91 Ford F150 Larado XL,302 V8, pw,pl,cruise, 60,000mi.
My issue is, when I am running my defrosters, and the compressor engages, my charging guage drops below the normal range, and when it disengages, it returns to the normal range. My question is two fold. Why or what is making it do this, and why is it necessary for the a/c compressor to run while the defrosters are running.. I am mechanically challenged but it appears something may be astray. Can I unplug the compressor temporarily to keep it from cycling. Thank you in advance for any advice. I am including an image to better explain

http://i144.photobucket.com/albums/r...hett/GUAGE.jpg

 

The compressor is intended to run to dehumidify the air, which is important for keeping the inside of all the glass dry.

First, remove, clean, & tighten every terminal on both battery cables (total of 5). Then go by a parts store that tests starting/charging systems free in the parking lot & find out if you have any other problems. Your alternator/voltage regulator could be going bad.

 

If your idle speed is dropping slightly when the compressor load is applied, the problem won't be with your alternator, but the idle management system.

Often, this means the IAC valve is weak/lazy.

Steve

 

Thanks guys

 

Yes, it's very common for bad battery connections to cause this symptom on these trucks.

 

Steve: I am looking for the voltage regulater (black box on a fender well?) can't find. Is it incorporated in the alternater. Thanks, Sparky

 

Yes, '87~03 has the VR in the alt. But those '87-91 alts are known to crack diodes & desolder themselves, so just get it tested on the truck.

 

Thanks Steve