I know this issue has been covered before, but I did some testing on my truck tonight. I've been experiencing the "flashing light" sympton that seems pretty common in these trucks.

The steps I've take thus far:

1. Replaced Headlight Switch
2. Replaced dimmer switch
3. Wired HOT and Low Beam directly together with female spade connectors.

None of the other lights shut off.

That leads me to believe that the issue is between the power wire from the headlight switch to the dimmer switch connection.

I know a lot people are recommending the "heavy duty headlight wiring upgrade" which I plan on doing but im afraid that If i spend the money on that right now and it doesn't fix the issue, im going to be in the same boat im in right now.

I'm going to test the wire continuity but I feel that is going to lead to a dead end since the lights work on and off.

My second thought was to pull the power wire from the headlight switch harness all the way down to the dimmer switch and replace it completely. New wire and new prongs. Re-assemble it with some dielectric grease.

 

I wouldn't go to just heavier wiring. Do a relay system. Then your headlight switch is only tripping the relays. Then you run heavy gauge wiring from the battery to relay to headlights. That way you take most of the load off of the headlight and dimmer switch. Either way you may want to consider replacing your headlight and dimmer switches.

 

can you give us more detail on what "flashing lights" means? how long are they off (assuming they go completely off), when it happens, when they come back on?

 

If your headlights are flashing on and off, that's the internal circuit breaker in the headlight switch interrupting. It happens when wiring downstream of the headlight switch is pinched and shorted out. The headlights are protected with a circuit breaker instead of a fuse so that you can fix the problem on the side of the road without having to replace fuses for which you may not have spares.

None of your troubleshooting has had anything to do with the actual problem because you have been focusing on everything upstream of the load. The problem also has nothing to do with deficiencies in the original wiring configuration that some people upgrade with a relay setup. Check all wiring downstream of the headlight switch for frayed insulation. This is really a very simple issue.

I wouldn't call this "common." Your truck has a problem and this is its reaction to it. It's not any different than if it were to overheat because the coolant was low.

Of course, this all assumes that your lights are flashing on and off at a periodic rate. As dlburch explains, you need to be more specific in regards to the actual problem. Your post mostly consists of steps you've taken, but it really doesn't describe what the actual problem is other than alluding to some sort of common issue with which we are assumed to be already familiar.

 

By "flashing on and off" i mean: sometimes they'll go off for a few seconds then come back on, other times, they'll flash off and on, almost strobe like.

I've checked the wires going to each of the headlights and they all seem fine. I didn't see any pinches or frayed insulation.

 

Check that your voltage regulator is mounted/grounded to clean shiny sheet metal.

As a side note...this HD harness is a nice upgrade (only 30 bucks).

Heavy Duty Headlight Harness

 

Oh I know. I'm planning on getting that harness. Overall it'll be about $4 cheaper through LMC.

Like I said in my original post. I hope that solves the issue.

 

I too have a issue with flashing lights or I should say they just go off when I use high beams for a while. I haven't been able to find any bad wiring. It's funny I didn't have this problem till I ruined the headlight switch one day and had my wife grab a new one at advance. I think it might be the culprit which is no surprise. I have a new Motorcraft one that I'm going to try next.

 

Do you hear the headlight switch clicking or buzzing when the headlights go out? If not, that may rule out the internal circuit breaker. If that's the case, in the absence of any further clues, you might have to wait until they go out again, pray they stay out, and use your multimeter to start looking for the place where power drops out.

 

I've never heard any clicking. Right after it started it I replaced the dimmer switch too but still same thing. They usually stay out when they get kicked off till I switch back to low beams. It just seems odd that I didn't have this problem till I had to change the headlight switch.

 

Well I finally got around to trying out a new headlight switch. It turns out the Motorcraft one I had was not right and for a big truck so I tried a Napa one. The lights stayed on tonight on hi beam which was never possible and did so for about 45 min then went out. I switched back to low beams and they stayed on to get home.

 

I was just reading in another post that has me thinking now. I added the factory cab clearance lights a while back and I read that they are supposed to be powered through a relay? I can remember exactly how I did it without looking but I don't remember using a relay. I know I robbed a wiring harness out of another truck and thought I used all the factory components.

 

I had a flashing light problem, but it was with the marker lights. It was caused by a short in the wiring on the trailer I was towing. Repaired the wiring on the trailer and everything was fine. For PA74F250 it sounds like your problem is in the high beam wiring because the low beams did not go off. I also have cab clearance lights and they are on the clearance light circuit not on the headlight circuit. Hope this will help you find your wiring problem.

 

The 1974 wiring diagram does seem to show a relay used for the roof lights: http://fordification.net/tech/images...aster_6of8.jpg

The factory switch is under-rated even in the absence of roof lights. If the problem goes away with the roof lights disconnected, it would give you added confidence that the roof lights are overloading the switch.

 

That's interesting and probably is my problem. With my new switch I'm back to when the problem first started. Eventually
Even my low beams will start flickering. I guess the switch gets wore out over time. I will unplug my roof lights one of these nights. Thanks for the help.