M1010 upper alternator belt vibration

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On my M1010, the lower alternator belt runs to the crank pully. When the engine is running, that belt shows no vibration at all.

The upper alternator belt goes around the crank and fan pulleys. When the engine is running, it vibrates like a plucked guitar string. The lower belt tension is about 67 lbs. The upper belt is new. Tension is currently 80 lbs. I'll loosen it once it's broken in, if necessary.

Is this vibration normal? Does it indicate a problem? The alternators are charging properly, 14.2V on the front battery and 14.4V on the back. We did the Plan B Mod, so the upper alternator is 12V charging the front battery, and the lower is 24V charging both.

https://youtu.be/FaQa3nKBVqk

 

[m38inmaine]

Mine does it.

 

[cucvrus]

Normal. Push on the throttle a wee bit and it is (belt) running tight and smooth. No worries. You know I tear M1010's apart? Made several plow trucks out of them over the years. No fewer than 6. Just had to tell you that. Good luck. Maybe some day if you need some M1010 parts we can meet up.