6V at Heatlight
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Junior User
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From: Versailles, KY
6V at Heatlight
1990 F250, 7.3 diesel has headlight issues that make no sense to me. The high-beam indicator light on the dash is on all the time. On one foot switch setting (don't know which since indicator doesn't change), I get 12V on one lead and 0V on the other and essentially have fine low-beams. On the other setting I get 6V on the other lead and 0V on the other and get very dim output. I can't imagine what the conductors are running through to cut the voltage by half. There has been some previous owner re-wiring, but I don't see anything as yet compromising the headlight circuit; but there must be somewhere.
Any ideas?
Tom
Any ideas?
Tom
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Joined: Jan 2004
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From: St Louis
You likely have a bad connection that has overheated and corroded. That would cause the high resistance that would drop your supply voltage.
Inspect your dimmer switch and its connections first.
Steve
Inspect your dimmer switch and its connections first.
Steve
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From: Versailles, KY
Thanks guys. Good idea to check the switch; it gets lots of abuse over 17 years. I checked the switch and was getting some resistance between selected connectors, so I replaced it. Unfortunately, that wasn't it. While I was there I checked the input voltage from the line connector and got a good 12V+ on both sides, so the problem isn't at the switch. That does help me narrow it down though. I've got the wiring diagram and can eliminate the pre-switch connections.
It's weird at the bulb, though. In one switch position I get 12V on one side and 1.9V on the other (seems like it should be 12V and 0V, not 1.9). In the other position, I get 5.88V on one side and 5.88 on the other. This'll be interesting...
Tom
It's weird at the bulb, though. In one switch position I get 12V on one side and 1.9V on the other (seems like it should be 12V and 0V, not 1.9). In the other position, I get 5.88V on one side and 5.88 on the other. This'll be interesting...
Tom
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From: Versailles, KY
Hey Steve,
I've got .01V at the ground when I turn the lights on - bright or low-beam doesn't matter. With the headlight switch off I get 0V at the ground.
On a related note, I just figured out how the wiring diagrams work. Didn't get the zone thing at first. Should help me track some of this stuff down. The only problem is that it is pretty much only a logical schematic and I have no idea where some of the stuff is located. But it should get better...
Tom
I've got .01V at the ground when I turn the lights on - bright or low-beam doesn't matter. With the headlight switch off I get 0V at the ground.
On a related note, I just figured out how the wiring diagrams work. Didn't get the zone thing at first. Should help me track some of this stuff down. The only problem is that it is pretty much only a logical schematic and I have no idea where some of the stuff is located. But it should get better...
Tom
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From: St Louis
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From: Versailles, KY
Part of the trouble seems to be with the guy typing this... When I originally tested the headlamp leads I didn't know which was which; I was getting 5.88V by measuring the high and low beam hot leads (circuits 12 and 13) and leaving the ground out of the equation. On high, both circuits 12 and 13 are getting 12V; on low beam circuit 13 is getting 12V and 12 is getting around 6V. Seems like the high-beam circuit shouldn't be getting any juice when the low-beams are on, though.
I'm left with two problems. First, on low-beam the lights are seriously dim on the road. In my shop on a dark day with the lights off they make little difference; high beams are much brighter, but not like real brights. I'm beginning to think that the problem is with the headlight diffuser/lens. It doesn't look too bad, but I'm not getting much light. The bulbs when lit outside of the headlamp housing are pretty brilliant; just unfocused. It isn't an aiming problem; it's just dim on my shop wall. Second, the high-beam indicator light is on permanently when the headlights are on; no matter the stetting. I've not found this in the wiring diagrams anywhere as yet. Having gone over the headlamp circuits, I don't see the connection.
Suggestions?
Thanks,
Tom
I'm left with two problems. First, on low-beam the lights are seriously dim on the road. In my shop on a dark day with the lights off they make little difference; high beams are much brighter, but not like real brights. I'm beginning to think that the problem is with the headlight diffuser/lens. It doesn't look too bad, but I'm not getting much light. The bulbs when lit outside of the headlamp housing are pretty brilliant; just unfocused. It isn't an aiming problem; it's just dim on my shop wall. Second, the high-beam indicator light is on permanently when the headlights are on; no matter the stetting. I've not found this in the wiring diagrams anywhere as yet. Having gone over the headlamp circuits, I don't see the connection.
Suggestions?
Thanks,
Tom
Last edited by tomherrick; Nov 13, 2007 at 03:03 PM. Reason: error
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i ain't rite
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From: Marlboro Mental Hospital.
do the lights flicker if left on for a long time?
the 87-91 trucks had a bad problem with the headlight switch and switch harness overheating, and eventually burning up.
on my fleet of trucks, every one had the harness and switch replaced at least 3 times due to overheating. the first sign was dim lights, then they would start to flicker, then go out all together.
the 87-91 trucks had a bad problem with the headlight switch and switch harness overheating, and eventually burning up.
on my fleet of trucks, every one had the harness and switch replaced at least 3 times due to overheating. the first sign was dim lights, then they would start to flicker, then go out all together.
