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A/C through defroster and floor vent sometimes

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You have a ruptured vacuum line in the diverter system circuit. Check for a rub-through condition below the battery tray. Also check the vacuum lines from the AC Programmer behind the glove box.

Edited by KHE

'93 Fleetwood Brougham
'05 Deville
'04 Deville
2013 Silverado Z71

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For what it's worth, my 95 etc was doing that (along with the parking brake failing to auto-release) and the vacuum line that failed was against the passenger side wheel well near the firewall. Just another place to check if it's not where KHE suggested.

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I didn't see any problem with the line going down the frame by the battery. I disconnected the vacuum line check valve by the firewall and I could hear the air rushing into the accumulator so I think it is getting enough vaccum. The air going from the dash vent to the defroster vent is intermittent. But it does go to the defroster more than the floor vent. Should I just assume it is the AC Programmer or is there a way to test it? Do I have to remove the dash to replace the programmer or am I not reading the FSM correctly? How about routing a direct vaccum line to the vent motor and bypassing the programmer? My parking brake is releasing as it should.

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No, don't assume that it is the programmer, they are expensive. Pull the vacuum line at the firewall and put a hand held vacuum pump on it to see if it will hold pressure. If you don't have one, just suck on the hose and when you create a vacuum, plug it with the tip of your tongue. If it does not hold a vacuum, there is a leak somewhere. A stethoscope will help greatly in locating it.

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