Back in the day the controls on your vehicle were manual. That is, when you pulled a knob, it was mechanically connected to operate and control a function. The operator actually physically moved a valve or lever. The system worked well although there were draw- backs like seizing components and limits to functionality. As technology progressed and cars became more sophisticated, controls become more automated. Manufactures started to control functions using vacuum motors. These units seemed quite robust but lacked controllability. With the introduction of computers, manufacturers had the best of both worlds - the accuracy of a cable and the remote functionality of a vacuum system. Unfortunately all this complexity has rendered today's vehicles unserviceable for the average backyard mechanic. Servicing these units requires extensive test equipment and serious knowledge of the technology.

Q. I have a 1997 GMC Safari Van. My problem is we have no real heat in the vehicle. The gauge on the dash does show the engine is running normally. Moving the leavers has no effect on heat coming out of the vents. There is always air coming out of the defroster vents. Can you help?

A. What you are describing is actually a vacuum leak. The heater controls on older models are powered by vacuum. The dash controls are basically designed to switch engine vacuum to various vacuum motors operating the heating system in your car. Just as you described, if the vacuum source fails, the system defaults to the defrost setting. Have a tech check the system. Don't be surprised if they have to dismantle the dash.