I've read some posts regarding a lack of heat (interior heater) and see something called a mono valve or ACC mono valve. My '87 420SEL comes up to operating temperature but there isn't much heat going to the defroster or heater vents. The temperature is luke-warm at best...

Is the mono valve the most likely culprit?

Thank you,

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Richard / E320

2001 E320
1987 420SEL