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73-87 Chevy _ GMC Trucks => Interior & Equipment => Heating, Ventilation & Air Conditioning (HVAC) => Topic started by: jasnake on September 21, 2022, 06:55:57 PM
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I have the dash off as I'm replacing that and trying to figure out my defrost. Vent and floor works but no defrost. Put new vacuum line on under the hood but no luck yet. What does this connect to, its blue? I don't have the manual, it's just floating around with 2 vacuum lines to it. It's behind the carb and one line going to the vacuum to the throttle and other to the canister next to firewall
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That's one-half of a vacuum check valve. Is the connected canister a ~5" round black plastic ball?
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Yes. It's round and on the driver's side, but right now it's attached to nothing. What should it be attached to?
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Manifold vacuum connects through a hose to a port on one side of the vacuum check valve. The port on the opposite side of the check valve tees into two hoses. One hose connects to the vacuum reservoir (the black plastic ball mounted on the firewall) and the second hose runs through the firewall inside the cab where it connects to a vacuum selector valve that is integral with the HVAC control head mounted in the dash (the panel with the A/C, heater, and fan controls). The reason defrost air doesn't switch from the floor nozzles and blow onto the windshield is that the system depends on a continuous supply of vacuum to power pneumatic actuator motors that selectively move air doors within the HVAC housing behind the dash to direct airflow out the various nozzles, depending on what is selected.
So, the solution is fairly simple. Replace that broken check valve.
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After you said Vacuum Check valve I knew that was the cause. Went this morning and installed and works like a charm. Thanks for the advice!