Hey y'all,

My 1991 F-250's check engine light comes on when I turn on the heater, A/C, or defroster, and when I turn off the A/C the light goes back off again. I'm thinking it might be a bad blower motor (or maybe my battery?), the A/C has always kind of bogged the engine down for a minute when I first turn it on, so I'm wondering if this problem has been coming on for a while. Any ideas?

Thanks

 

Check pcm for codes...

 

Sounds like the alternator is weak.
Couple of thoughts here for you....

'91 would have a 2G alternator. It's garbage. It's fire prone.
You can pull the alternator and inspect the brushes. If the brushes are worn far enough, that could be causing your CEL. If the output voltage drops below 12.5, then every sensor that has 12 Volt's is gonna start acting weird.
When the brushes wear to far, they can chew into the slip ring and once that happens, it's time for a new alternator. My ex-wife didn't tell me the battery light was coming on and off in the 2000 Expedition we owned. Well, I found out and it was to late. One brush destroyed the slip ring and it was time for a new alternator. You can rebuild the alternator, but I wouldn't waste my time with a 2G.

If you have 1 or more diode's failing, that can cause a similar problem with the alternator not able to keep voltage up and that will drain the battery.
My '92 had a 2G and one night I was making a right turn with headlights on and blower on. The whole truck bucked and jerked, CEL came on, headlights dimmed and blower slowed.
Pulled alternator next day, found 2 bad diodes in it.

And if you ponder replacing your alternator, then I HIGHLY recommend you do a 3G swap. You can find plenty of posts mentioning how to. And once your done, you won't have to worry about the fire risk and your voltage will keep up regardless of blower, AC, and headlights being on.