Got a blinker problem that I cannot figure out and need some help. I have a 74 F250 and the left rear blinker doesn't work at the taillight. Basically, all other blinkers flash correctly when I have the blinker turned on to either left or right, except for the left rear. If I have the headlights on, the left rear light works, as does the side marker, but it does not flash if I turn the blinker on.

I used my test light on the wire that runs power to the left rear bulb for the flasher, and it does not show any power. I tested this on both sides of the plug where the rear wiring harness plugs into the front, near the engine on the firewall. I also checked the ground at the left rear bulb and it looks good. I assume it would have been the cause if none of the left rear lights worked.

From testing the wiring harness where it plugs into the rear harness, it seems the problem may lie deeper under the dash, but I'm at a loss here. Anyone have any ideas where I should check next?

Thanks
Chris

 

From what you say I take it your Front Left T/S and dash left indicator lamp works.
If so and you have no power on your Y-BK wire coming out of the steering column then I would say you have a bad Turn and Emergency Signal Switch.

 

Yes, my left front turn signal and dash indicator work fine, just the left rear doesn't. I wasn't sure if I was missing a ground somewhere, or if a bad turn signal switch would allow the left rear to quit working, but not the left front.

Thanks

 

Do you have power coming out of the harness to the left rear?


You said that you did not have power at the left rear lamp. Thus, if you have power at the harness and not at the lamp, you've got a broken wire somewhere between the two.

Also, did you check the bulb?

 

I the left rear brake light works, then it's almost guarenteed there is something wrong with the turnsignal switch. The left rear brake light uses the same filament in the bulb, and the same wire all the way from the turnsignal switch to the rear as the left rear turn/blinker circuit does.

 

Yes, the left rear brake light works, and it also workes when the headlights are on. Since I tested the wire to the rear left blinker before it plugs into the rear harness, it makes sense that it would be the turn signal switch.

However, if the front left blinker works, as does the dash indicator, would the signal switch still be the problem? Does the wiring harness for the switch have independent wires for the left front and left rear?

 

Yes. There are three sections to the turnsignal part of the switch.

There is one section for the front turnsignals. It works like a simple switch, putting the flasher on either front bulb and disconnects both when in the center position.

The other two sections are for the rear turn/brake bulbs. This is more complicated, since when in the center position, both rear lights have to be hooked to the brake switch. But when the signal lever is moved, the brake signal wire is taken off the bulb, and the turnsignal flasher is connected in it's place. All this happens while the other side is still hooked to the brake light switch. Flip the signal lever the other way, and it all happens again, only the sides swap.