W140 ACC - No Heat

[jgl1]

'93 500 SEL, 125K miles... climate control producing NO heat at all, regardless of setting. Heater core temperature essentially same as vehicle's interior temperature, per pushbutton control unit.

Fault code check revealed two codes (E074, E075), both indicating a faulty auxiliary coolant pump. Approx. 11 volts present at aux. pump harness. Connected new pump to harness and visually confirmed pump operation, prior to installation... pump noted to cycle on/off, relative to temperature thumbwheel position.

Cleared fault codes and installed new aux. pump... but still no interior heat. Fault code recheck reveals no ACC faults. Aux. pump warm and duovalves hot to the touch, with engine at operating temperature.

Any suggestions? Duovalve assembly at fault? Is a diaphragm repair kit available for the duovalve? (None listed at Fastlane). TIA.