91 A/C problem

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The A/C system is only blowing air out of the floor vents, nothing from the dash vents in any setting. Is there by chance a fuse that could cause this, or is  it a mechanical problem?

Before I take it to the shop would appreciate any advice or knowledge  that is out there  in this  fine  Group..

Thanks in advance

Hank, in Biloxi Ms. where we are cruising the coast with over 8,000 registered vehicles  this year.

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Pretty sure your a/c is the same as my '90. The air distribution is controlled with vacuum through what is called a programmer, which is located in on the far right side under the dash. It stands up vertically under the dash above the removable panel below the dash. It is black plastic with a cardboard cover on one face. There are vacuum connections on the bottom of the unit. The upper end with the temperature control rod is visible with the glove box liner removed. The most common failure is loss of vacuum feed to the unit, a small black plastic line, which causes air to be discharged primarily from the defrost vents. The vacuum source is from a tee on the engine side of the firewall fed from a line that runs toward the firewall from the vacuum block on top of the intake manifold. It sounds like you have some vacuum present, but there may be a hole or kink in one of the vacuum lines from the programmer. They are all 1/8" colored plastic lines. Do you hear a hissing sound from the right under dash area when the engine is stopped? The programmer itself could be faulty. Did it ever work properly? 

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