quick alternator question(pulley size)

[mustangGT85]

i have a friend i believe is running an underdrive pulley set-up on his 89 5.0. and i'm sure its killing his battery, cuz he can push start the car once the battery has been charged for a while, but after driving it for a short while it dies.(i'm assuming the alternator isnt putting out enough juice to charge the battery)

here is a pic of his engine bay: http://www.torquecentral.com/attachment.php?attachmentid=14684
my question is, first off, is that an underdrive pulley? (he bought the car modified to where its at now) and second it looks smaller than a normal alternator pulley. to me. maybe i'm wrong. but the second question is, would putting on a stock size alternator possibly solve his problem. i'm going to go by tomarrow and check out how many volts the battery is putting out and test this therory.

but the thing that is perplexing me, is, if we were to put a stock, larger size pulley on, wouldnt that slow down the alternator and make less charge?

please inform me? thank you


~Steve

 

[srothfuss]

first off, is that an underdrive pulley? (he bought the car modified to where its at now)

It doesn't appear to be an underdriven pulley from that photo. It looks stock

second it looks smaller than a normal alternator pulley. to me. maybe i'm wrong. but the second question is, would putting on a stock size alternator possibly solve his problem.

Actually, the smaller the pulley the faster the alternator will turn. This in turn puts out more voltage at idle for higher load situations. The real question that needs to be asked: Is the crank pulley stock or underdriven?

i'm going to go by tomarrow and check out how many volts the battery is putting out and test this therory.

To me it sounds like the alternator is just bad but the only way to tell will be with a DMM. when the car is running, the alternator should be putting out at least 14volts by 1500RPM's.

 

[Marlboro_Man]

Use a Digital Volt Meter to check voltage at the battery at idle and at 1500 rpm like Srothfuss said.

In my case the stock 3g pulley was too big and it wasn't charging at idle (could have something to do with the crank pulley being underdriven). What i did was use my smaller 2g pulley along with a washer to "overdrive" the 3g alternator at idle. Since then it charges like a champ.

 

[mustangGT85]

The real question that needs to be asked: Is the crank pulley stock or underdriven?

so if the crank pulley is underdriven, that might cause the alternator to be spun too slow to charge?



~Steve

 

[srothfuss]

Yeah. Since an underdriven crank pulley slows down all of the other accessories, you could see a drop in alternator RPM's.... Just a thought though, so get out your DMM and test the output first.