Rij1

My airconditioning comes on as well as the heater. No matter which area the dial is the ac comes on. the the heat comes on for a few seconds then the ac cool air over powers it. I noticed there was something going on a few months age but if I turned the dial a few off and on past the ac settings the heat eventually worked with out the ac kicking on. What is wrong? Has any one had a problem like this before?

97z28k&N

Your heater core is probably blocked.

bobdec

First make sure the AC compressor is running. The AC runs with dash switch in MAX, Norm, Bi-Level, Heat/Defrost, and Defrost positions. I know you have DataMaster, you can start it up and select the AC Request and AC Clutch Status and AC Clutch Relay status bits in the upper right corner of the display. AC request comes from the dash switch, AC Clutch Relay is the PCM to AC relay activate signal and AC clutch status is the AC has relay picked OK signal. You should never get the AC request in Vent or Heat mode on the dash switch. Here is a schematic ref shbox.com http://shbox.com/1/hvac_compressor_control.jpg.