Back again...I have an 85 300DT. Seems like when I have the heater on, it only works while driving. When I come to a stop, the air goes cold, especially when I have the fan setting on high. Any ideas??

Aux. water pump or control for aux. water pump?

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1986 300SDL 201K Daily Driver
1984 300D In Progress
1989 240GL 196K Swedish Brick
1984 300SD -- Sold 289K
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It is ONLY at idle? Do you notice any difference when going highway speeds (engine @ constant speed)?

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1989 300E 144K

at highway speed, the air is hot....while driving in general it's hot......when I stop, it goes cold

When at idle, test the auxilary water pump (or just put your hand on it to feel if its humming).

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1989 300E 144K

Yeah, sounds like the aux pump. At idle, you're not getting enough RPMs on the main water pump to push through the heater core.

That aux pump can also cause a blockage in your coolant system if it isn't working.

When you turn your key to ON, with the hood up, do you hear the aux pump running?

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'85 300TD "Puff The Magic Wagon" - Rolling Resto
'19 Mazda CX-9 Signature - Wife's sled
'21 Morgan 3-Wheeler P101 Edition
'95 E300d - SOLD
'84 300TD "Brown Betty" - Miss this one
'81 240D "China Baby" - Farm grocery getter

where is it located??

There should be some heater hoses going into a little box on the passenger side firewall (monovalve). One of those hoses should go toward the front of the car inton another little box (aux water pump). Basically, it should be near the front passengerside of the engine. Someone correct me if I'm wrong, as I haven't popped the hood of my w123 lately.

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1989 300E 144K

Here's the aux pump on my CD, the accordion tube from the air cleaner to the top of the front fender has been removed.

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83 SD

84 CD

X3, sounds like aux water pump.

Try running the engine at say 1200 rpm when in stopped traffic to see if this makes a difference.
speeding up the engine will pump more coolant to the heater.

Here are some pic of my 85 300D

first pic is the aux pump. the J hose comes from the water pump into the aux pump.
the hose on the left connects to the black pipe.

pic two shows the aux pump lower left, black pipe runs towards the battery.

pic three shows the Mono-valve to the right of the red hose.
Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Hi Charlie,
Couple things:
1) Great help for OP.
2) That wrap on your air filter seems to be doing its job nicely! I remember seeing that at the GTG.
3) I envy all aspects of your engine compartment . We should all strive to keep ours that clean.

Happy Holidays!

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"

Thank You Dan, Yeah it`s a full time job. the wrap is easy to make and change, amazing how much it traps. But I am thinking about installing the
Cali air filter, just have to do some custom fabing.

I tried not to get a straight shot of it, didn`t want to flame broiled over it .
I wish the sheet metal looked as good .

Pictures are always better than trying to describe where/how something is.
when is the next GTG?

Charlie

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there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

I'm not sure. Our top notch coordinator has fallen off the face of the earth.

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1981 300TD "The Green Lantern"
1980 300TD
1983 300D Euro "China Cat"

Count your blessings you have a 123 body. I've changed out the aux pump on my 126 body (85 300SD) and it's a pain!! It's between the insulating wall of the engine and the firewall, UNDERNEATH the ABS electronic stuff and underneath the monovalve!! Very well hidden. Also, on 126 if the aux pump fails and seizes up, the electrical spike (amps? not sure) will very likely take out your climate control module in the dash. Had to replace that too!!

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R Martin
Dallas, TX (Addison)

86 560SEL, 128K

May have another issue. The air is really only really hot on low fan speed. When I switch to high, the temp decreases. At idle, the air is warm at best.