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Old 01-26-2008, 08:27 PM
Registered User
Join Date: Sep 2004
Location: Metamora, IL
Posts: 60
No heat at idle but cooks you at highway speed


I have been reading everything I can find on heating and it all seems to be reversed of my situation so I have to ask.

I have a 81 300SD it will burn you out at road speeds with no problem but when you slow down like in town to say 30mph or lower or stop the heater will cool down to freezing you out.

I have just replaced the aux water pump.
I have reworked the control unit and have even replaced with a known good one.
I also just flushed and filled the antifreeze while I did the aux pump.( And I used the Zerex antifreeze).
The engine temp stays at normal. ( And always has ever since I have owned this car)

The blower motor runs like normal not dragging or anything.

Also if you let the car idle with the blower on low you can get very little heat out of it. But not enough to stay warm.

I was thinking of the mono valve but I am not sure as all the articles describe just the opposite problem for that.

Any help will be greatly appreciated.


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Old 01-26-2008, 09:02 PM
babymog's Avatar
Loose Cannon - No Balls
Join Date: Jan 2006
Location: Northeast Indiana
Posts: 10,767
Find a high point in the heater line and bleed, sounds like there's air in your heater core.

Gone to the dark side

- Jeff
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Old 01-26-2008, 09:16 PM
Jadavis's Avatar
Registered User
Join Date: Oct 2005
Location: Tennessee
Posts: 995
Still sounds like the aux cooling pump. Try hot wiring it to a 12 volt source and run it with the engine hot. You should get heat sitting in the drive way with the engine running and hot if it is working.

1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.
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Old 01-26-2008, 09:17 PM
rg2098's Avatar
Detailing Moderator
Join Date: Jun 2004
Location: Rochester Hills, MI
Posts: 2,415
Aux water pumps bad.
Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section
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Old 01-26-2008, 10:57 PM
Registered User
Join Date: Jan 2002
Posts: 18,355
Bad auxilliary water pump.
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
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Old 01-26-2008, 11:47 PM
sailor15015's Avatar
Reverse lights! Score!
Join Date: Apr 2005
Location: Norman, Oklahoma
Posts: 1,184
Chock up another vote for the aux water pump. Its amazing how much of a difference it makes at slow speeds. It wouldn't hurt to add a one amp fuse inline while you're in there. It just might save your CCU one day.

1984 300D 225K
1985 300D Donor body
1985 300D Turbo 165K. Totaled. Donor Engine. It runs!!!
1980 300SD 311K My New Baby.
1979 BMW 633csi 62K+++? Dead odo
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Old 01-26-2008, 11:55 PM
Registered User
Join Date: Jan 2000
Location: PA
Posts: 5,440

See if U can feel the vibration of the aux water pump when it should be running. If it is not running, check if it is getting 12 volts.


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