General Maintenance & Repairs - Voltage drops at idle

I have a 99Z, when the car is first started the voltage meter is at about 15, but as soon as I put in gear it drops below 11 or 12 and when I get over 30MPH it goes up again and then back down if I come to a stop. Replaced the Alternator already and the battery post. What else could it be? Also have the under drive and overdrive pulleys.

battery have you replaced that??? and the meters on these cars arent really reliable so you might want to check it with a hand held volt meter to see what your volts actually are. what kind of mods or system do you have on the car???

yes the battery is new. I have a mild cam, all the bolt ons. I also have a big stereo system but I have it all unpluged for the troubleshooting. I've heard of people installing a bigger wire from the battery to the alt. do you think it will help? And do you know were I can get one. Oh I also had the computer reprogrammed (sent it off) would the comp have anything to do with it?

it's a pretty common problem with fbodies, and the pullies make it even worse. I had a LT1 that did it even with the stock pullies put back on, just not as bad. In my opinion, you're going to have to get a high output alternator or just put up with it.

or he could rebuild the alternator for much cheaper, there are kits they sell i think at autozone where you can clean the alternators wires and make it like brand new with great results from what ive heard from other board members. so many people just buy new alternators and they suck and then they just end up getting a new GM one that is alittle bigger in terms of voltage. dont know how much a new higher outputing one from the Dealership costs.

OK thanks guys I think I will try to find a high output alt.

I got my high output from afterthughts auto- sponsor to the right. also I would take off the overdrive pulley. bad on cars with big stereos. it might even fix the problem.
good luck

i thought the overdrive pulley was suppose to spin the alternator faster and produce more volts increasing voltage, is that true?

Black01_06 - Did the high out put make a difference and how much out put did you get?

i thought the overdrive pulley was suppose to spin the alternator faster and produce more volts increasing voltage, is that true?

yep, good to counter balance the under drive pully.

then why is he saying to take it off?? unless the guy doesnt have a underdrive pulley

hey guys yea i ran into the same problem with my 02 Z28. i replaced the alt with a 160 amp and everything was cool. i also just recently put on the APS underdrive crank pulley and alt and now it drops, i know its b/c of the underdrive alt pulley. my question is if i use the stock alt pulley am i not going to get as much HP as advertise on the underdrive crank pulley, they claim 12.5 hp, i dont know if that is with the alt too or just the crank.

Recently had the same problem with also a new alternator and good battery (Optima Red Top). I checked all the grounds and everything. With a voltmeter I realized that at times there was a big difference between the car gauge and the voltage at the battery, I'm talking a difference of 1volt!! The car manual refers that a reading between 11 and 16 is normal. If once you get rolling the voltage goes up your good. These car's charging system is not at all too well engineered to meet the demands of the electrical system.

do you mean you got a overdrive alternator pulley, if not then why did u get the underdrive alt pulley??

well my stock alt pulley is way smaller then the new one i got. and it doesnt charge like it did with the smaller pulley but i didnt have the undercrank pulley on either.

I just talked to one of the guys at an H.O. alternator shop and he told me to put the stock alt pulley back on first and that should fix the problem. He said the bigger the pulley the more HP you get but, you lose the battery recharging power you get with the stock alt pulley. If that doesn't work then im going to get a HO alt and try that... I reply when I find out if it works.

I got the bigger alt for the stereo and my volts jumping around. I had a overdrive and underdrive pully, was having the same problems, and when i took off the overdrive pulley it all settled down. It worked for me, may not work for you. I tried every combination I could think of, or replaced everything I could think of. these cars are very jumpy when it come to the voltage system

now i'm confused, the overdrive pulley should be larger correct? And this larger pulley should cause the alternator to spin faster, hence more charging power?

Thanks,
Adrian

Yes overdrive is bigger, the bigger the pulley the less rotations it gets, so it charges less. The reason for the bigger pulley for the alt. is to free up horsepower for the crank. I finally bought a 2"(smaller than stock) pulley for the alt and now everything is fine power wise. Still have something drawing 35 mA from the batt while the car is off. I pulled all the fuses saving the power accy for last and as soon as I pulled that one it went down to 8 mA, any idea's.:bang:

Ok, so I finally figured out where my battery drain is coming from. My Eclipse stereo. My next question is, do all after market stereo's draw 35-42 mA. or is it just mine? I hooked up my factory stereo and after the car settled down it dropped from .08 to .01-.02 mA

Where you got the smaller pulley for the alternator? You kept the damper stock?
maybe the Eclipse stereo has a lot of more functions requiring more juice, you know memory wise functions...

I got it from an alternator shop around town, now since ive put that on, the idle is real low. About 700 rpm in park, and when i put it in gear it acts like it wants to die, idle drops down to about 400 rmp just for a sec then goes back up, anybody know what would cause this? It just started doing it when I put the underdrive pulley for the alternator on.

Did you perform the idle relearning procedure?

No how do you do that?

1: Disconnect battery for 2-5 mins aprox.
2: Start car
3: Holding the brake put it in "drive"
4: Let it idle in "drive" for 5 mins.
5: Put it in "park"
6: Let it idle for 5 mins more
7: Turn engine off for 30 secs
8: Have a beer......