Been having an ongoing problem for a few years which I'd like to clear up. My heater control valve works OK if vacuum is put to it at the end of the line at the control switch unit (under right vent). Everything else appears to switch OK the rest of the year. To keep heat off when unwanted I put a manual shutoff on the heater hose under the hood. That is a pain to control in winter, either too little or too much heat...open the hood and adjust!
So, is there a way to diagnose if the vacuum switching unit that controls the heat flow (#11) without using dealer test boxes etc.? Have the switch out and all looks to intact, no cracks etc, or could the problem be elsewhere? How would one check the operation of the unit?