Cooling Temperament of an Air Vent

Times Staff Writer

Question: I have a 1970 Ford LTD. In the last several months, I have been having a problem with the controls that regulate the defroster, heater and air conditioner. When I’m using the air conditioner and I accelerate to pass a car or climb a hill, the air shuts off from the vents and comes on my feet. No one seems able to help.--E.J.B.

Answer: Your problem is enough to give you cold feet, literally. It’s a fairly simple problem to understand, but it’s a lot more difficult to repair.

The heater and vent controls on your Ford and many other cars are vacuum controlled, meaning that the power to open and close the various baffles that direct the air flow is provided by engine vacuum. These vacuum controls are usually operated by a push-button panel on the dashboard.


The control system gets its vacuum from the engine intake manifold, which is in a natural vacuum state when the car is operating.

Engine vacuum usually drops when you have to accelerate or suddenly depress the accelerator pedal to climb a hill. To maintain vacuum for the dashboard controls in those situations, a vacuum line runs to a canister under the dashboard, which stores up the vacuum when engine vacuum drops.

The problem you are experiencing is probably caused by a vacuum leak somewhere in the vacuum canister, the dashboard controls or in the various vacuum hoses that run to the various baffles.

So, when you accelerate, the engine vacuum temporarily drops and a baffle suddenly changes position. It probably is not a severe problem, but it may indicate that you have other problems. For example, when operating the air conditioner, it’s important that the baffle that blocks the heat be fully closed. Otherwise, you’re simply reheating the air you just refrigerated.

Your problem is relatively simple to fix, but you have to get to it first. Often that requires removing the entire dashboard of the car, a major job that runs into the hundreds of dollars.

Frankly, I have never liked vacuum-operated controls. I much prefer a simple cable-operated system, which is more reliable and provides more positive opening and closing action on the baffles.

If you decide to have your problem fixed, I suggest that you take it to an air-conditioning repair shop. The mechanics there probably have a lot more experience with your specific problem than a general mechanic at a garage.

Q: I have a problem with the cruise control on my 1979 Corvette with 22,000 miles. I took it to the dealer, who replaced the transducer. After a long trip, the speedometer failed. The dealer replaced the transducer again. After 60 miles, the cruise control and speedometer failed again. Do you have any suggestions?--E.D.M.

A: The cruise control is operated by the speedometer cable on the Corvette and requires a smoothly functioning cable to operate properly.

The speedometer cable runs from the transmission up to the cruise control-unit on top of the engine. It operates a transducer, which reads the speed and tells an electronic unit how fast the car is traveling. The cable then continues on to the dashboard, where it operates the speedometer.

If the upper portion of the speedometer cable is binding and turning with difficulty, the transducer can be repeatedly damaged. There is a bushing in the head of the speedometer, which could be causing the cable to bind or break.

To replace the bushing requires pulling out the dashboard. Usually, the speedometer unit is sent off to a special repair shop. It is unlikely to be a low-cost job.