GMC Troubleshooting Questions? Ask a Mechanic for Answers
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.
Are you working on a truck or a van?
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.https://dl.dropboxusercontent.com/u/104923897/Vacuum%20pump%20Sept%2021.pdf
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