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Re: 4wd system operation 98up Blazers
If the air flow changes to defrost on acceleration, (low vacuum) it's an indication of a vacuum leak either in the HVAC system, the 4WD system, or a faulty vacuum check valve. You'll need a hand held vacuum pump and some time and patience to find the leak. Start by applying vacuum to only one of the systems to determine which system is leaking. Then trace the line to locate and repair the leak.
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You have to know how it works,
to figure out why it doesn't.
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