Pulsing / shaking serpentine belt

[andrewwynn]

Trying to determine if this is normal behavior.

Mostly only notice when engine is cold but the serpentine belt is getting a pulse about 3x a second.

I thought maybe my harmonic balancer is worn but I don't think the timing matches.

I was thinking might be an open diode or winding on the alternator so I actually bought a tool I've been wanting for years: pocket oscope.




There is a giant spike of voltage I'm guessing from some device pulling current in spikes that seems to match the pulses. (or a signal sent to the alternator to boost output but in 330 millisecond period).

It looked like the pulse was synced with the windings at first then I realized it is not but it was periodic. (330 milliseconds).

Anybody familiar with what's going on? I'd love to not have to replace my belt tensioner every 13 months.

I suppose the next thing I should do is measure the signal to the alternator see if it's actually getting commanded to output more. (Unfortunately it's more likely integral to the VR if there's some feedback loop that takes 1/3 of a second).

Symptom of failing alternator? The charge when coasting system?

My E70 is 2011 35i with N55.

My wife has an F10 2015 N55 so I will take some measurements on that next.