I have a pair of 1991 F150s (5.0L and 5.8L) and I'm finally getting around to fixing the worse of the two (5.0L). I just got a battery for it and started her up for the first time since probably September when I got it. I discovered that the signals don't work unless the brake pedal is depressed. When it is depressed the indicator on the dash is solid and does not blink. Both left and right signals do this.

Basically, I want to know where should I start to look for what could be causing this. I've searched around and I couldn't find anything related to this.

On a side note: I know this isn't electrical, but when it is in gear the idle drops from about 800 to 4-500 rpm. What could cause that?

Thanks in advance!

 

Start by checking for burnt out bulbs. Faulty flashers. Messed up wiring - especially trailer wiring.

 

I just checked the bulbs. All 4 are good and I found out that the hazards work and flash as needed.

Edit: Managed to get a glance at the tail lights when holding the brake pedal with the signal switch in the neutral position. The brake light faintly flashes and the (at least driver's side) signal flashes as if I was turning. Would this be a faulty flasher relay?

 

Well, flashers are cheap, so it's worth a try. Trailer wiring is also common. Your signal's crossing somewhere.