I have an 01 Supercrew Lariat. A noticed my right low beam had burnt out then about a day later my left went as well. I purchased new bulbs but when I pulled the old ones they looked fine. The new bulbs did not work as well. High beams work fine so do the flash to pass lights.
I have daytime running lights which when activated only have the parking lights and high beams. Fog lights work as well.

Now I don't know much about electronics but I know how to do a search and how to read. So I found a few threads that provided some trouble shooting. So this is what I've done so far:

I've checked Fuse # 3 30A in the power Distribution \ Battery box - has continuity and meters for power

Then I checked low beam Fuses # 26 & 28 10A - both checked out for continuity and metered for power.

I also checked the hi beam fuse # 16 20A which checked out the same.

Next I removed the steering wheel column and metered the red w\yellow wire going into the Multi-Function Switch for power which it did. Lastly I metered the dark green w\orange stripe (right headlight) and the light green with black strip (left headlight) going into the headlamps, these both metered fine.

Signal lights, hazards, break lights and so on all work fine. The only think that doesn't work is my trailer lights but my 7 pin round connector is in rough shape so that might be hte problem there.

So there you go.. Help! Why don't I have any low beams?? Does it look like my MFS? Or something else?

 

Hmmm... Puzzler. I can see why you're stuck. Well researched and pretty well executed.

Since your fogs work correctly, the MFS appears to be working properly.

During DRL operation, the high beams are not turned on. The DRL circuit only runs the low-beams and the fogs at reduced voltage. If the headlight switch is turned to PARK, the parking lamps are added at their normal intensity.

Based on your description of what you've done, my logical conclusion is that you've got four defective bulbs unless....

When you checked the DB/WH and DG/OG wires at the bulb connector, what was your voltage reading and was it taken with the bulb in or out of circuit? The LG/BK wire is the high-beam lead....

Check it with the bulbs in-circuit and low-beams on. Should read full supply voltage. If so, I'm back to the "four bad bulbs" scenario in the absence of any new data.

BTW, did you make a resistance check of the bulbs' low beam filaments?

If you'd like the schematic and the diagnostic procedure from the factory service manual, shoot me a PM with your email address and I'll send the stuff back as a pdf file(s).

Steve

 

Sorry it was the DB/Wire I tested for the left low beam not the LG/BK.

I tested with the bulbs in and with the headlight switch turned to on and the MFS in low beam setting. I actually used a circuit test light not a multimeter so I don't know the actual voltage reading.

I didn't check the low beams resistance on the bul, I'll try that next.

I'll also sent a P for the diagnostic PDF, thanks.

 

Steve, I'm not allowed to send you a PM, the board won't let me. can you send me one and I'll respond with my email address?

 

Check your email. Board rules likely require a minimum number of posts before PMs are permitted if your Control Panel options are properly set.

Steve

 

GeoFan,

I almost feel useless with this type of detail

With the main headlamp switch on, MFS is in the low beam position, and power found at one side of the fuse socket for CJB fuses #26 & #28, this tells you the MFS is operating correctly. This is the low beam output of the MFS, and as Steve pointed out, the fog lamps operational.

Q1. Background question, did you modify your main headlamp switch with the fogs in low or high and / or the ramp job on the switch ? ( FedEx pilots mod )

With power to the low beam fuses in the CJB ( Central Junction Box = Pass Compartment fuse panel ), The only part left is from the CJB to the bulb connector, and one other thought.

Other thought = can you get a DC meter, and check for resistance on the ground side of the bulb connector, as well as check for power on the low beam pin ?
If you are having problems with the ground on the bulb connector, the high beam might be deriving ground from the low beam side of the circuit, causing the high beams to light, at a daylight un-noticeable lower wattage ?? ( Huge SWAG right now ).
Best to get a Meter ( cheap one would work if you can't lay out cash for a good one about 10.00 at Radio Shack / Walmart, etc ) and check the bulb pins for the correct voltage, and continuity to frame / battery ground.

Diagrams for the headlamps :
https://www.f150online.com/galleries...028-167986.jpg
https://www.f150online.com/galleries...028-167987.jpg

I would pass on the MFS testing at this point, with voltage being at F#26 & 28, but in case you want it, MFS testing diagram :
https://www.f150online.com/galleries...028-172815.jpg

If you got a hold of Steve off line for the other stuff, that might be more useful, I am just looking at the CJC fuse to the bulb connector, which seems to be where power is not making it through ( BTW : Did you change a fuse for the heck of it ?? Just another thought = SWAG again )

 

There is no possible ground path for the high-beam current via the low-beam circuit. Since the high-beams both work and the low-beams failed separately, the grounds (they are separate) are not at issue.

Has to be a huge coincidental failure of the original bulbs along with a problem with the replacements....

Steve

 

Thanks guys, you've been a lot of help.

I'll re-test everything with a multimeter. I did borrow a auto ranging meter but I'm a bit emarrassed to say it has so many options I didn't know which setting to use to check voltage. This is the model I have:

http://www.canadiantire.ca/browse/pr...romSearch=true

I didn't do any modification to the fog lights, everything is factory.

Since I have my steering column off I'll also check all the MFS circuits just to make sure I didn't miss anything.

 

While I will agree that this is a HUGE SWAG ( I mean REALLY big here ), the grounds are common at the bulb connector, and at the bulb ( only 3 pins & 3 wires to the bulb connector ). Each side is grounded to the truck on separate lugs, which would bring into case that both ground lugs would have to have issue, at to a close to the same time.

The question I cannot answer ( not in the same city as my '06 to check ) is if reference ground is at the fuses #26 & 28 in the CJB when the MFS is in the high beam position.

With power at the CJB fuse positions, that only leaves wire and connectors to the bulb itself.

 

The URL above does not go right to the meter, it asks for a CAN postal code ( got me what one is ) to see it ??

The meter might have a DC V or a V with 2 dashed under a solid line ( that is some times used for DC voltage, where AC is a Sine wave ).

Here is a page that might help you with desyphering the face of the meter:
http://www.allaboutcircuits.com/vol_6/chpt_2/1.html

 

Each bulb has a separate grounding point. They are not in common. Each bulb has three terminals: Ground, low, and high. Therefore, there is no common ground except the section of the body that they screw into.

There's no "reference ground" in this circuit or the high beam, much less at the CJB fuses.

Gotta be the bulbs in the absence of any additional data.

I sent Geoff some stuff to help him out. We'll see how he makes out.

Off to work. later.

Steve

 

Well Steve was bang on, the issue was the bulbs. I re-did the dianostic this time metering the voltage at the fuses, relays, MFS, and finally the bulbs - all checked out at 12V (or close to it , 11.98V etc..)

So I bought a couple more bulbs and everything works as it should.

Very wierd coincidence though. The original bulbs blow within a couple days of each other then both replacement bulbs are bad as well. Funny thing is the bad replacement bulbs physically looked just fine so it must have been a bad batch.

Anyway thanks for all the help I learned tons from this little exercise.