Does the new motor fit the manifold block? I doubt you purchased the CORRECT motor from Amazon. HWH has about 20 different motors over the years and some have been discontinued, especially a 20 year old motor. The motor you purchased is probably a BI-Directional motor, that is the reason it has a Pos and Neg terminal. HWH does not use a BI-Directional motor system. The Ground(negative) comes from the frame of the HWH pump/motor mount. But, IF that motor fits and the keyway fits the pump fitting, then YES, you must connect that Neg terminal to a chassis/frame ground for the motor to work. Doug