10 Pete 388 High Beam Issues

Grounding problem ?

 

Unplug it again.
Check the pins in the harness.
Chheck that the wires are tight in the pins and that there is no play when gently pulling on the wires.
A pin or wire that isn't tight enough can cause this issue.
Another cause, could be a damaged wire that could be broken or shorting out.
Removing / moving the wires may allow wire to make connection just long enough to work.
Check fuse box if there is a fuse for the lights as well, make sure in place of fuse is a circuit breaker (looks like a fuse, but made out of metal and resets automatically after blows from over heating, replace relay.
Lastly, check cab connection... Where harness goes thru firewall.

 

I have ran into quite a few problems with the plugs under the radiator.

 

On the driver side framerail right at the starter are 2 Deutsch connectors in the chassis harness. These are known for melting the ground pin and causing hood lighting issues.

Another spot to inspect.