i have an 83' 300d. when i turn the a/c on. it will not blow cold air and it also will not blow through center vents in the dash. the compressor comes on. i cleaned the diaphram valve on the firewall. this normally was the cause, not this time. i do not think the a/c is being told to turn on and blow through correct vents. is this done by vacuum or from climate control switch?