I've replaced so many parts the truck is gong to be new......Anyhow I have a 1987 ford 150. When I trun on the blower/fan switch the battery starts to discharge. After an hour or two it will drain the battery. The Blower is new, alternator new, battery new, blower relay new. If I leave the switch off its fine. ( but I live in tampa and is 99 degrees today) Any ideas what this could be>? I really can't believe that its the blower switch, which I have not replaced. Also the new blower seems to get very hot quickly. (I did replace the blower relay too) Turning on the headlights or wipers does not effect the battery. Only the fan switch when turned on. please advise me

 

My guess would be the blower is defective, the motor coil creating a high resistance, or there is some screwed up wiring sending your battery power through a high resistance from the blower positive to ground once the blower is activated. Have you blown any fuses? Often these things ride just below the threshold of the fuse, enough to draw excess current, but just shy of blowing the fuse. Can you return the blower? Is it original equipment or some no-name blower, they can be very inconsistent. Hope this helps, Dave

 

One more thought, although unlikely. I'm not familiar with 87, Is the relay a one piece plug or multiple color coded individual connectors. It's always possible to cross a wire, but that would usually create a short and blow fuse.

 

Are you sure it's the blower and not the A/C compressor clutch that's drawing all the current ??