I bought a sweet 82 300sd. When I drive it the climate control switches on and off intermitently. I have no idea how to diagnose this and my searches are fruitless. I will be going along heater blasting and then it willjust stops only to come on again later in the trip.

so the fan stops? is that what happens?

at that age, the brushes in the fan motor often are worn out. you can replace them. george murphy of Oak Ridge, TN has the brushes and sometimes he has rebuilt motors too

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

XYZ - next time the fan stops try something. Grab your key with engine running and fans stopped, wiggle your key slightly left, right, left right. If the fans come on your problem is dirty/worn contact points in your ignition.

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Fuse may have poor connection.

excellent point! on my 84 300SD the ignition key doesn't always spring all the back to the little notch on the ignition. you have to turn it back just a hair for the climate control to work.

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

Is it possible that the system is functioning normally and just responding to the interior temperature wheel setting?

Yep, the ignition switch is a likely possibility. That said, don't replace parts on possibilities or other folks experience. Troubleshoot scientifically.