Left turn signal works, then doesn't, then does, then doesn't
 

2008 F-150 SCREW, Limited, 68,000 miles, bought new in 2006. (Oops...I mean 2007!)
Never a problem of any kind, until this pesky little deal: the left turn signal gets an intermittent rapid flash, as if there's a burned-out bulb. When this happens, the left rear does not flash. Left front rapid flashes, as does light in left outside mirror.

I took the left rear lens off, and bulbs worked fine. Just to be sure, I swapped bulbs with right rear, and all still worked fine. Buttoned it back up yesterday, and drove all around. Turn signals all worked perfectly.

Then today, left signal again began that rapid flash. Twenty minutes later, it's fine again. Then another hour later, the rapid flash is back.

I give up! Don't know where to start looking for a fix. It's only broken sometimes - not all the time.

Any ideas where to start?
Maybe the flasher module itself?

All wiring is stock from factory, with one exception: I added an electric door lock to tail gate. It locks with doors. Been there for 4 years, and is not involved with turn signal circuits anyhow.
BTW, I set the steering wheel tilt when I bought the truck, and have not adjusted it since. (Some say tilting wheel up/down over the years will short turn signal wires.)

Any suggestions will be greatly appreciated,

Thanks,
CW

You will need to do a visual check for damage to the ground wire from the bulb connector to the ground termination point. While you are at it, also check the circuit for the S/T part of the bulb.

This sounds a lot a wire that is crimped somewhere, and it is making and breaking contact.

Drove for 2 hours yesterday. Turn signal began working properly 10 minutes into drive, and worked fine for rest of day. We will see what today brings.

CW

BTW, what is the "S/T part" of the bulb? (Sorry to be obtuse).

I had previously rotated the left and right side bulbs to eliminate possibility of a bulb fault. I don't know what you mean by the S/T part, so I may have missed something that should have been obvious.

Fortunately, these bulbs are very easy to get to.

CW

If you were being obtuse does that mean I was being acute about it ? ( the 2 make up a straight line )

Rim Shot
Thank you, thank you I'll be here all week, prime rib is on special, don't forget to tip your waiter...

Sorry been one of those days.

S/T I was trying to get at Stop / Turn. The bulb connector has 3 wires to it.
1. Black which is the ground
2. Brown which is the parking lamp section
- This may or may not have an issue at the same time as the turn signal, but you would have to have someone following you to tell you this.
3. The Stop / Turn circuit from the MFS ( MultiFunction Switch ). The color is going to be different on each side.

Well, it's been 4 days now since the last malfunction. Turn signal seems to have fixed itself. Whew! That sure is a relief! For a while there, I thought it might require a visit to the dealership.

Got to love trucks that repair themselves!!

CL

I had this happen earlier this year.....turned out to be a bulb that was going bad....just replaced the bulb.....hasn't happened since....

Could be the multifunction switch (the whole turn signal lever/switch assembly on the steering column). I had similar symptoms with mine a few years back (on a '97). The cold temps can make the situation worse, so it may not work initially on a subfreezing cold start, then start working as the cab warms up. I think it has to do with expansion and contraction of the connectors, so cleaning and reconnecting them may fix it, but I just replaced mine rather than trying to fix it.

wait you bought a 2008 in 2006?:confused:

But have had that happen to me as well. only happened twice in the 2.5 years i have had mine.

Oops...I meant bought new in 2007...November 2007 actually.

Anyhow, the blinker failure returned yesterday...continued today. I may take it to dealership tomorrow. I'm all banged up after my bike wreck, and am in no mood to chase down the problem myself. Trouble is, I don't have any hope the dealer tech will have any better luck than I would have. Maybe I shouldn't be so cynical...but I really miss that left blinker!

When it fails, turn on the hazard lamps.

do all 4 work correctly ?
- if one is out, you won't get the fast flash, you will have to take a look at the Stop / Turn lamp itself.

The fail is still with me. When I activate the 4-way hazard switch, only 3 corners work. The left rear does not light, and does not flash. all others do. I have changed bulbs and problem persists. Next I will plug in my trailer to see if left trailer light flashes. If not, I'm not sure what that tells me, but it's easy enough to try. Decided to wait until next year to go to dealership for fix. Hope I don't get hit or get a ticket in interim!

CL

If the trailer works on that side, that tells you the MFS is good, and the output to the Stop / turn fuse for the trailer is good, so it is the other way to the bulb socket.

Did you get a chance to check the wires at the back of the truck ?
- You have the symptoms disappear, and no mention if you checked this.

This would be using a pointed probe ( or ice pick ) into the insulation back from the bulb connector, to see if there is power there ?

Trailer lights work just fine; will try probe tomorrow to see if tnere's power to the bulb. If so, the I must have a ground fault somewhere. Either way, at least I'll know which wire to trace: POS or GND.

PITA for sure!

OK, you know the MFS to the CJB ( Central Junction Box aka Cab fuse panel ) is good, at least to fuse # 42.

The MFS goes to the CJB, inside the CJB is a T tap, that the trailer tow adapter fuse is on, and then the circuit exits the CJB, and continues to the rear of the truck. Both the rear stop / turn and the Trailer tow adapter stop / turn use the same contacts in the MFS.

Worth checking both the ground & the light green w/ orange stripe wire at rear of the truck at least 12" away from the bulb connector. If you can follow in under the truck, might want to check there.
A small ice pick or test light should be able to pierce the insulation to allow you to test the wire itself. There are pin adapters for meters, but I don't think you need to spend the money on it, if you have a test light or ice pick.