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Pete, Cadillac Technician
Category: Cadillac
Satisfied Customers: 27883
Experience:  17 years Cadillac experience as a technician.
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I have a 1985 Fleetwood Brougham. The heater comes on while

Customer Question

I have a 1985 Fleetwood Brougham. The heater comes on while the A/C is running and hot air comes out of the A/C vents.
Submitted: 9 months ago.
Category: Cadillac
Expert:  Pete replied 9 months ago.

Hi my name is ***** ***** I will be happy to assist with this today.In some cases I may need more info to assist better.I apologize for any delay.

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.

If more help is needed, use the reply tab to continue our conversation. If no further assistance is needed, kindly rate my service,your 5 star rating is appreciated.You can rate at any time and we can continue to work on your question as this will not close out your question.Keep in mind in some cases it can be difficult to fully diagnose or help repair your vehicle without seeing it and I work on delivering the best possible answer from what I see to work with.

Thanks Pete

Customer: replied 9 months ago.
If you are talking about the in line valve on the heater hose, I have already replaced it. I tried putting a positive vacuum form the carburetor on it, and that kept it shut. But then, when I accelerated, engine vacuum drops and the heat is back on until the heater core cools down again. Where does the vacuum to shut this valve come from and what controls it?
Expert:  Pete replied 9 months ago.

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.

Let me know if you need anything else and feel free to rate my help here as this is how I receive credit for my time.Your 5 star rating is appreciated

Thanks Pete

Customer: replied 9 months ago.
Not being a mechanic, I'm not sure that I understand the diagram. I'll need a few days to look under the dash to see if I can find the hose and I assume, a dash pot. There are no vacuum hoses that go to the climate control module on the dashboard. Would I look behind the glove box, or what is the easiest way to get to this?
Expert:  Pete replied 9 months ago.

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.

Thanks Pete

Customer: replied 9 months ago.
I will try to locate it this weekend. Do I pay now, or when I'm satisfied the you have helped me?
Expert:  Pete replied 9 months ago.

The decision is up to you.I would wait until your satisfied and let me know if you need anything else here.

Thanks Pete

Customer: replied 9 months ago.
Thanks. I will try to get back to you by Monday.
Expert:  Pete replied 9 months ago.

Your welcome

I should be here Sundays through Thursday

Customer: replied 9 months ago.
It took me longer to get out to work on the Cadillac than I thought. I traced the color coded vacuum line from the heater hose valve to a dashpot under the dashboard area just above the transmission hump. I couldn't see any other vacuum lines under there that would bring a positive vacuum to either that dashpot or the heater hose valve. There is another dashpot under there, but it controls the flow of air from the upper vents to the floor vent. It does work. When I push the temperature up to 80, the air flow switches from the upper vents to the floor vents. There must be some place that a positive vacuum can get to the heater hose valve. If I pull a vacuum directly off of the carburetor, it closes the valve and cold air comes out of the vents. This is great, but when you accelerate, the engine vacuum goes to zero and the valve opens and hot air comes out of the A/C vents. Is there another vacuum line that enters that dashpot?