low idle charge, at times alternator at low-speed whistle high pitch sound?

Hi,

Is the sound loud enough to record and upload for me to hear? Also, I never heard of the diodes causing this type of issue. A diode is a component that is designed to allow power to flow in only one direction and rectifies an AC signal to DC.

How low is the idle speed? If it is too low, that will cause the fan to slow. I wouldn't think it would cause lights to flicker. It could cause them to dim.

As far as the alternator you have, were all three the same brand? Were they new or remanufactured?

One last thought. If you can, check the alternator output and let me know what you find.

Here is a link that explains how to do it:

https://www.2carpros.com/articles/how-to-check-a-car-alternator

Let me know.

Joe

I can hear the shrill squeal, from the alternator from inside the vehicle. It doesn't do this all the time, but quite a bit of the time it does. When it happens my A/C blower fan slows down and when i accelerate it speeds up. My original alt. bearings went out, so i purchased one online from a manufacturer that only deals in alt. and starters. After i installed it i was told there were smaller pulleys at a later date to help with low idle voltage. The distributor told me when it was making a loud squealing noise it was the diodes fighting each other. They did not have another alt. in stock, so they refunded my purchase price. I then went to AutoZone and purchased a $189.00 re manufactured alt. After a couple days it started again. At idle without a load, it puts out 13. 30 volts, when i rev the engine to 2,500 RPMs it puts out 14 plus volts, at idle with A/C, light, blower fan and condenser fan kicks on. it puts out below 12 volts or a little less at idle. With engine revved up it does fine. Didn't have this problem until i replaced the original alternator. i understand that the computer controls the alt. functions, as not to overcharge or under charge. sometimes i crank it up, it is fine but after driving around town it starts again. i purchased a new serpentine belt, thought that might help, but the same thing happened. For some reason it at times it fails to put out at idle speed. My engine at idle is turning 550 RPMs in gear, 800 out of gear. I am very frustrated from dealing with this ongoing problem . A new OEM alternator for this vehicle is over $400.00. I would not like to purchase one and still have the same problem. Hope you can help.

Hi,

The voltage output sounds good except with the load placed on it. Does the noise seem to happen more often when the electrical system is loaded? Also, I realize you replaced the belt, but is the tension correct? Some of these have an automatic belt tensioner and others are mechanically adjusted with a bolt.

Could you record the noise and upload it for me to hear? Also, if the new AZ alt stopped the noise for some time and then started again, in my mind the belt is slipping when the alternator is under a load.

By any chance, do you have the part number and brand name of the alt installed? The OEM part number is 3730037405 and I would like to see if I can cross reference the one you purchased.

One other thought. If it is a shrill sound, there really isn't anything in the alternator itself that would cause that unless maybe a bearing is bad.

Let me know.

Joe

Yes, the AutoZone part # is 11020 110 amp. My 2.7 v-6 engine has 1 serpentine belt with a tension pulley. Sorry I can't upload the whistle noise at this time. It should not be a bearing because 2 alternators from different manufacturers alternators did the same thing. I'm wondering if my idle speed is too low or i have the wrong belt for the smaller puller. The belt seems to be tight enough. Have you had to deal with a problem of this kind in the past? Could the temperature sensor have an effect on the computer telling the alternator when not to put out certain voltage at low idle. I have noticed on cooler days it is not as bad. Just a thought.

Hi,

The alternator appears to be the correct replacement. As far as the temperature sensor is concerned, no, that wouldn't cause this to happen.

I wish I could hear it. Could it be the belt? Try this. When it is making the noise, pour a small amount of water on the belt to see if the sound changes. Just protect yourself from water flinging.

Also, I found the idle speed is too low. In gear, it should be between 720 and 920 (see pic below). The low idle could be caused by a couple of things such as a dirty throttle body, the idle air control valve, a vacuum leak, and so on.

Let me know what happens with the water test.

Joe

Thank you for you patience and time, i have already replaced the idle control valve ,cleaned the throttle body and replaced the throttle position sensor . when it makes this sound and the idle voltage drops ,as i accelerate and it goes through the gears is when the shrill noise is louder. I will adjust the throttle cable to idle it up a few rpms I will idle it up a bit and replace the belt to make sure i have the correct belt for the smaller alt. pulley. if that doesn't help ,guess i will have to live with it. Thank you so much for your help.