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From: Virginia
Seems like the high-beam circuit shouldn't be getting any juice when the low-beams are on, though.
A poor ground would explain the dim light, and the dash indicator staying on, and the 6v on the highbeam side when the lowbeams are on. When the power goes out to the lowbeam prong, it goes through the lowbeam filament, and then it is supposed to leave on the ground terminal and go to ground on the sheetmetal of the frontend. But if the ground path has a high resistance, part of the current will decide to go backwards from the lightbulb ground/common, up through the highbeam filament, and then backwards up the highbeam wire and light the highbeam light in the dash.
If you want to make double sure this is or isn't the problem, take a scrap piece of wire and hook it to the battery negative terminal. Then shove it into the ground connection of the headlight and see if the headlight gets brighter, and the highbeam dash light goes out.
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From: Versailles, KY
I went right out to the shop and did it. It didn't work. I used the Ohm meter first to make sure that there was a connection. I put one lead on the neg. terminal and the other on the temporary ground wire and got .04 Ohms of resistance indicating that there was an electrical connection through the grounds. I took the meter off and put the temp. lead on the battery and there was no difference in the brightness, nor did the high-beam indicator go out. Thanks for the suggestion; it was a good one. Narrows down the options...
I've spent time polishing the headlamp lens today to see if that made any difference. I wet-sanded with 600-grit, then 1200-grit, then buffed with toothpaste for a while. The exterior surface is gorgeous, but there's no difference in the amount of light. When I turn the lamp on I can see lots of what look like bubbles in the plastic; probably southern California desert sandblasting...
I did manage to get the engine hood lamp and the passenger door dome lamp switch working today while trying to chase down loose ends...
Thanks to all for the input. I'll keep working on it.
Tom
I've spent time polishing the headlamp lens today to see if that made any difference. I wet-sanded with 600-grit, then 1200-grit, then buffed with toothpaste for a while. The exterior surface is gorgeous, but there's no difference in the amount of light. When I turn the lamp on I can see lots of what look like bubbles in the plastic; probably southern California desert sandblasting...
I did manage to get the engine hood lamp and the passenger door dome lamp switch working today while trying to chase down loose ends...
Thanks to all for the input. I'll keep working on it.
Tom
Cargo Master
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From: Lynnwood, WA
Where exactly do you measure voltage? Put in the connector about 3/4 of the way (or a bit further, but leave some space for the probes) and measure what actually goes to the bulb on the bulb's leads while the light is on.
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From: Versailles, KY
At the bulb while on I get these readings:
Low beam setting - 11.26V on lt. green high beam circuit #12 and 6.35V on the red low beam circuit #13.
On the high beam setting - 10.90V on lt. green #12 and 10.96V on red #13.
Given that on the low beam setting I've got one filament burning at full strength and ther other at half strength I shouldn't be having any trouble seeing the road in front of me; but I do. And I still have the brights indicator light on all the time...
Where should I look for the headlight system ground? There are grounds all over the place; I've not found the one used by the headlight system. Although, the ground tested fine with .04 Ohms of resistance.
Maybe it's just the headlight lens that's diffusing the light so much. I hate to spend the money on new ones just to try them out, but don't know what else to do at this point - except maybe to work on the suspension for a while...
Thanks,
Tom
Low beam setting - 11.26V on lt. green high beam circuit #12 and 6.35V on the red low beam circuit #13.
On the high beam setting - 10.90V on lt. green #12 and 10.96V on red #13.
Given that on the low beam setting I've got one filament burning at full strength and ther other at half strength I shouldn't be having any trouble seeing the road in front of me; but I do. And I still have the brights indicator light on all the time...
Where should I look for the headlight system ground? There are grounds all over the place; I've not found the one used by the headlight system. Although, the ground tested fine with .04 Ohms of resistance.
Maybe it's just the headlight lens that's diffusing the light so much. I hate to spend the money on new ones just to try them out, but don't know what else to do at this point - except maybe to work on the suspension for a while...
Thanks,
Tom
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From: Virginia
When in lowbeam mode, you have 11.26v on the low filament, 6.35v on the high filament. I was just thinking, if it was a ground problem, usually it will take the 12 volts and split it up. So you may have 8 volts on one, and 4 on the other. Yours adds up to 17.5 volts. So if we ditch the bad ground theory, the only other thing it could be is melted wires touching each other in the harness.
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From: Versailles, KY
Talk about melted... I just toasted something in there real good. Pulled out the main power lead in the headlight switch and it got away from the pliers and grounded out on the metal mounting bracket. I couldn't react fast enough and within seconds there was lots of smoke after the sparkler effect was complete. Got no lights at all now. Hopefully it's just a matter of replacing fuses and fusible links, but I've got a bad feeling about the instrument cluster...
Tom
Tom