The original Borg Warner installations were controlled by a relay and a governor. The governor did not allow the overdrive to activate until the car reached a speed of about 35 mph. This prevented any damage from backing up with the overdrive engaged. It also prevented the solenoid from being powered when the car was parked. They also had an over ride switch associated with the gas pedal that would disengage the overdrive temporarily if you wanted to accelerate.
Most Model A modifications did not include any of these features and often the overdrive was damaged when attempting to back up with it engaged. There were often incidents of a dead battery when a Model A was parked and someone did not turn the solenoid activation switch off.
I had three Borg Warner overdrives installed in Model A's back in the day and I took advantage of all these features when I installed them. See the attached link
Tom Endy
http://www.santaanitaas.org/wp-conte...r-Apr-2005.pdf