Crazy w126 heater

[jonnyboy]

Hi. I've an '88 300 se, with a crazy heater, sometimes hot, sometimes clay cold. I've checked the voltages at the duovalve, and anywhere not max or min it keeps clicking and the voltages jump up & down (both sides).
Any suggestions?

 

[Ian B Walker]

Have you bled the heating system?

 

[jonnyboy]

Sure have, when I had the cylinder head off.