I say mono valve too. While you are at it. my 90 and brotheres 92 would blow hot and cold when in AC mode. Usually in 5 min cyles. This went on for years. We both cleaned the 'Main Ground" going from the engine to tranny bell housing on drivers side of our 300-E's. The whole climate control system on both cars went back to flawless performance. It also corrected his jiggling enging temp guage to 'no more jiggle' and it restored my heat sensors that trigger my Electric Aux. Twin Fans. I don't think cleaning the ground will do anyting for your problem (Monovalve) but if an old car, clean that ground and many thing got the right electricical flow again, and go back working.