Hopefully someone has run across this problem before. I am well versed in the blend and mode door motors going out on these vehicles. I own eight 4300's and have seem most problems. The mode door gets stuck on defrost but it is not a door problem. I can take the blend door wiring pigtail plug it into the mode door and move the temperature knob to change air flow direction. I have tried two spare control heads, they work a couple times then stuck back on defrost. Unfortunately I do not have a service manual and my heavy duty scanner does not see this system. With a digital voltmeter I get a quick voltage reading and then back to zero when not turning the mode door knob. Any thoughts or diagnostic help appreciated.

Bob Hamilton
Owner/Technician
Springdale Automotive
Cincinnati, Ohio, USA