1988 300 SEL - on a long drive last week, the temp in EC mode would either be all cold in the MIN position or jump to hot air once I moved the dial to the 18-22 range (coming from the center vents).

Now (a few days later after the trip) it has gone to all hot in any mode, all the time. It will cycle to the proper vent in the modes - as if the climate control is telling the air where to flow from, but it's always hot. For instance, if you crank the temp dial up to the hot range it will start kicking it out the floor vents - cold range will send it through the center vents.

Even if I cold start the car with the climate control in the off mode, it will flow through the center vents at full temp once the car has warmed up. If I turn off all the center vents it will just flow through the defrost vents at full temp (obviously).

Is this a sign of a bad monovalve, bad CCU, bad temp sensor? Please help! I am supposed to be leaving for a 10 hour trip tomorrow and this will be like driving in a sauna

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(2) 1988 300 SEL W126