My car's heater works fine as long as the RPMs are above 2500, but there doesn't seem to be any heat when RPMs are lower than that. It hasn't been a problem on long trips....but around town, it gets to be annoying. Anyone want to take a stab at this one? I've searched the forum and found lots of posts on the opposite problem -- heat at low RPMs, none at higher RPMs......but that doesn't describe my problem.

Thanks in advance!

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1983 300SD "Helga"

Funny, I've only read about the exact problem you're having...not the opposite.
Check the auxiliary coolant pump, is supposed to come on when the climate control unit is trying to heat the cabin. Its on the passenger side of the engine near the front, under the plastic air intake tube.
Although, mine was leaking so I replaced it with a brass elbow fitting, and I still have heat when just sitting and idling.
Maybe also check the water pump...squeeze the upper radiator hose with the engine running, you should feel pressure build up fairly quickly.

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1985 300TD-euro 352,000 mi
1974 240D (1?)52,000 mi - has a new home now

Aux water pump.

The reason you still have heat at idle with the brass elbow is that the non working pump acts as a restriction. With it out of the circuit the hot water flows easier.

It could also be the Mono valve. If it is not working right the pump can spin all it wants, there is just nowhere for the hot water to go. Scratch that. You have heat above idle so it must be working.

Definately the aux water pump.

-Jim

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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.

The aux water pump on my 83 300 D is no good (the motor is open circuit per ohm meter). I am getting heat. The big questions:

1. Should I leave it as is?
2. Should I replace it with a pipe?
3. Should I replace it with a new unit (which is $100 I don't really want to spend)?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

I've replaced mine (when it was starting to make noise), but it's not really critical (but incorrect stuff bugs me). If you want it correct, replace it. If you don't mind having slightly reduced heat at idle, either leave it alone or put in a pipe.

Do you still have the old pump? I need just the motor (if it comes off).

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Somewhere, waaaaay back in the archives, there is a great thread with pictures of rebuilding those pumps. Sorry, I can't remember the membername who posted it. 300SD Superturtle, or Superturtle 300SD, or something like that. Maybe somebody else can remember her username.

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Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

Try this:

Member's name is Pete Burton

use advanced search for his post:

How to disassemble aux water pump-part 1

Good Luck!

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'84 300SD sold
124.128