The book calls them hi and lo beams...I will go with it.

Anyhow, my truck only has hi beams and fog lights. The low beams never work at all. I'm not really sure when it started, but it isn't that fun. I checked both fuses (9 & 11 under hood) and they were good. I replaced both bulbs as well.

I do have a snowplow and the lights were working fine this winter. I also did the mod that makes the fog lights stay on with the hi beams. These are possible sources of error. There is something strange going on with the fog light mod because they come on with the park lamps. I copied a couple other guys who talked about the mod and did a write up on it. Maybe you need to tie into a different wire? Maybe this couldn't cause a problem anyhow.

The only guy who replied on the Super Duty site said it might be the MFS. I assume that is Multi Function Switch...the turn signal, hi/lo beams, wipers, etc switch. Should I just buy a $50+ MFS from RockAuto and take my chances??? Any thing else to look at? TIA!

 

Don't buy the MFS unless you are sure that's the problem. I didn't see a model year in your post, but here is the headlight wiring diagram from 1999:



I don't know where your plow lights were wired in or how the fog light mod was done, but hopefully this diagram will get you started.

Reply back if you have daytime running lights and I will post the correct wiring diagram for them.

Chris

 

Sorry, I am still running the truck in my sig. 2001 F-250 Ext Cab

Followed the link in my sig for the fog light mod. It worked for a long time, but maybe it fried the switch???

IIRC the lo beams were on with the hi beams for a long time. Maybe the switch had been going out for a long time?

What else could it be if it wasn't the MFS? It couldn't be the "Main Light Switch" because the hi beams work. I guess the question is where should I start trouble shooting?

 

Woops, I missed the truck in your sig.

We know the main light switch is getting power because your parking lights work (I assume), so I would start by looking for power coming out of the main light switch to the MFS (red/yellow). Next, make sure you have power coming out of the MFS (red/black) to feed fuses 9 and 11. If you're good to there, follow the dark green/orange and dark blue/white wires coming from fuses 11 and 9, respectively. They should lead you either to your low beams directly, or, more likely, to the plow control or light mod. From there, look for power going to the low beams on the same color wires (DG/O and DB/W). If any of those colors don't match up to my diagram, it might be due to harness changes between 99 and 01.

Chris

 

It looks like the problem is in one of the plow relays. I cheated and went straight to the 9 and 11 fuses and I had power there.

I was at a stand still because my plow stuff was between there and the headlights. I reviewed the plow wiring diagram and saw there were two relays. One for hi and one for lo. I didn't know what to do from there so I swapped the hi and lo beam relays. I now only have low beam headlights. That must mean the relay that follows the problem is bad??? I haven't bought one yet, but I would guess that is the problem.

 

Looks like you found the problem .

Chris

 

I'm practically an electrician now. Is there any other way to test a relay?

 

Is it a bosch style relay like this one?

Relays

 

Yes. A relay is nothing more than a switch that operates when a voltage is applied to a coil. The current flowing through the coil causes a magnetic field which pulls the switch contacts together, it's as simple as that. They come in varying switch configurations and current ratings, and the coils come in various voltages, but that's all there really is to it.

To test the relay you need apply the correct voltage to the coil while checking the resistance of the switch contacts to make sure they closed. Using a multimeter set to measure resistance, you should see an open circuit (infinite ohms) across the switch contacts with no coil voltage. When the relay is energized you should see a closed circuit (zero ohms, or close to it). If you see a closed circuit when the relay is not energized it is shorted. If you see more than a fraction of an ohm when it is energized it had bad contacts.

Keep in mind that there are lots of diferent relays, and some may have several sets of contacts. There are a few threads on here that have the pinout for some of the more popular relays, such as the Bosch.

Chris

EDIT: Check out the link that carltonwebb posted ^^^ as I was typing!

 

Yes, it is. (but mine has the top on it.)