It wouldn't affect anything, so long as it quit slipping - or just slipped occasionally. All the serpentine pulley does is turn the accessories - but it also contains the TDC timing mark.... so if the pulley had not slipped when the timing was last set, the ignition and valve timing would still be fine - even though the serpentine belt pulley had slipped on the hub. The problem arises when you try to set the ignition timing AFTER the pulley has slipped on the hub.

If you want to try a shadetree fix, align the sprockets exactly on TDC and don't touch the crankshaft. Install the timing belt cover and the crankshaft pulley assembly, but barely tighten it so the crankshaft doesn't move. Put a spot of white paint on the serpentine pulley where it aligns with the "0" mark on the plastic timing gauge. Now you can finish tightening the crankshaft pulley assembly (and hope the pulley doesn't slip on the hub). If the pulley didn't slip and the crankshaft didn't move (you should be able to tell by looking at the timing mark on the passenger side cam sprocket), you can now set the distributor. To set the ignition timing, ignore the notch in the serpentine pulley and go by the white paint mark.

I don't recommend the above method, but it might work in an emergency.... although the next mechanic might scratch his head long enough for his hair to fall out. <img src="/forums/images/graemlins/lol.gif" alt="" />

Frank