Headlight Wiring W900

Low beam: outside lights lit, inside lights NOT lit.
High beam: all 4 lights lit.

 

Sounds like a poor connection in the harness. Both drivers and passenger side harnesses see a lot of flex at the hood hinges over the years. That clicking is most likely the headlight circuit breaker. They should be self reseting as a safety feature so you don't lose your headlights completely if a power wire grounds out at night. Once you sort out the wiring, I'd replace the breaker since they weaken with age and repeated tripping.

 

I agree with NM re the harness. Those voltages don't make sense, except under the condition of bad grounds, or power wires shorting to ground. Should be either 0 volts, or 13.7 (or whatever your system runs at.) 5 and 10 volts is indicative of a very bad connection, or as I mentioned, a partial short to ground. The very first thing I'd check is an ohm check right at your headlight plug in. (Unplug from the headlamp to do this check). One of the three should be 0 ohms when measured to a good ground spot on the frame or engine. If you don't have that, they're not going to work right. If you have good grounds, then you need to work back on your harness and see if you can find a bad spot. It's probably not going to be easily visible, will look ok on the outside, but be rubbed through, cracked, broken, or green on the inside. Good luck! If you need help on how to do the ohm check with your meter, just ask, be glad to walk you thru it.

 

I know the old A model KW's had just a single switch with no relay powering the headlights. The switches were known to burn up, causing dim lights or loss of lights. If you have no headlight relay I would check and see what you're getting for power supplied to the switch and through the switch before tracing the harness.

Ohms is not always an accurate test TBH. I've been fooled once or twice by a harness or component that showed 0 ohms. You need to have a load on the circuit. Best way is stick a battery charger on the truck and turn on the headlights. Then measure the voltages. Anything less than battery voltage at any given point in the circuit indicates a resistance. Maximum voltage loss for a 12 volt circuit is 0.5v (this includes positive and ground side).

 

Have you tried swapping the relay with another? Since that headlight relay is more than likely attached directly to the headlights, I would check power there as well. Have you checked the back side of the fuse panel as well? If its mounted to the firewall below the windshield, the connections and wiring on the back side of the panel can become corroded if the windshield has ever leaked.

 

This might be a better idea than trying to fix the crusty old wiring. We redid an old 379 this way and the headlights are phenomenal now.