FuzzFace2 Posted August 27, 2022 Share Posted August 27, 2022 The system defaults to Defrost w/o vacuum. So my guess is that something in the illustration below is disconnected or broken. The vacuum hoses shown as 17A543 need to be connected, and it isn't unusual for the one going through the grommet at the firewall is broken. https://forum.garysgaragemahal.com/file/n124016/hvac-vacuum-hoses-1980-81_orig.jpg Gary do we have a part number for the Tee check valve 19A563? The ones I have picked up from the parts stores dont seam to work because every time I am on the throttle it goes from AC vents to defrost. Thanks Dave ---- Link to comment Share on other sites More sharing options...
85lebaront2 Posted August 27, 2022 Share Posted August 27, 2022 The system defaults to Defrost w/o vacuum. So my guess is that something in the illustration below is disconnected or broken. The vacuum hoses shown as 17A543 need to be connected, and it isn't unusual for the one going through the grommet at the firewall is broken. https://forum.garysgaragemahal.com/file/n124016/hvac-vacuum-hoses-1980-81_orig.jpg Gary do we have a part number for the Tee check valve 19A563? The ones I have picked up from the parts stores dont seam to work because every time I am on the throttle it goes from AC vents to defrost. Thanks Dave ---- Dave, I have some single line ones that could be used with a tee. Before I would condemn them, I would check for leaks, pull a vacuum on the line going to the inside and see if (a) you can get a vacuum and (b) hold it for at least several minutes. Link to comment Share on other sites More sharing options...
FuzzFace2 Posted August 27, 2022 Share Posted August 27, 2022 Dave, I have some single line ones that could be used with a tee. Before I would condemn them, I would check for leaks, pull a vacuum on the line going to the inside and see if (a) you can get a vacuum and (b) hold it for at least several minutes. I do think the HVAC switch is a issue a little. If I go from say defrost to MAX AC it will not move the doors but if I go from defrost to Normal AC and let the doors move then to MAX it will go. But I also found when on MAX even less throttle make the door move back to defrost so I been leaving it on normal AC and been working just fine. I also have a heater core vacuum valve and that could be part of the problem and it dose not work when on normal AC I have not had time or its been to hot to look into this deeper. Dave ---- Link to comment Share on other sites More sharing options...
mat in tn Posted August 28, 2022 Share Posted August 28, 2022 Dave, I have some single line ones that could be used with a tee. Before I would condemn them, I would check for leaks, pull a vacuum on the line going to the inside and see if (a) you can get a vacuum and (b) hold it for at least several minutes. I do think the HVAC switch is a issue a little. If I go from say defrost to MAX AC it will not move the doors but if I go from defrost to Normal AC and let the doors move then to MAX it will go. But I also found when on MAX even less throttle make the door move back to defrost so I been leaving it on normal AC and been working just fine. I also have a heater core vacuum valve and that could be part of the problem and it dose not work when on normal AC I have not had time or its been to hot to look into this deeper. Dave ---- especially here in the south. i have seen vac switches with the amber plate bowed up and away from the black main housing creating an internal vacuum leak. these too are fragile when 40 years old Link to comment Share on other sites More sharing options...
Gary Lewis Posted August 28, 2022 Share Posted August 28, 2022 The system defaults to Defrost w/o vacuum. So my guess is that something in the illustration below is disconnected or broken. The vacuum hoses shown as 17A543 need to be connected, and it isn't unusual for the one going through the grommet at the firewall is broken. https://forum.garysgaragemahal.com/file/n124016/hvac-vacuum-hoses-1980-81_orig.jpg Gary do we have a part number for the Tee check valve 19A563? The ones I have picked up from the parts stores dont seam to work because every time I am on the throttle it goes from AC vents to defrost. Thanks Dave ---- Yes, we have part numbers: D7OZ 19A563-A from the page at Documentation/HVAC/HVAC Systems. So it looks like we are using a part that was engineered in 1977 and was used on the cars as well as the trucks from then to at least 1989, so should be easily found in a salvage. Our truck MPC doesn't say what the Motorcraft part number is, but the 1973-79 car MPC says it is YG-193 and Amazon has those for $10. However, they look a bit different from those on our trucks, so if originality is an issue... Link to comment Share on other sites More sharing options...
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