I was wondering in what ways could an alternator partially fail such that it would create excess drag on the engine ( enough to cause some shaking / lugging ) but not make any immediately apparent noise?

  • Did you come across such an issue, or is it a hypothetical question? Jun 10 '16 at 12:59
  • @IhavenoideawhatI'mdoing Kinda of. I'm having some weird problems with my 98 Mazda and trying to narrow things down. It's kinda lugging/shaking lightly at idle, and it get's allot worse when I turn on electrical loads and the a/c. I'll probably write another question about it after I've gathered a bit more data. Jun 12 '16 at 19:28
  • Did you check how much current is drawn? Alternator creates "drag" when the current demand increases. Jun 13 '16 at 6:53
  • @IhavenoideawhatI'mdoing I'm trying to find a local store which has a decent clamp meter, but I may have to order from aliexpress and wait two weeks.... I've already measured voltage at idle with all loads on ( electrical + AC ) and it gets down a little below 12 volts, meaning it's drawing from the battery at idle to supply all the needed current. The question is why is that happening. Is it the alternator, a clogged IAC, or the ECU not responding sufficiently to the load? The IAC is only going from 28% to 41% open, and rpms are falling from about 750 to 650. Jun 13 '16 at 10:15
  • Yeah, that's way too low. Though I'd first blame the alternator itself and check the voltage regulator, brushes. Hard to believe a difference of 100rpm could cause insufficient voltage. Jun 13 '16 at 13:02

A partially bad alternator can create an excessive drag and cause your serpentine belt hydraulic tensioner to go bad or become noisy when AC is on., I have had my local mechanic replace my belt tensioner 6 times in 10 months, we never knew the drag was from the alternator until the alternator eventually stopped working and was replaced also, After alternator replacement we noticed the car automatically stopped shaking and vibrating when ac is on. No more belt tensioner noises, car is now smooth and fun to drive yeeehhaaa..


You could have bent the shaft. That would cause it to shake and add drag.

  • Bent what shaft? Jun 10 '16 at 7:38
  • The alternator is basically a motor. The pulley is on a shaft that runs the length of the alternator. If that's bent, and the pulley sits crooked, it would cause the symptom you describe. Jun 10 '16 at 12:35
  • How do you even bend it anyways? I think it would quite be noisy because of the excessive vibrations. Jun 10 '16 at 12:58
  • The last one I saw with a bent shaft was because the alternator was dropped, but I've also seen it happen from someone putting too much weight on the pulley while working on the engine. I doubt it would have been bent from belt tension though. Jun 10 '16 at 13:04
  • Actually, it could also be a bent mounting bracket causing the alternator to be out of alignment with the belt. If the bracket is cast aluminum, the bracket could also be cracked. Jun 10 '16 at 13:08

The bearings inside could be going bad and cause some load,but normally there would be some squeaking or grinding sounds. There could be some mechanical load produced by a large electrical load on the alternator too.


It seems like bad diodes in the rectifier could force the voltage regulator to run the alternator at a higher duty cycle than normal at idle which would create extra drag on the engine, especially if the electrical load is high enough to cause the alternator to be close to full fielded in order to get enough output through the remaining good diodes.

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