I have had my BD-36 for a couple years and ride my modified "Giant" everyday to work and back. about a year ago the controller got wet and stop working. I bought a new one and the same thing happen even after I sealed it with a protective wrapping.
Since then I have taken out the controller and connected a direct switch to power and have been using it this way for months now. I have yet to understand the importance in having a controller but at the same time know I need to have one.
The controllers are the 36v 20a controllers sold with the W.E.(Wilderness Energy)kit. I don't know how to fix them or whether I should even attempt to. I am also considering jumping to more volts and amps and wonder if a different controller would be wise at this point.
Ecrazyman seems to have the good generic controllers on Ebay. Some fun though getting the wiring matched up. One issue with no controller, is it can send more power to the motor than is best for your batteries or motor or both. A big hill is where this would happen. Even with a controller, I managed to fry a BD36 motor in 400 miles climbing a big hill in weather too hot, 105 F. Older golf carts had no controllers, but used several resistors to get three speeds and controll the wattage. It may have been hard on the batteries though, and definitely drove jerky. On a golf course job I once had, I drove one with nothing left but high speed. It was fun.