Hey guys,

Ever since I went to underdrive pulleys, my alternator has been putting out about 11.7 volts during idle. This is even with a high output alternator that I got from DB electrical. Can I change out the alternator pulley to something smaller to basically overdrive it to restore normal output at idle? Any recommendations on a pulley? Thanks in advance.

 

Could you just put your stock pulley back on?

I have had underdrive pulleys on car for well over a decade now and never had any battery related issue. I have never checked my voltage at idle, so I can't give a like for like comparison. Are you concerned about killing the battery? How often are just idling your car?

 

I mostly drive my car to and from work and a little jaunt on the weekends but there's not really much stop and go traffic so I guess it doesn't idle very long. My battery sits at 11.9 volts when the car is off. I think that's a little low. I have an Optima red top.

 

Your problem is a bad alternator and the only fix is to replace it.

 

My aftermarket high output alternator has always been like this since new. It puts out over 14v when cruising. It just drops to about 11.7 at idle because of my underdrive harmonic balancer.

 

I've had Steeda underdrive pulleys on my car for the last 7 years and had no such problems. At idle my multimeter shows 14.2v but then I still have a stock clutched alternator pulley. My alternator is a reman. 135A Motorcraft unit.
It's possible that your alternator has too low a current output at idle/low rpm. Although it's rated at 205A, it's possible you're only getting anywhere near that when your engine's at ~2000rpm (highway cruising speed). Raising the idle speed from 750 to 850rpm might help if you have an unlocked handheld tuner.

 

I have the SCT tuner and I've thought about raising the RPM but never got around to it. I'll try it and see how much it improves charging at idle.

 

Well I guess I can't adjust the idle speed after all. The "91 street emissions" tune doesn't give me options before programming. My other tune called "91 street" does have options and I can change the idle in there but I'm not using this tune. Oh well. Back to getting an overdrive pulley.