I have yet to see this truck but a friend reports his front left signal on his '08 is not working. He replaced the bulb and same results. He says it DOES work with hazards.

I'm wondering if the flasher relay behind the knee bolster can cause this? The fact that the filament is capable of flashing with hazards makes me doubtful of ground or wiring problems at or near the bulb socket?

 

Flasher relay cannot cause a single turn signal to be inoperative.

Probably a failed MFS.

 

Thanks for the reply! I was also wondering about MFS but seemingly every complaint I could dig up on them was due to mechanical failure (ie, floppy, doesn't click, doesn't cancel) or high beams stay on/come on at random.

I'm not saying that means it's impossible, I was merely searching for typical, common failures.

 

The MFS is what sorts out what bulbs work whenever. There are numerous things that can go wrong with it internally besides just what you've dug up.

Consider:
1. The flasher's input would affect front and rear, left and right equally.
2. Since the LF signal works with the hazards, we know that the circuit from the LF signal output of the switch is good, therefore the problem has to be between the flasher input and the switch's output.

FWIW, the failed portion is indicated here:

 

Thank you SHO!

I was wondering if the relay had separate pinouts for front vs rear....it didn't make sense to me that it would. It makes more sense to be in the MFS if they're going to break out front vs rear.

Thank you for the schematic as well!

 

I think SHO called it (as usual) because I just got hands on the truck and indeed the mirror signal and the green arrow in the instrument cluster do not come on with left signal. They both DO work with hazards...so I'm leaning toward MFS

 

Ok well thanks to SHO's kindness in providing a schematic I was able to backprobe the harness entering the MFS. The LG/WT (front left) wire was dead with left signal on. The LT/OG (rear left) was flashing.

Of course at that point I was 99.8% positive of the problem but just to be 100% I swapped in a known good one from my '07 F150 sitting right here and of course the problem was resolved. Options are limited so I'm replacing with a Standard Ignition from O'Reilly. Obviously Motorcraft would be preferred but like most shops I'm limited to what's available today in this case.

You can just make out one of my backprobe pins here: