The way I've heard it explained here before is that it cycles the valve open and closed depending on how hot you're telling the CCU to make it. More heat equals a longer open cycle.

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Seth

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo