So, I think there was a similar question on here a couple of weeks ago, but I don't think it was ever resolved. Here's my story:

I go to start the truck up ('90 F-150 5.0) yesterday, and notice I have a burned out headlight (low beam only; high worked fine). No big deal, I get replacements (silverstars, 9004 I think) this afternoon on the way home from work. I check the number against the old ones, they match. I install them, both high & low now worked fine.

A couple hours later, I start the truck, make sure both lights are still working, and head out to meet a friend. I'm doing 60 down the highway, and suddenly both lights go out. I go to click the high beams on, they came on for about 2 seconds, then go dark. The blue light in the dash, and all other lights seemed to be working. I got off at the next exit, parked in a park 'n' ride, and opened the hood. There wasn't a heck of a lot I could do, I thought maybe I didn't click the connection in properly or something, but it was all still tight.

I locked it up, and it will have to sit there until tomorrow afternoon at the earliest. Anything I should be looking at when I finally get my hands back on it? Did I do something stupid and burn the new bulbs out? Old/bad fuse? By the way, where is that fuse? Is it with the switch? I had pretty limited light, so I couldn't really do anything beyond what I've described.

Any help is appreciated!

 

When you installed the new bulbs, did you avoid touching the glass part and getting oil from your hands on them?

 

Yep, I made sure not to touch them.

 

I have a 1990 F-series owner guide right here. Two things I could find pertaining to the headlights.
1. Headlight and high beam indicator. 22 amp circuit breaker. Integral with headlamp switch. If the blue light in the dash you mentioned was the high beam indicator light, it's most likely not that.
2. Headlamp switch and fuse panel feed. 18 Ga fuse link. Near R. H. fender apron and dash panel. (don't ask me where that is)

I don't know if either of those would be the problem. Good luck.

 

I have a similar issue on a similar truck. 1990 F150 5.0L V8, PT 4WD. Headlights sometimes turn themselves off, whether in high beam or low beam. If in high beam, the blue tell-tale in the instrument cluster DOES go out with the headlights. The instrument cluster lights themselves (e.g. tach, speedometer) do NOT go out; I'm not sure if the running lamps go out or not. Also, sometimes when the low beams are on, the blue "high beam" tell-tale lights up dimly. I replaced the dimmer switch, but problem remains. I've also wiggled every wire I could find without being able to replicate the problem. Any insight appreciated.

 

In your case, when they do go out what does it take to get them back? Do they just come back when they feel like it?

 

They turn off and on as they choose, once the switch is in the 'on' position. Last night I was stopped at a red light and while I was sitting there they turned off, waited perhaps two seconds, then turned back on.

 

So, I went back to the truck after work, checked the lights again (they still didn't work). Started it, drove home. At some point, the battery light came on, and the voltage spikes. It wasn't all the way against the peg, it was probably 17-18v or so. I got home, took the bulbs out, and they're definitely burned out (chunks of filament rattling around in there). Started the truck again, voltage read normal, checked voltage at the connectors, it was ~14.

I'm assuming my voltage spiked last night and burned the bulbs out (what else could I have damaged?). I'm thinking voltage regulator or something? You'll have to forgive me, I'm pretty stupid when it comes to electrical systems, so if it sounds like I don't know what I'm talking about, it's because I don't ;-).

The battery is only about 3 months old, if that matters.

 

If nothing else quit working, then the voltage spike probably didn't hurt anything else. Might have shortened the life of some other bulbs, but MOST electronics are fairly well hardened and have input power conditioning/regulating. If nothing is fried now, it's fine.

I'd say that voltage spike was caused by a bad regulator. You can buy just the regulator, or replace the entire alternator.

And I was fixin to say that the lights turning on/off like that tells me the breaker (which is built into the main light switch) is cycling. Either it's weak, or your SilverStars are drawing too much current. There's a headlight relay mod you can do, if you search on here you'll find it.

 

Unplug and take a good look at the harness plug at the alternator.
The connector over heats and makes poor contact with the regulator.
The insulation of the wires can also melt which can cause the wires to
short together.

The headlight switch plug also had melting/over heating of the wiring
problems. You should remove the headlight trim bezel and examine that plug.

The pigtails/plug for both are sold at most auto part stores.

 

You were absolutely correct. I pulled out the switch plug and disconnected it from its socket, and the socket has obvious heat damage. Thank you very much!

 

After replacing the headlight switch and connector, install a headlight relay setup. This will take the load off of the switch and provide more power to the lights.

I have the RJM relays. You can also get them from LMC. Another option is making it yourself.