How to emulate potentiometer to control electric vehicle?
I had the exact same problem, with I think a Curtis 1234 motor controller for a hybrid vehicle project I was on as an undergrad. 

It turned out, as a safety feature, the controller sent out a current through the pot and compared sourced current to sunk current to check for a broken wiper or other throttle faults, similar to a GFCI breaker. 

You could enter a programming mode and change it to look for just the voltage, but the programmer was very expensive.

In the end I wound up using a digital potentiometer to interface between our computer and the motor controller.