1991 Park Avenue Climate Control Question

Hi there,
on my '91 Park Avenue the climate control suddenly can not be switched to "Auto" anymore but stays on or goes to "Econ" instead if I press the
"Auto" button - meaning that I cannot cool the car anymore.
All other functions of the dashboard control unit seem to work.
Does this mean that probably the dashboard control unit is defective?
Does anybody know how to replace this control unit? It seems to be fixed with 3 screws (2 at the sides, 1 at the top), but how to remove the long black dashboard covering?
I would be very grateful if some of you could give me some hints.
Thanks a lot
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This happens as a default, the a/c circuit is a problem, usually a loss of freon prevents the compressor from coming on, and it defaults to econ. Have the compressor looked at, I believe you will see it does not engage.

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... thanks a lot. I had the freon checked and there is a lot of pressure in the system.
How to check now the compressor if the control unit does not allow this?
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If the compressor doesn't come on , jump the high press switch and see if does, actually there may be 2 switches low press also, not sure. This should turn on the compressor, if the relay is ok, you really should have an operation diagram here, this can get complicated. When it comes on see if the LP pipe gets cold, the larger of the two, this will confirm there is some freon in the system.

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