So yet another chapter unfolds in the A/C gremlin saga of my LS1 Chevy.


  • A/C compressor is relatively new (2 years old)
  • One of the belts is squeaking (not sure which one)
  • Recently performed a thermostat swap (the only reason why I mention this is because the thermostat housing is mounted directly above the compressor and a bit of coolant did spill on the compressor)


  • The alternator outputs ~14.5 V with the A/C off.
  • With the A/C on, the alternator outputs 10-12 V, the battery symbol lights up, and a 'Check Alternator' warning shows up on my display. After some time the steering goes hard as well, indicating loss of power steering. Cooling is abysmal.

My Thoughts

The symptoms appear to suggest that the compressor is inducing a parasitic draw on the engine (seized bearing, perhaps?), which surprises me since this is a two-belt setup (one for the A/C, one main).


  • Is it reasonable to expect that a seized compressor bearing would cause loss of tension in the main belt to the extent that alternator and power steering pump functionality is compromised?

  • Could there be some non-mechanical issue that explains this behavior (such as electrics)?

  • Year, Make, Model? Jul 17 '14 at 14:03
  • How much does the engine RPM drop when the A/C comes on? Is the voltage drop with the fan blower on high, or just on the low side? Jul 17 '14 at 14:17
  • could maybe be a short circle somewhere in A/C system. Check the Amperage at turning on/off the A/C. It can not be, that the alternator is not strong enough.
    – Watsche
    Jul 17 '14 at 14:34
  • @Larry, 2004 Chevrolet Lumina SS (aka Pontiac GTO)
    – Zaid
    Jul 17 '14 at 22:02
  • 1
    Just asking ... It would provide the answer to all your issues. Also, don't think that just because the PCM is set to ~650 rpm it's going to absolutely keep it there. I've seen other GM vehicles not idle right for various reasons, and they were all set the same as your LS engine. Sometimes we cannot see the forest for the trees. If you have preconceived notions of what the issue might be, you may never figure out what the real problem is. Jul 18 '14 at 11:17

As highlighted in the question comments, the culprit was the vibration damper on the crank pulley that had failed, causing the pulley center and outer ring to have some limited slip relative to one another.

I'm happy to say that the following symptoms were resolved.

  • With the A/C disengaged, the crank pulley was spinning at a limited speed. This was also causing issues slight engine overheating and reduced power-steering assistance that I did not believe were related at the time of posting the question. With the new crank pulley in, things are much better; no overheating, A/C runs as before and the steering is easier.

  • Engaging the A/C provided additional load on the crank pulley that was enough to cause it the outer ring to stop rotating. This was what caused the 'Check Alternator' message to appear and battery voltage to drop.

  • Belt squeal is vastly reduced, so I probably will still have to replace them.

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