I lost heat last week for no apparent reason. I replaced the monovalve and it works, and I am getting vacuum from the whit line from the servo, but apparently the other white line that t's in near the servo isn't holding the vacuum. I can manually open the monovalve and still have no heat. The only thing I have changed in the last week is i had to take the AC belt off and haven't replaced it, but I can't see why that would matter. Any ideas??

Please post model and year of vehicle.

sorry. thought it was on my signature. 1980 300d

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1980 300d on veggie

Is the aux. water pump working?
To eliminate any remaining doubts about the monovalve, disconnect the lead to it...no power to it = full heat.

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

I had a monovalve default to the closed position rather than the open position. When connected it also pushed down to close the valve when it shouls have gone up. I replaced the monovalve and it works fine.

I just replaced the monovalve, and I hooked up the auxilliary water pump directly and it works fine.

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1980 300d on veggie

Did you replace the whole unit, or just the valve body, or the solenoid?

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

the whole thing.

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1980 300d on veggie

That's not god. I was hoping for a less expensive answer. Did you get a new unit? How much?

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Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

Am I missing something? I believe that if you have the ACC servo, you don't have a monovalve. Am I mistaken on that? If I'm correct, what have you been replacing that you're calling a monovalve?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

The unit I am talking about bolts to the firewall and has a small coolant line that goes through it. I may be calling it te wrong thing. Either way, the old one immediately leaked down when I put a test vacuum on it. I got another used one from a guy I know and it doesn't leak down under a test vacuum. It cost $25. The crap servo that was on the car when I got it was bad and I got one off another guy I know for $10 (yes, I practically stole it) and it has all worked like a champ until now.

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1980 300d on veggie

Your car doesn't have a monovalve. The thing on the firewall with water and electric and vacuum to it is the notorious ACC servo unit, the bane of that model.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970

ACC servo is on the inner right fender liner near the front of the engine. I don't know what the other vacuum part is.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I know what the servo is, and that is not what I'm talking about. I took the old servo piece of crap apart tonight to see what made it tick. The part I'm talking about that bolts to the firewall has no electrical connections. It only has a valve which is controlled by vacuum. Whatever it is, it has something to do with the heat because it has coolant flowing through it and changing the controls on the climate control panel makes it open and close. Another thought I had was that the coolant valve on the bottom of the servo is bad, but the lines going to the heater core seem to be getting hot. The only other thing I can think of is that the flap that allows air to pass over the heater core is stuck closed. This doesn't make much sense to me because if the system fails it is supposed to default to heat. Also I really don't want to pull the dash to check it. Any Ideas?

Thanks

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1980 300d on veggie

My 300 CD 1980 parts car has one. Heat worked last i used it. PM or email me an offer and ill send it to you this week.

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What Would Rudolph Do?
1975 300D, 1975 240D, 1985 300SD, 1997 300D, 2005 E320 , 2006 Toyota Prius