Ford F150 - high and low beams out

penney75 · Posted: Thu May 08, 2008 5:24 pm Post subject: high and low beams out

My low and high beams keep blowing a fuse (20amp). Fuse does not blow right away takes a day or 2 to blow. Park, fog and interior lights work fine. Any thoughts? Really hard to diagnose when fuse doesn't blow right away.
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Edge, CBA exhaust

TONY4848 · Posted: Thu May 08, 2008 6:05 pm Post subject:

Which 20 amp are you blowing, there are F40 controls the Low beams and the High Beam Relay thru the Light Switch, F25 and F23 are for the low beams and F35 is for the High beams.

Since both are out I will take a stab at F40, it you worst one to trouble shoot because it goes thru the Headlight Switch and Multifunction switch plus some other area AutoLamp control and Body Security Module, Daytime running lights if equiped. I had a Headlight switch fall apart inside and short to ground but it was constantly shorting, you got an intermintent problem, most likely a wire shorting when you hit a bump. Also you could check the High beam coil but if they fail only when you have the low beams on then it is not that.

Fuse do not blow in a day or two. they are not timed delay fused in most trucks, once you hit that magical number printed on the fuse it opens. granted if you were at 19amp it would get hot and maybe break on a bump or something.

You say it happens 1 - 2 days, do you have any pattern like to drive home at night and there is a bump you hit with the lights on. When do they go out?

Low beams go out then you try the high or you turn the high beams on and both lights go out?

What about your parking lights? Dash lights?

penney75 · Posted: Thu May 08, 2008 6:17 pm Post subject:

I would have to check but I thought it was fuse 35 (20amp) which is supposedly for high beams. However low beams also go out when this fuse blows. Dash and park/fog lights still work. Don't really see when the fuse blows as it has always light outside when it blew. Does anyone know where I can get a wiring diagram for the headlights?

penney75 · Posted: Thu May 08, 2008 6:27 pm Post subject:

Sorry it is fuse 40 that is blowing and I do have daytime running lights

07_F150_07 · Posted: Thu May 08, 2008 10:31 pm Post subject:

I would say the DRL relay is probably bad which if i had to guess, that Relay comes off of the same fuse as the main light power. Not sure but just a guess, try unhooking the DRL's I think you can just pull the relay for them but not positive. I dont have them and i dont know anyone that does.

TONY4848 · Posted: Fri May 09, 2008 2:24 am Post subject:

both Haynes and Chilton manuals have the headlight ckt and they are the same in either one. But if they blow durig the day.
Relay for the DayTime Lights is C1439, it is located in the Aux Relay Box 1 (engine compartment near the battery, left fender). As Mr.07 indicated, it the first thing you can elimated from your troubleshooting.

If it stll blows, make sure you do not have your lights in Auto Mode.