86 3.2 Mondial heater/defroster

To clarify, the fan works when the square button switches are set to either defrost or dash vents, and the round temp setting is set to cold, and your fan speed is on high. But if you turn the temp switch to heat rather than cold the fan stops?

 

Check vacuum hoses to the vent selector switches

 

Doug, does the fan motor work but no air comes out the vents? Or does the fan motor stop when you press A/E or Defrost?

 

OK Doug: The push button unit connects the HVAC ecu and sensors etc to ground when you press AC/AE/DEFROST, and disconnects it all when you press OFF.
Air distribution is handled by vacuum depending on which button you select. No need to look at that, it's independent of the fan question.
Since your fan stops when you press any other button button than AC I suspect the push button unit's electrical switch is not operated while you press AE or DEFROST.
Disconnect the electrical connection (white green) and ground it directly with a bypass wire. If the unit's switch is broken, the fan would work in all button positions.

 

AC, AE or Defrost push button switches all should cause the fan to go on, with either heat or cold on the thermostat knob setting. If you push the AC button, but set the temp control to full heat, do you get the heat? You should have full heat, and if you slowly dial back the temp setting, you should hear a click and the ac compressor should come on, providing the cooling.

Each push button switch itself does not affect heat or ac, it just changes the flaps in the unit to direct the air (which unusually are actuated a vacuum hoses, not electrically actuated). The AC means recirculation (not air conditioning per se, although recirculation of interior air is more effeicient for the cooling), the AE means fresh outside air is being brought in, and defrost means the window vent. Heat or cooling should work in all these button settings.

It does sound like the push button switch unit (it is all one unit with the four push buttons) has partially failed. Maybe the wire connectors to switch on the underside are corroded. You might first spray some electrical cleaner around the switch (don't stain you plastic console with the cleaner, that stuff can melt the plastic), and with the key off press the buttons a whole bunch to see if there is some internal contact that can be revived).

Here is a used ebay switch available, you can see how the connections look on the underside in the picture. On the right underside are where all the little vaccuum hose connects with a special connector.

https://www.ebay.com/itm/133691555592

As long as you are getting the heat and cooling, even if in only the one AC button positition, that's not so bad.