HVAC air flow switching to defrost under heavy engine load on Plymouth Acclaim

The heating, ventilation and AC (HVAC) system may  change to defrost mode under heavy engine load conditions when low engine vacuum is produced. This can often be corrected by installing an additional vacuum check valve with reservoir to help stabilize the vacuum supply to the HVAC system.

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Average mileage: 62,500 (30,000–95,000)
6 model years affected: 1990, 1991, 1992, 1993, more1994, 1995
9 people reported this problem
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problem details
1990 Plymouth Acclaim 95,000 mi,
Problem as described, but not repaired.