S-Class (W126) 1979-1991: 300 SE, 300 SEL, 380 SE, 380 SEL, 420 SEL, 500 SEL, 560 SEL, 360 SEC, 500 SEC, 580 SEC, 300 SD TURBODIESEL, 300 SDL TURBO, 350 SD TURBO, 350 SDL TURBO

Heating issue

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Old Mar 9, 2016 | 09:14 PM
  #1  
WindChaserX's Avatar
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300SEL 1990
Heating issue

Hi folks,


Practically no heat, which is not to say none, but almost none. If I stop the fan for a couple of minutes, the heat builds up somehow. But as soon as I start the fan, I get a momentary blast of decent heat but then it soon cools down to nothing.


I replaced the mono valve. Things only got colder. So I pulled out the mono valve, put it back again and removed its power leads. No effect. Then, even though the temperature gauge showed a steady 82F temperature, I changed the thermostat to an 87F model. Made sure that coolant level was topped up. No effect.


I'm at a loss. Does anyone have any ideas? Thanks!
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Old Apr 27, 2016 | 01:07 AM
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V12Stealth's Avatar
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From: San Jose, CA
w201,w124,w126
Was it like this when you bought it or has it slowly happened? I believe some cars had a axillary water pump for the heater. Might want to look into that. Sorry, I don't know much about the 300SEL.
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