1989 Toyota Supra Automatic Cruise Control

My cruise control works only when the ambient temperature is below 40 degree F. (?) The select switch (marked "Main") will only actuate when the temp is as stated. If the temp is higher it won't allow me to set a speed. I can't find a repair manual to direct me to what might be the possible reason. The "main" switch is located on the left side of the steering wheel and the "set speed, coast and cancel" switches are located on the right side. When it works, it works okay, but only when its cool enough. Weird, huh? I don't have a picture