Underdrive Pulleys
 

I am about to order 3 pc underdrive pulley set. a friend of mind has an 84 mustang with a set on and his battery voltage is almost at 0 during idle and his lights may even dim, my memory isn't too fresh on that. i have a sound system and what not in my truck so i was wondering if i am going to have a problem with that or if the problem he having something uncommon

It depends on the pulley's and how you operate your truck.

Most pulley sets (though not all) have a disclosed charging RPM which is the RPM the engine must be turning for the alternator to work properly. Race only pulleys have a very high charging RPM.

Any pulley set will raise the minimum RPM the alt needs to operate properly. How this affects you depends a lot on how you operate the vehicle. If it idles a lot, or you do a lot of stop and go, and your minimum RPM is 1200, any time you're below that, the battery will be supplying the electricity and you will be loosing power. But once the RPM rises above that mark, the alt will start charging again. Be warned, however, aftermarket pulleys have ill affects on alts and batts. Some will never have a problem (due to a mild pulley set), others will have nothing but (wild).

I had a system in my truck, and in had installed some pulleys on there as well. Didn't like the result at all, unless you buy a higher amp alternater along with the pulley kit, It will not be satisfying.