1990 k5

[Justin Fleming]

Any idea what would cause the head lights to go from working fine for awhile and then to flashing on and off repeatedly

Then the next night they will work fine for a little while and start flashing on and off again

 

[diesel4me]

Headlight switch is supposed to have a built in thermal overload that will do that if too much load is applied,(added any aftermarket lights to the original wiring?)--or there is a short circuit in the wiring to the headlights..(This is for square bodies,some newer trucks might have a relay or circuit breaker in the headlamp circuit instead)..

 

[Justin Fleming]

Yea I am running some plow lights through the system.

 

[diesel4me]

That may well be the culprit then..the switch cant carry the load and the breaker kicks off and on again..

Fisher plows have a whole separate harness & 2 way switch for the plow lights ,to switch from regular headlights to the plow lights and still use the stock dimmer switch for high & low beams on both..

I have wired plow lights up by making a separate harness (aka as "3 wire extension cord ) and I used a push pull switch that had two positions for high and low beans ,and a continuous duty solenoid that has a metal case and can be left on constantly ,to take the load off the switch...
Dont be tempted to use a Ford solenoid with a plastic case--they will work but melt if left on long enough and trucks have burnt some customers used them on--use the metal cased ones only and mount them on something metal,they may get warm..