So putting my '91 F350 4x4 on the road, the head lights flicker on and off, and twitch on it.


Only does it once the *engine bay* warms up. If I start it cold, give it a few min to warm up, take off and get on the highway with it, they'll work non stop for as long as I stay on the highway, soon as I start hitting red lights, or not moving fast enough, i'm guessing when the engine bay gets hotter, they start to flicker and not really work.


If I shut them off for a few min and driving on the park lights, and turn them back on after, they'll work again for another 5-10 min before starting to flicker again.



I moved around and played with the wiring real quick, doesn't do anything different when I do.


Any ideas?


Also, again when the engine bay gets hot, the dash temp gauge flickers as well, goes all the way up to overheat, or twitches same goes for the red Engine Light. It flickers and twitches with the gauge.

 

Alternator/regulator? Highway speeds has the RPMs up high enough to keep up but not at city speeds, running with them off allows battery to charge back up. (Not sure why it wouldn't do it on startup though, unless its a heat related problem with alternator/regulator)

 

I had the exact same issue and it turned out to be the light switch was shorting out somehow so i got a new one and problem solved.

 

Have you checked your body grounds ? Could be a ground issue . Just a thought !

 

Yes , headlamp switch and plug should be checked . Seen that before also . But that won't explain the temp gauge issue , maybe multiple issues .

 

Is this the same issue on the 83-87 body styles? Have the same issues..

https://www.ford-trucks.com/forums/s...070&styleid=19

 

This won't be an Alternator issue, it charges great, and it's only the head lights alone that flicker on and off, they don't dime, they simply turn on and off non stop.

 

The head light switch witch is also the high beam switch on the floor, looks brand new.


JWC 3, the plug your talking about, is the plug on the floor high beam switch correct?



Haven't checked grounds yet. If I remember correctly, theres suppose to be one from the frame, to the drivers side floor right? I think that one isn't even there anymore.


The entire cab was redone on this thing years ago, new floors, rockers, cab corners on both side.

 

Check the high beam switch and plug for corrosion . Also check the headlamp switch and it's plug , they can overheat and melt , causing a poor connection . All grounds should be in place and in good condition , or you can have odd issues .

 

Check the fender solenoid, the main power run for the headlights comes off there and could be loose or corroded, also could be a weak circuit breaker in the headlight switch though doesn't explain why they work fine on the highway, but one thing that may be doing it is the daytime running light module (if you have one) its on the front of the rad support and would have the change in temperature your talking about.

Also there is about 6 grounds that are all tied together for just the engine bay harness, one on the voltage regulator mounting plate, one on the fender solenoid, one on each fender up top close to the rad support, one ground right from the battery and one more ground going to the fender to cab upper mounting point on the drivers side.

 

The fender solenoid is new, and i've had to redo a couple wires on it, cause the old ones, the ends corroded off.


Daytime running lights module is probably not there, truck has a new rad support, and the connector for the daytime runners is hanging down in the bumper.


I will check the grounds around the engine bay, and recheck my wires I had to make for the old corroded off ones.

 

Had the same problem on my 94. Wasn't real bad,but they would just go out. I could hold the dimmer switch all the way back and have lights. I replaced the multi-function switch, but didn't fix it. Replaced the headlight switch and no more problems. The plug had been hot. Some said that it might be the switch and wire harness.

 

I agree, it's one of the switches (dimmer switch on floor or on / off switch in dash). I heard guys talking about a kit sold by LMC that eliminates this problem. I think it changes the wiring so the headlights are triggered by a relay or capacitor or something like that. I'm not an electrical guy, hate wiring problems! The kit is designed specifically for this problem.

 

I disagree about the switches. The LMC kit, or roll your own kit with relays is meant to keep the load off of the headlight switch to keep it from melting. I don't think it, or the headlight switch is actually the problem here, unless there is a relay under the hood that is failing.

If you look at the symptoms, its the ENGINE BAY temperature that is causing the issue. Now I might be wrong, but I'd be willing to bet that the dash switch has nothing to do with this because its in the cab, not the engine bay. This sounds totally like a wiring/connection issue under the cab either at, or near the headlights. Plenty of cooling air flow going down the highway, but not in stop and go traffic. Also, if the lights are off, the wiring/connections cool down without the current flowing through them.

I would start at the lights themselves and move back along the wires from there. Check to see if either plug is loose at the lights, unplug them and check for corrosion. Trace back to the next connection and repeat. Could also be a broke connection in the wiring, so wiggle them as you go along. Since the problem occurs only when the engine is hot, try troubleshooting then and see if you can find the issue, might make it easier since they seem to be working fine when cold.

For the engine temp gauge/light, check the wiring to the engine warning light sensor. NOT the temp gauge sensor. I can't remember which is which, but I know the picture is all over on here, and you've probably even posted it... The overtemp warning light will override the temp gauge and peg it to high if it goes off. It happened to me before when I overheated and the overtemp sensor stuck constantly pegging the gauge with the engine light on. Unplugged the overtemp sensor and the light went out and gauge worked fine.

 

So update, the flickering engine light/temp gauge, was just a bad sender for the temp light, I simply disconnected it, and the gauge works fine now.



As for the flickering head lights, I think it's a wire that I added with a female end connector that goes from the fender module - to the fender solenoid. Once it's warmed up and starts flickering, if I play with that wire I can make the lights flicker as I play with them, so hopefully that is the problem, I'll redo that wire with a better connection on the solenoid, and we'll go from there.




On a side note, I kind of regret not using this truck as my winter truck in the first place.


Fueled it up today at 13.2mpg compared to 11mpg in my F150, for twice the truck.

Not bad for a 4x4 F350 flat bed with 166k miles on it, engine appears to be all original, injectors and all. Does have the E4OD with 4.10's and boy does it hit 2nd gear really hard when it's warmed up.


I might hit over 14mpg after the alignment, and injector pump advance, the engine is a lil quiet, so i'll advance the timing on it abit, see what it does.