I have a 93 f250 351 4x4 ..... The headlights will go out and come back on constantly while driving ..... I have replaced the headlight switch and the block that plugs into it but still have the same problem also it does not matter if its on dim or hi beam.....to my way of thinking it acts like a relay getting hot and opening and closing but i checked the relays in the power dist box under hood and none are for headlights .... Is there one somewhere else idk about.... Or any suggestions what to look for i am lost at this point... Thnks

 

Full headlight power goes through the dash switch, I am not sure where the hi low relay is but it would go through that too.

It's possible the daytime running light plug is giving you fits, it's on the front of the rad support, drivers side, just a few wires that end at the plug.

 

Ok .... This truck doesnt hace drl's

 

The wiring is in all trucks, they just put a shorting plug in place of the module and I 'think' all headlight power goes through it still.

 

Ok so far no luck i checked the above items i am wondering now if problem is in turn signal arm?

 

check the plugs on the back of the HL bulbs, if the lock clip is broken they can vibrate loose. also check grounds going to lights could be broken in harness. Occurs when still, moving, bumpy, on long time or short, any & all?

 

I suspect a problem with the Multi-Function switch as well:


Each headlamp has a separate ground. If they both are flicking it is most likely on the supply side or common connector such as C202

 

Check grounds and check for loose wires going into connectors.

 

Ok i will do that .... I replaced the multi switch and that wasnt it .....this is quickly beckming a nightmare the eletrical side is not my strong suit

 

Did you test the voltage going into then back out of the multi-function switch? I would suggest a simple test light since a high impedance DVM may give you phantom voltage readings since it does not load the circuit enough.

 

When lights are on and engine not running its showing 9.7 volts at fuse ..... Gonna try to get them to go off and on and c if i lose power at fuse block then try to work out from there

 

9.7V? that's way too low. Either you have a very poor connection or something is pulling a ton of current. Which would explain why the circuit breaker is opening and closing.......

 

You can isolate exactly WHERE the offending voltage drop is actually occuring by performing the tests outlined here:

http://www.aa1car.com/library/voltage_drop_testing.htm

It's easy! Don't let those automotive electrons scare you too much. The battery can cause trouble, but just be careful. Take your ring and watch off, and don't short the posts.

All that's needed here is a voltmeter, nothing has to be disassembled or disconnected. Anything more than a couple few tenths of a single volt on either side of a circuit is considered excessive. Since electricity always takes the easiest route, by bridging our voltmeter across any two points in parallel while the circuit is under load, the voltage being lost will show up there as a positive number.

From just the battery post itself to the battery cable terminal, or, all the way from the post to the headlight terminal, and everything in between, each connection that drops voltage adds up. This resistance is cumulative of course, and is ultimately what roasts switches and causes breakers to trip. Let us know what you find.