no heat, bad amp?

[erubin]

1980 450SL:
I replaced the cracked and defective ACC servo with a metal bodied rebuilt unit ($350).

Unable to get heat in any mode (including defrost). I read that the amplifier is often taken down by a bad servo. Could a bad amplifier prevent heat from coming on? I thought getting heat in defrost mode is the fail safe mode. The aux pump is OK and anyway that is only needed when idling. Blower works and air moves to different zones when different modes are selected. AC does not work because of a refrigerant leak (will deal with this later).

Will a bad amp possibly cause no heat even in the fail safe defrost mode? or should I look somewhere else to fix before pulling out the amplifier?