This started to happen relatively recent. It's 2003 Chevy Trailblazer. What is happening when the engine is idling and I turn the A/C on or the defroster (which also uses the A/C I believe), the engine starts to bog down.

What could be causing this extra load on the engine? What could it be in the compressor that is so much resistance? Could it be that I just need to recharge my A/C?

  • Does it only do this at idle or is occuring at highway speeds?
    – mikes
    Mar 11, 2012 at 21:00
  • @mikes well it's more noticeable at idle due to the lack of engine load, but I can also feel it at highway speeds. Mar 11, 2012 at 21:19
  • Can you confirm that it's the demisting function of the A/C system (that blows cold dried air on the windshield) and not the rear window heater/demister/defroster thing?
    – kahbou
    Mar 12, 2012 at 11:58

2 Answers 2


The compressor is a considerable load on your engine. In order to stop the engine from bogging the A/C usually has a way of raising the idle by letting more air past the throttle body. This function is where I think your problem lies.

This can be a separate solenoid specific to the A/C system with a (fairly hefty) vacuum line to both before and after the throttle body. I've seen this solenoid referred to as an A/C Idle Up Vacuum Switching Valve (VSV) in some manuals, but Chevy's terminology may be different. If it is a separate solenoid, there will also be a separate idle-up solenoid for the power steering as well as for large electrical loads (headlamps, large electric fans, rear defroster). You should find this solenoid, verify that it's not working and replace it.

The alternative, I guess, is that there is one ECU controlled valve that performs the idle-up function for all the systems that require it. In that case, unless your engine also bogs at other times, I would suspect the electrical pathway that informs the ECU of the compressor operating.

Please update your question as you find out more, and I'll add to this answer.

  • Since the Trailblazer is Drive By Wire (DBW), I doubt it would have a VSV, and more likely this action is controlled by ECU directly to the throttle. I don't know that for sure, though. Jun 21, 2015 at 18:59

Have your system pressures checked, if the high pressure side is way up it could be putting excess load on the compressor, and if so, make sure the condenser fan is blowing fast enough to pull the correct amount of air to prevent the system from getting too hot and creating too much pressure. This just recently happened on my Honda civic, tapped on the fan a few times and it started bowing a lot harder and the pressures came down to normal

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