Heater air flow is stuck in Blend mode and AC doesn't not work

[MikeZ34]

I have a 93 lumina Z34. The heater is stuck in blend mode. The fan works on all speeds, the Cold/Heat control slide bar also works and the rear window defroster works and the off button works too. If I push any of the heater or AC buttons the fan comes on but the air flow is in blend mode. If I push the ac button the ac compressor clutch does not ingage and the air doesn't come out the dash board vent. I have good heat but can't change the air flow. I had some people tell me its the heater control panel and some say a vaccum leak. Anyone out there who might think what the problem is ?

[richtazz]

I would bet it's a vacuum leak. AT the throttle body, there is a vacuum block with 3 lines coming off. One goes to the Map sensor/PCV valve, one goes to the charcoal canister, and the other goes to the HVAC unit and cruise control servo. Follow the line for the HVAC and check for cracks (it's a hard plastic line) or a burned spot(it may have fallen against the rear exhaust manifold). The Control unit uses vacuum to switch from vent to defrost to floor, etc... and defaults to floor/defrost blend when it receives no vacuum.

[MikeZ34]

Originally Posted by richtazz

I would bet it's a vacuum leak. AT the throttle body, there is a vacuum block with 3 lines coming off. One goes to the Map sensor/PCV valve, one goes to the charcoal canister, and the other goes to the HVAC unit and cruise control servo. Follow the line for the HVAC and check for cracks (it's a hard plastic line) or a burned spot(it may have fallen against the rear exhaust manifold). The Control unit uses vacuum to switch from vent to defrost to floor, etc... and defaults to floor/defrost blend when it receives no vacuum.

Thanks for the info. I will check it out . Could that be the reason the AC compressor clucth is not ingaging ? I would figure the AC would still work if it was stuck in blend mode.

[MikeZ34]

I checked it out. You were right . What i found is the HVAC line coming off the Map senor is just hanging there down the back of the firewall and back head. The line is only about 8" long but the line looks good but it looks like someone might of cut it or it fell off a connection point. The end is sliced on an angle so to me it looks like it might of been cut. Next question is there a fitting connection it goes on and if so where is it located or does the line run through the firewall? Also does the vent controls, control the ingaging of the ac compressor ? I bought this car used and there was alot of stuff wrong with it and since you mentioned the HVAC also controls the cruise control my cruise doesn't work either.

[MikeZ34]

I forgot your mention it would be a hard plastic line. The line hanging off the map senor is a 8" small rubber hose. Does the hard plastic line plug into the rubber hose ?

[MikeZ34]

Thanks again , I found the hard plastic line and plugged it into the small rubber hose on the map senor and that fixed the problem.The air flow changes on all the modes but the ac compressor clutch still doesn't ingage ,any input on why its not ?