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Ron, ASE Certified Technician
Category: GMC
Satisfied Customers: 35599
Experience:  35 years in the Automotive repair business.
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My hvac has defaulted to defrost. I just checked under the

Customer Question

My hvac has defaulted to defrost. I just checked under the passenger side and it seems to be ok. I pulled the battery and got to the reservoir and it looked like there is a line missing off the reservoir and I found a hard line with a 90 connection on the end of it not connected to anything lying down with the other hard line that is connected to the reservoir . I went and got a Reducer and a piece of vac line, connected it and nothing. I took multiple lines off and don't feel any vac/suction anywhere.
Submitted: 1 year ago.
Category: GMC
Expert:  Ron replied 1 year ago.

Hello, my name is ***** ***** I am a professional here at Just Answer. I have noticed that your question was not getting a response and thought I would see if you still need help with this.I apologize for the delay and I hope I can still assist you with this here.

The system is designed to divert to defrost when the system looses vacuum, if you do not have vacuum at the reservoir then that is likely your problem. Trace the vacuum supply line back to the engine where it connects and make sure it connected and had vacuum to it. If so then you should have vacuum at the reservoir , if you are not getting vacuum then check the line for break causing the loss of vacuum.

Customer: replied 1 year ago.
Hi Ron thanks for getting back to me. Where does the supply line that's connected to the reservoir connect to in order to get pressure. Is there a pump somewhere or does it pull it from the engine. Also you should know I pulled the dash apart last night and checked the lines at the mode switch and I don't have any pressure there either. I also replaced the blend door and that didn't do anything . I'm gonna trace the supply line but it not easy because there's very little room being a van but it would helpful If you could tell me where it might be connected to. Does it connect to the engine block?
Expert:  Ron replied 1 year ago.

Are you working on a truck or a van?

Customer: replied 1 year ago.
2007 gmc savana van 3500 w/6.6 duramax
Expert:  Ron replied 1 year ago.

The Duramax uses a vacuum pump and I would start first with checking the supply line to it and with the engine running check and see if the pump is running and producing vacuum. See its location the link below. Copy and paste the link to your browser to open and view it.

Customer: replied 1 year ago.
Thank you I will try and check it tomorrow after work. I'll let you know. Thanks
Expert:  Ron replied 1 year ago.

Sure thing, Im here if you need me.

Customer: replied 1 year ago.
Ron, I haven't forgotten about you. I haven't had time to check the vac pressure at the pump and see if that resolves the issue. I will let you know as soon as I do in gonna try tonight when I get home.
Expert:  Ron replied 1 year ago.

No problem, I'm here if you need me.