low beams at the same times?

Generally, lows are 55w, highs are 65w.

Although, curent draw is usually higher, than what ME posted above here, because you are likely at about 14v, not 'nominal' 12 volts.

So, if each light was one combined, it'd be 120 watts @ 12 volts. 10 amps at 12 volts, it'd be just under 12 amps at 14 volts. Still, should be less than 24 amps in that situation of running both highs and lows.

At turn on, there will be an inductive spike that a few times normal draw, especially when cold. So it could easily be over 30 amps, but you'd expect it to blow quickly.

A decent 30 amp fuse shouldn't be blowing until over 33 amps anyway. Just seems odd.

I don't see any way whatever happened blew your headlight. Likely, the opposite happend, the filament failed and shorted the terminals.

Have you measured your battery voltage, everything ok there? Alternator hasn't slipped out of regulation....
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Old April 14th, 2009, 01:53 PM   #9
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Something is odd there. I would check all of the wiring to make sure nothing is shorting. That would be the most likely explaination.

Are you, by chance, powering both the high and low beams at the same times?

Generally, lows are 55w, highs are 65w.

Although, curent draw is usually higher, than what ME posted above here, because you are likely at about 14v, not 'nominal' 12 volts.

So, if each light was one combined, it'd be 120 watts @ 12 volts. 10 amps at 12 volts, it'd be just under 12 amps at 14 volts. Still, should be less than 24 amps in that situation of running both highs and lows.

At turn on, there will be an inductive spike that a few times normal draw, especially when cold. So it could easily be over 30 amps, but you'd expect it to blow quickly.

A decent 30 amp fuse shouldn't be blowing until over 33 amps anyway. Just seems odd.

I don't see any way whatever happened blew your headlight. Likely, the opposite happend, the filament failed and shorted the terminals.

Have you measured your battery voltage, everything ok there? Alternator hasn't slipped out of regulation....


I thought about running lows and highs at the same time but decided not to.

I'll test the lights to see if they still work or if the filament is blown. And then check the batt and alt voltage... The alt is new and is one of those 160 amp reworked jobies fwiw.
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Old April 14th, 2009, 03:15 PM   #10
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I circumvented the relay and confirmed both headlights do still work. Then I replaced the blown 30A with a 20A fuse.

I set my tester to the ohms setting and according to it's manual was looking for it to change from 1. to 0.0 when touching tips, which it did. I then tested the ground screw of each headlights' ground wire against the motor and frame and all measurements came out to to the 0.0. I did the same to the ground screw for the relay and also got 0.0.

I then tried to measure the amp draw by placing the red lead onto the high beam spade of the pass light and the black against my block ground. I was expecting close to 10 amps (12.6 volts at the time) but the meter immediately shot up to 16-17amps and then promptly fried itself

So, now I'm off to grab another multi meter and another relay or two. Also, all wires look in good working order without any obvious ground issues So if it's ground, it's hidden...

Last edited by Nuzzy; April 14th, 2009 at 04:34 PM.
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Old April 14th, 2009, 08:26 PM   #11
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I had mine wired exactly the same way. I even jumped the hi/low switch so the lows stayed on when I turned on the highs. The only time I ever had a problem was when one of the grounds went bad.
What gauge are your grounds at the headlights?

What happens if you run them off just the battery for 5-10 minutes? Just the alternator?
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Old April 14th, 2009, 08:32 PM   #12
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Weird. Because the only draw is the light(s), which sets its own current. So it just can't really draw excessive current.

The 87a terminal is just left open, correct?
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Old April 14th, 2009, 09:01 PM   #13
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I then tried to measure the amp draw by placing the red lead onto the high beam spade of the pass light and the black against my block ground. I was expecting close to 10 amps (12.6 volts at the time) but the meter immediately shot up to 16-17amps and then promptly fried itself

So, now I'm off to grab another multi meter

you do not measure amperage like you would voltage....you disconnect the circuit, and place the amp guage in series, not parallel...basically, you just connected the positive directly to ground, and used your meter as the fuse...



unplug the fuse and both headlights, and then measure the resistance of your power wires to ground, any that are not infinte resistance (open, or what the meter reads when set to ohms and neither lead touching anything) are shorted to ground...you do need to remove the fuse and lights from the circuit though, as those will read resistance to ground and confuse the results..
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Old April 14th, 2009, 09:17 PM   #14
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I bet if you were troubleshooting in a tin foil outfit you'd have better luck
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Old April 14th, 2009, 11:00 PM   #15
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Well, I think I may have a likely culprit...

I grabbed a handful of new relays figuring spares wouldn't be a bad thing, and put one of them in. However, then I got to looking more closely at the back of the lights themselves through the access cutouts in the backside of the grill and noticed something. It appeared the light prongs might be closer to my radiator than I'd anticipated. I flexed the grill and sure enough, contact.

To fit my elec fan I had to move the rad forward in the grill a few inches. This caused an interference with the back of my light cups which I then modified. I thought there was enough clearance, but now I'm pretty sure that's not the case.


The solder on the exposed plug was just close enough to the rad to make contact under certain conditions.






Stuck elec tape over the solder, put heat shrink over the connector, and used 1/4" thick nuts to space the whole light forward a bit.





Did the same to the other side and then left the lights on for 15 minutes without issue. This would make sense since the lights hadn't popped last night until I hopped in and put the jeep in gear and started moving. My guess is that movement flexed the grill just enough so that the powered prong made contact and shorted the whole deal.

I suppose time will tell, but I'm hoping this was it!
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Old April 15th, 2009, 08:58 AM   #16
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they need to be wired in parallel. there is too much resistance with both headlights sharing the same power source. Each headlight needs its own independent power feed from the relay.

Too much resistance would cause a lower current draw. Wiring in parallel drops the resistance and allows more current to flow.
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Old April 15th, 2009, 09:05 AM   #17
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Too much resistance would cause a lower current draw. Wiring in parallel drops the resistance and allows more current to flow.
He's got brand new 12 gage wire. The resistance is negligible, and that wire can support well over 20 amps in free air applications. People tend to get too hung up on this.
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Old April 15th, 2009, 09:08 AM   #18
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He's got brand new 12 gage wire. The resistance is negligible, and that wire can support well over 20 amps in free air applications. People tend to get too hung up on this.
But that has nothing to do with what I quoted.

A circuit wired in series has a higher resistance, and lower current flow, than a circuit wired in parallel. I said nothing about the capacity of the wire.
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Old April 15th, 2009, 09:08 AM   #19
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Well, I think I may have a likely culprit...

........



Did the same to the other side and then left the lights on for 15 minutes without issue. This would make sense since the lights hadn't popped last night until I hopped in and put the jeep in gear and started moving. My guess is that movement flexed the grill just enough so that the powered prong made contact and shorted the whole deal.

I suppose time will tell, but I'm hoping this was it!
Sounds good, that is the kind of thing I would expect, and that is also a scenario that could damage the light itself as well.
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Old April 15th, 2009, 09:11 AM   #20
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But that has nothing to do with what I quoted.

A circuit wired in series has a higher resistance, and lower current flow, than a circuit wired in parallel. I said nothing about the capacity of the wire.
Well, the guy you quoted didn't say there was high current draw because they were not in parallel. (they are in parallel, so the guy was wrong, anyway).
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