The service manual has a good selection of documents in the climate control section that covers step by step troubleshooting of almost all of the components of the system. It can seem a bit tedious to go through the tests but when you're done you'll know if component X is working or not. Documents 83-604 and 83-605 are a good place to start. If you Google "124 83-605" you should have good luck find a pdf file on the web.

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'82 300D - Light Ivory, 2nd Owner (Back in the wind April 2013!)

'95 E300D - White, grey interior. (Suffering from stuck/broken glow plugs)

Deuteronomy 22:4-
"Thou shalt not see thy brother's ass or his ox fall down by the way, and hide thyself from them: thou shalt surely help him to lift them up again."