The Modutrol Motor is a M9165D1004.

I have a customer with a large heat recovery unit in a nursing home. There are a few Modutrol motors in the unit but there is one that operates a hot water heating valve. It is operated by a Honeywell T991A potentiometer control. The valve stopped working. I can make it open by jumping from the 24 volt Red to the Blue. It has a spring close. If I remove power, it closes. The T991A is supposed to modulate the valve open to control the air temperature inside the air exchanger. I got the manual for the Modutrol online but it doesn't tell me how to test it to see what might be wrong with it.

Any electrician that I ask gets that glazed look in their eye. I understand how it works, just not how to tell what it is doing wrong. I can't figure out how to test it electrically. Or if it can be fixed. The motor works, it just won't modulate and control the temperature in the building.

How do you test it and what do you look for?