It seems like the AC shuts off above 50 mph. What's going on?

Dear Car Talk

Dear Car Talk | Jun 01, 1996

Dear Tom and Ray:

Our 1985 Lincoln Town Car -- which rides like a charm and is in excellent
condition -- is now giving us problems with the air conditioner. It cools
very well if I don't exceed 50 miles an hour. But on the higher-speed
interstates, it intermittently switches to "vent" and stops cooling. When I
slow down again, the cooling goes back to normal. Since we travel the
interstates often, and live in the Southwest, we need constant cooling.
What's wrong? -- John

TOM: Sounds to us, John, like you have a vacuum deficiency. The car uses
vacuum power to operate things like the power brake booster, the cruise
control and the ventilation system. That vacuum power is produced by the
engine: Every time a piston sucks in air and gasoline, vacuum is produced
in the intake manifold and routed to these other devices.

RAY: When you turn on the air conditioner in this car, vacuum-operated
"motors" open the cooling vents or "blend doors" and send cold air in to
mix with fresh air (the fresh air's there so you don't choke to death on
your wife's cigar smoke, John).

TOM : But if there's not enough vacuum for some reason, those "blend doors"
can't stay open, and what you get is mostly un-air-conditioned air.

RAY: This problem would be the worst when traveling at high speeds, when
the throttle is wide open and the least amount of vacuum is being produced.

TOM: So you've got to ask your mechanic to find out why you have low
vacuum. The worst-case scenario is that your engine is wearing out, and you
have low compression. And that's a possibility in a car that's going on 12
years old.

RAY: On the other hand, it could be something as simple as a hole in a
vacuum hose, which would cost you 10 cents to replace. So it's either 10
cents or $2,000. How's that for an estimate, John?

TOM: Or maybe you have bad vacuum because it's time to change the bag!

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