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Re: 1987 300z ac air only comes from the defrost. ...
Locate vacuum hoses that go from inside switch to actuator doors. Either switch is leaking, hoses are leaking or kinked or actuator is leaking. Listen for hissing noise under dash at idle or obvious bad hoses.
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You defroster may not be block the vent controls that switches vent position is vacuum controlled there is a hose from the intake manifold that goes under the dash to control the vent position,you may find a broken hose under the hood on the passenger side it a long palstic vacuum hose these have tendency to break just use a vacuum tube to repair it.
not knowing what type or year of this car i would say look under the hood for removed or cracked vacuum lines,the line would run from the intake vacuum port to inside the car, look towards the pass/side under the hood on the firewall,also listen for any hissing noise that may help you locate the bad vacuum hose.
There is a vacuum problem under the hood,look where it goes into the firewall and follow it,there will be a check valve in the vacuum hose,and it could be broken,or the hose is melted into,or the vacuum hose has disconnected.
THAT BLACK HOSE IS PROBABLY COMING FROM HOOD LINER,THATS A BROKEN WASHER SUPPLY HOSE FOR YOUR WASHER NOZZLES TO SPRAY WINSHIELD OFF. HOWEVER YOU PROBABLY HAVE ANOTHER SMALL BLACK LINE THAT GOES THROUGH YOUR FIREWALL BEHIND DASH RIGHT UNDER PASS SIDE AND VERY BACK OF ENGINE COMPARTMENT. THAT IS A VACUUM LINE THAT SUPPLIES VACUUM TO YOUR A/C PLENUM TO ALLOW MODES OF OPERATION TO WORK WITH YOUR MODE SWITCH TO DIRECT AIRFLOW TO FLOOR,VENTS ETC... THE LINE IS BROKEN,DEFAULT IS DEFROST AND THATS WHAT HAS HAPPENED.
The AC functions of defrost, vent and floor are controlled by vacuum motors, you have a vacuum loss problem from the engine, check for a small disconnected vacuum hose under the hood from the engine to the firewall..
Check your vacuum lines under the hood and particularly the vacuum reservoir which is a canister that supplies a reserve on vacuum to bridge the times the engine is pulling hard.
These often have plastic nipples with pushed-on hoses that can snap off.
The directional control of air flow is most often accomplished with vacuum 'motors' that move vanes to change direction.
LOOK AT THE WHITE PLASTIC HOSE GOING TO THE RECIRC VACUUM ACTUATOR UNDER HOOD. THIS IS ON TOP OF A/C BOX BY RIGHT SIDE OF FIRE WALL YOU WILL FIND THIS HOSE IS STARTING TO COME APART REPAIR HOSE AND PROBLEM FIXED.
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