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.
HVAC air flow switching to defrost under heavy engine load on Plymouth Acclaim
Average mileage: 62,500 (30,000–95,000)
9 people reported this problem
1990 Plymouth Acclaim 95,000 mi, Visitor
Problem as described, but not repaired.