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ps there must be other issues since it would only run full speed briefly and no alteration care of the auto/ max slider control. Who knows! It’s a right little monkey.
Have recently taken a Series II wagon back from my son which has exactly that problem. My son says it's the slider control that is cactus and I have no reason to doubt him. We think it's essentially stuck on Auto
To test if it's stuck on auto, try moving thermostat to an extreme (ie coldest setting if car has been sitting in the sun), then listen carefully to see if the fan runs faster - mine definitely does, but it is easier to tell if the carpet trim that covers the fan has been removed first.
Cheers
Alec