Ive heard that pullys will mess up am oem alternator, is it true?

[Mr 5 0]

The underdrive pulleys turn the stock alternator slower so it puts out less amps and this can strain the system, especially at idle with lights, stereo, A/C fan all going at once.

Generally, a stock 75-amp alternator in good condition (with a good battery) can handle it but older ones often have problems.

I would suggest that if you install the pulleys prepare to upgrade to the 130-amp alternator. If you don't need to, great, but if you do, you'll be ready.