cthomasparr

Hi,

Initially my son's 2000 Mustang had charging issues and we found the battery was not very old, so we suspected the alternator might be the issue. After replacing the alternator with one from Advance Auto it seemed like things were OK. The battery was charging at 14+volts at 1500 rpm. I went through some diagnostics to check for lose wires/bad grounds and all seemed OK.

My son told me shortly after that there was an issue at higher RPM that the battery light would come on briefly. I thought he could ignore it. On a recent trip home he had the battery indicator come on and the voltage gauge show between low and middle the entire trip. Now it starts up fine but the battery light stays on. The gauge reads in the middle as expected. I didn't try and of the tests that I had done prior. I was hoping to get some suggestions.

i also noticed that the accessory drive belt seemed kind of lose and the new alternator had a smaller drive pulley. I purchased a replacement tensioner, but it was hard to remove the mounting bolt. I was afraid to break it trying to remove it so I left the old tensioner on. I have never heard any sounds of slipping on the belt.

Appreciate all the help.

It appears that a blown 20amp fuse in the engine compartment was causing the charging issue with the battery light staying on. Not sure what caused the blown fuse or the high rpm battery light flicker.

PNYXPRESS

Did you have the old alt checked before replacing it?

Pulley diameter difference could be someone had installed underdrive pullies previously and now you arent spinning the alt fast enough to charge the battery at high rpm.

fuses blow for all kinds of reasons also.

cthomasparr

My son was supposed to do that before picking up the rebuild at Advance Auto but never did. The new alternator definitely helped. Just those two issues are a concern. The new alternator came with a smaller pulley. I didn't change anything, so it is running at a speed that it was designed to run at I suppose. I saw a post somewhere that said the fuse that has blown was to protect the voltage regulator, so there is possibly a short there.

PNYXPRESS

If the car has underdrive pullies and you didnt swap the pulley off the old alt then you now arent spinning the new alt fast enough. At high RPMs this will cause a drop in voltage

cthomasparr

Thanks for the responses !!

The new alternator has a smaller diameter pulley, so it would spin faster. I am not sure if that is considered an "overdrive" pulley. If I put the stock pulley on that alternator I would expect it to spin too slowly. My thought is that the rebuilt was of lower quality and had to be spun faster to produce the same output and that is why it had a smaller pulley.

Chromeshadow

I had an underdrive pully on my 2000, It came with a smaller belt. My kit did not have a larger pulley for the alternator, it used the stock alternator pulley. I don't think changing the pulley size on the alternator gets you anything, the alternator will try to keep up regardless of it's speed.

PNYXPRESS

Ive seen sets with 2 and 3 pullies. I think the motoblue 25% ones came with an alt pulley, it was to aleviate charging issues. This would also account for the slack in the belt. you should be able to tell if the crank and water pump pullies are aftermarket or stock.

cthomasparr

I will have to check the crank pulley, but i would be surprised if the original owner changed anything. The intake and exhaust were stock. I get the underdrive as a smaller crank pulley to free up some power from the accessories. My alternator came with a smaller pulley than the one I replaced, so that was doing the opposite.

I get the spin of the alternator may not be too important as electronics will control how much electricity the alternator produces until the voltage regulator requests the max output and then you can only increase that by turning the alternator faster, I think.

Anybody understand what that fuse in the power distribution box is all about? It is a 20 AMP that is there to protect the voltage regulator?? is this the source for the electricity that sets up the field in the alternator to draw more electricity?

cthomasparr

Today the fuse blew three times. Luckily the battery had enough power to get me home. I removed the alternator and will have my son take it for testing.

PNYXPRESS

If it keeps blowing fuses, you need to check the wiring for frays and shorts.