Auto Air Conditioning & Heater Repair: Cold / Heat control, silverado ls, logic controller

Question
My 2001 Silverado LS extended cab with 74K miles and is in showroom condition is only blowing heated air. Everything has been working fine until this morning when I turned the defrost on I found out that I had no temprature control. The air will change from all the position and blows good from, dash vents, defrost and heater but only blows heated air. I tried to turn the AC on but it's still hot air no cold even with max AC. Is this a vacuum system or electronic control? Help

Answer
Mike,

First things first -- providing you have C68 (Auto) A/C, then Technical Service Bulletin (TSB)# 04-01-39-007 deals with the replacement of the HVAC Control Module. Apparently, the logic controller locks up, and the temperature will not change. The only way to fix it is to replace the HVAC module -- the part number is 15126603 or 04, depending on the temperature metric (minus celsius? the bulletin is a bit cryptic).

Also, TSB# 01-01-39-007 outlines an issue with poor terminal contact at the control head due to stress from a misrouted harness. Basically, you R&R the upper IP pad, and check how the control harness is routed past the air distribution duct -- if it's over, the all is well. If it's under, it's misrouted, and needs to be rerouted over the duct. Before reconnecting the harness, you should inspect the terminals for proper integrity and alignment.

In answer to your question, the blend door (temperature mix) should be operated by an electric motor. Some of these motors are easy to get to, but it always seems that the one that fails is the one that requires the HVAC case removed. Aside from these two issues, there's still other possibilities, but addressing TSBs is always the step after verifying the problem and before further diagnosis. Good Luck!