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I have a 99' F150. Over the last 5 years I rarely drove it due to the job I had which required extensive travel time. About a year ago during the summer, I got in it, turned on the AC and it worked fine, as soon as I started off in 1st gear, the AC cut out. When I am cruising along, it works fine, its just when I initially start off in first gear it cuts out and you hear a little knock in the dash which indicates when it cuts out because the air goes right to hot as soon as you hear the knock. My mechanic thought it was the thermostat, no go, his next plan was to replace the entire mode control unit but I'm suspecting he's at a process of elimination and I don't have the money to play around with that. Any ideas?
Symptom: Loss of air cooling when accelerating or while cruising.
Problem: A/C clutch slips or won’t stay engaged causing loss of cooling.
Solution: Excessive A/C clutch gap due to clutch wear. Usually, the problem can be resolved by removing a shim from the clutch to bring the gap back into the desired .014-.030” gap.
Problem: Air conditioning cuts out under heavy acceleration.
Solution: On some models, it is supposed to so that engine power is made available for acceleration. Can also be caused by the clutch slipping due to excessive air gap.