I've had vacuum wipers on cars in the past, they had 1 push pull control for the on/off and speed. On my 46 pu, the push pull knob seems to control on/off and speed just as I've seen on other vehicles - but I'm not getting what the 2nd twist turn knob is for. I've messed with it, turning in and out, but it doesn't seem to have an effect on the wiper operation. (The unit has been rebuilt). I've tried the owner's manual and shop manual, but no mention is made of what the 2 knob does. Dumb question, but it's bugging me.
