I have a 98 Park Ave. and the HVAC system is shot. It will not switch between defrost, mid vents, and floor. Auto is just useless. If I set it to heat, it blows heat out of the drivers side and cold air out of the passenger side. If I set it to cool, it blows cool air out of the driver'* side and heat out of the passenger side. Sometimes the temperature will fluctuate. For example, if the heat is on it will blow heat out of the passenger side like it'* supposed to for a few seconds and then switch back to cool air. It repeats this every minute or so and the fan makes a different sound, like something is closing and then opening back up again. I know there isn't a problem with the door actuators or it wouldn't switch between heat and cool at all. When I have the air set to off, I can hear a groaning sound coming from the dash for a few minutes. I'm pretty sure the computer is the problem, but I would like to know if it could be anything else. Any help is greatly appreciated.

*I have never worked on a '98 Park Ave auto climate system.*

Not switchingi between vents is probably a vac line leak. If it were the primary incoming line from the engine then it should default to defroster only mode. That it is doing something else probably means either a vac line itself or the vac actuators for that section (particular vents) have failed.

I wouldn't be too concerned yet about heat on one side and cold on the other, except that you can't rule out the door actuators because they could be sticking intermittently, especially with changes in temperature.

You need to pull the stereo and/or glovebox out to find the source of the groaning. That isn't likely to be the computer. While you could be right that the dash head unit has a problem too, ensuring that the rest has no vac leaks seems necessary, then you can try to troubleshoot whether the head unit is cycling vac lines and blend doors by checking van line state and power going to the blend actuator at the appropriate times.

The other option is pull a new dash head unit out of one at a junkyard. I suppose you could buy new if you can find one but it could be expensive. It could help if you can find a diagram of the vac lines and general layout, for example indicating what is involved if drive and passenger have separate temperature zones/settings possible.

Usually when the HVAC head unit fails the system defaults to defrost, not the issues you are experiencing. Are there any signs of a vacuum leak, like high and/or rough idle?

This does not use vacuum at all....all electrical actuators....

What I want you to do is command full cold on the MAX position.......turn off or put the PS controls at midlevel.......should get cold on both sides.....does the compressor run?

Now command full hot, from the vent position........should get full heat on both sides......

In both cases air flow should be coming from the registers in the I/P.....

A scan tool, like a Tech II, would indicate if codes are set in the HVAC system....