Quote Originally Posted by Rich View Post
Already mentioned in this thread is lower output due to a slipping alternator drive belt. Not mentioned as a cause of belt slippage is an incorrect drive belt. Use a known-good belt that meets specs. A Gates 7495 is one such belt.

Citations below, slightly edited, are taken from the Knowlegebase article about alternators from DMC Classic - including how to prove belt tension is adequate (use a wrench):

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"4 - Set the correct belt tension. Nominally the belt should be tight enough that the belt will deflect 1/2 inch when pressing with your finger. A better indication is to try to turn the alternator main shaft clockwise with a wrench. You should see the belt turn the engine slightly rather than slip at the alternator pulley. Too tight is better than too loose. A loose belt will slip under load and greatly diminish the output of the alternator just when you need it the most.
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The # 1 Reason for Premature Alternator Failure Including "No Output" Is a Slipping V-Belt!

In the good old days when all cars needed was a 40-60 amp alternator, slipping belts were not a common problem. Now that we need 100 amps and up, it has become a real issue. The higher the amperage, the more resistance an alternator has, and it takes more power to turn the pulley. Therefore the belt has to be tighter to prevent slippage. Most of the time the alternator belt doesn?t make a squealing noise when slipping.

Signs of belt slipping:

  • Little or no output, discharged battery
  • Alternator pulley extremely hot and starting to discolor
  • Belt sticking to pulley or belt glazed or cracked
  • Rust on alternator shaft
  • Belt dust collecting on fan and front alternator housing
  • Belt riding too deep in pulley groove
  • Bearing noise
Yeah those are enough potential issues that its worth looking at the belt slippage, especially since I'm taking the alternator off. It seems tight enough based on the deflection. I don't know how one gets a socket on that pulley without removing the asbestos muffler heat shield, but what I have done is push on the alternator fan blades with a wrench and its extremely hard to get it to slip/rotate. If belt dust can get it to slip maybe that bearing old grease residue is causing some slippage. The sides of the pulley's where the v-belt grips are black from heat from belt friction I presume but unless you all tell me your pulleys are super shiny I assume that's normal. I'll consider all these things after I re-assemble after the alternator bench test.