QUESTION: My ventilation air has begun blowing only out of the defroster on my Ranger pickup. I wondered if there's some advice you may have on this before I start checking fuses, etc. It doesn't have air conditioning, so having air blowing at me is a good thing with the warm weather.

ANSWER: Based on your symptom, it sounds like the Ranger uses engine vacuum to operate the air delivery doors, as opposed to a growing number of vehicles which use electric servos. Vacuum (reduced pressure) occurs in the engine's intake manifold due to the pistons pumping air, and the throttle being closed or near to it — much of the time, causing a restriction. Several golf ball sized servos, each containing a rubber diaphragm, pull the air delivery doors to the desired position, and a spring subsequently returns them.

A slender plastic hose is routed from the intake manifold to a check valve, then to a vacuum reservoir (a metal or plastic container), and continues into the Ranger's cab, ending at the ventilation control panel. The check valve and reservoir help to maintain a consistent vacuum level as the throttle is periodically opened wide. Behind the control panel's mode control knob or lever is a vacuum switch, which directs vacuum through additional hoses to the appropriate servos.

Defroster-only operation is typically caused by a loss of vacuum to the control panel switch. The vacuum hose could be separated from a fitting, kinked, collapsed, or perhaps the vacuum reservoir has become damaged and is leaking. Take a look under the hood on the passenger side for this hose. After unplugging the section of hose leading from the reservoir to the cab, feel for vacuum at the reservoir nipple (engine idling) with the tip of your finger. If there's no vacuum, check again prior to the reservoir, and follow the hose as necessary back to the engine. If vacuum is present, visually follow the hose as far as it can be seen prior to entering the cab, checking for kinks or damage.

Diagnosis beyond this point will likely require disassembly of the instrument panel. It's possible the vacuum switch has failed and is leaking vacuum, or a hose is kinked or collapsed. Dropping the glove box may be worthwhile, providing a glimpse of some of what you'll want to check. Keep in mind the fingertip vacuum test is quick, but a vacuum gauge (16-19 inches of vacuum at engine idle) would be better.

(Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail under-the-hood@earthlink.net. He cannot make personal replies.)