Just thought I would share my experience today.

Tore into my heater expecting bad vacuum pods. All my pods are good, I didn't have any vacuum source. Traced it down to the ignition switch.
At some point in my dash swap I flipflopped the vacuum hoses. I never paid attention cause I figured one in one out its a simple switch, right? Wrong. One way holds the vacuum the other side doesn't.

Very simple fix, heater works as it should now.

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81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)

Atta boy! ( I love to read these kinds of posts. They give me hope.)

"Worry gives a small thing a big shadow"
Swedish proverb

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"Knowing is not enough, we must apply.
Willing is not enough, we must do." Goethe

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1951 Chevy 3100

2003 Indian Chief Roadmaster

1983 GMC 1 ton Dually

1982 Chevy 1 ton Dually, service body (sold)

'90 GMC Suburban 6.2 "SS Veg-Burban"
(single tank WVO\diesel conversion) SOLD

'81 300D ~ Mama's car...my job (now my car)(but still my job) SOLD

'83 300sd ~ rescue car SOLD

2005 Ford Taurus (Mama's new car)(NOT my job!)

my heater recently stopped working. I can put the "zone selector" on defrost or any of them, but unless the temperature roller selector on the left is clicked all the way on cold, no air blows. Does this sound like I could have the same problem (and fix) you do?

No, not the same as mine. What year and model do you have. Put it in your signature so others don't have to guess. It helps to keep some of the more grumpier members happy too.

Yours sounds more like a disfunctioning controller. May just need resoldering.

The symptoms on mine were just defrost in all positions. Heat or cold.

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81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)

sorry...I just updated profile info and sig (though for some reason it is not showing up when I post).

I think it might be a soldering issue, as when it did work (a couple days ago), it would make some weird clicking noises when shifting beetween heat and cold, and the rpm gauge would bounce up and down a little at the same time.

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1983 300SD ~250k

yep that clicking noise if from the broken solder joints. Mine did the same clicking thing before I soldered it.

__________________
81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)