Need help with the high beams

[Roachhill]

I was driving the bus to work out of town the other night and I turned the headlights from high beam to low beam. All was fine until I switched back to high beam and I got nothing but darkness, back down to low beam and everything was fine. Now the troubleshooting begins, the dash indicator says when the light is on high beam or low beam so the switch is operating normally. I unplugged the front light wiring harness from the main wiring harness then connected the high beam wire to the battery and the lights worked fine. Next I checked the voltage for the high beams at the main wiring harness and got 12 volts. It seems like it should work when the two wires are attached but still I just get dakness. When I attach a incandescent bulb to the high beam wire nothing happens but If I attach an LED light the LED lights up. Seems somehow I don't have enough power coming throught the wire to light the lights. I'm not sure where to go from here or what might even be able to cause the symptoms.
Anyone have any suggestions that might help?

[sportyrick]

It's probably the switch, you could get voltage but not carry enough current. sportyrick

[gseufzer]

I would agree , replace the switch, especially since its both lights.

[Redbear]

If there is a high resistance, like a worn-out switch or a wire that is corroded, it will still show 12 volts at the lamp socket with the minuscule draw of a meter or test lamp. Test it again when it is supplying current. Pull the connector slightly off of a bulb, but not enough to make the lights go out. Stick the meter probe onto the connecting blades when the light is supposed to be on. A loss of voltage will show you if a high resistance is burning off the voltage when the bulbs start drawing amps.

Depending on the type, you may be able to make the same tests at the dimmer switch connection. If 12 volts comes out on the low beam pin, and little or nothing when selecting high, it's definitely the switch. If there is plenty of high beam voltage out of the switch with the bulb connected, but none at the headlight, the wiring is suspect.

In that case, feel along the wire for a bulge, a hard spot, or an area that won't bend as easily as the rest of the wire. If you find such a spot, chances are that there will be green powder instead of metal inside there. All it takes is a pinhole in the insulation at a wet location.

I would still suspect a worn switch is the most likely culprit.

[Ob1]

Might want to see if your circuit uses a relay to handle the high amperage lighting power needed, similar to the way the starter switch uses a solenoid to run the starter motor. Sounds like a relay with scorched contacts.

[Roachhill]

How would I know if it has a relay and where would I find it? I was just thinking the wire size at the lights is much larger than the wire size at the switch suggesting some sort of relay system but I wasn't aware they used relays in high beam setups. Of course I don't know much so their are tons of things in there I'm not aware of, LOL.

[Ob1]

To find out if it has a relay, check the electrical schematic. If you dont have the schematic, start tracing the heavy gauge wire from the headlights backwards...

[Roachhill]

So far I've removed and tested the high/low beam switch and it works perfectly so I'm down to the relay idea and that looks like the most likely answer. Now to find it. I don't have the luxury of a wiring diagram or repair manual yet but the wire at the lights is large and the wire at the switch is awfully small sugesting there's a relay in there somewhere. I followed the wire up from the headlight but that disappears into the main wiring harness then into the firewall so that's a dead end. The wire from the switch disappears under the dash. So anyone have any idea where the most likely place would be to start searching? Would it be behind the dash or under the hood. I hate to tear the bus apart randomly.

[Roachhill]

Just an update if anyone is interested. The other day the poor old skoolie got a flat ( and I got another hit in the wallet) So the mechanic at the company where I work took the wheel off and drove me to the tire shop, on the way we chatted about the problem. I mentioned the International dealer said the relays should be under the passenger side dash but of course the bus doesn't really have a passenger dash to be under. The mechanic said the company buses have their relays on the firewall next to the engine. When I got home I inspected the firewall more carefully and still didn't find what I was looking for so I went to watch TV. Later it occured to me something was odd about the corner panel that holds the turn signal so I went to check it out. One side has screws but the other has a hinge, It's a little door, Duh. Behind that I found the corroded connector that was causing my problem. Happy day all the lights are back on!

[gseufzer]

dont cha love it when you actually find a problem!!!!! lol