My 1979 300D has 233,00 miles. Slow commuter car for 70 plus miles a day R/T.Great on the flats. No heater or A/C. Can live without A/C in summer even though work area is 90-100 degrees. But I'm freezing in winter and can't defrost inside. The blower fans work. To just get the heater to work without putting more than $300.00 into it, anyone have any idea what to check first? I'll get my husband right on it if any suggestion. He changed out all my glow plugs without a problem.

Hi Pismo300D , try doing a search on ACC servo or ACC amp. It's a complicated system to diagnose with vacuum controls and resistance values that need to be tested. Try this link for a starting point.
Good luck

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