Cadillac Repair Questions? Ask a Mechanic for Answers ASAP
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This model uses what is called a heater control valve or hot water shut off valve.When the ac is switched on this valve is switched to block coolant from flowing into the heater core to prevent hot and cold air operation like you have here.This control valve is common to fail and cause this issue here as I have seen and had to replace before.In some cases some of these are vacuum actuated and the vacuum line can become disconnected or damaged causing this here but most of the time it is the valve itself at fault.
Since this is an older model my info is not showing me a diagram of its location but does say it is located on the driver side of the engine cowl.Usually in line with the heater core hoses.
If this was already checked or replaced let me know.
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The vacuum comes from the electronic climate controller/programmer.This should hold vacuum to keep the valve from opening it under acceleration to allow only cold air with the ac on.Below is the diagram for this valve.It shows it as a white vacuum hose from the programmer to the water valve vacuum motor.
My info shows it behind the right hand center of the dash.It should have a bunch of vacuum lines going to it.Check and make sure these lines are o.k.If they are o.k. then I suspect the programmer has failed and is causing this here.This is common to fail and cause this fault.
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Usually it is located behind the glove box area near the heater box and should have several vacuum lines coming off of it.It may be black and rectangular.There is a built in check valve in the electronic climate control programmer that can fail and causes this here with the programmer needing replaced.
The decision is up to you.I would wait until your satisfied and let me know if you need anything else here.
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