79, 240, automatic, manual hvac controls.

When sitting still for a few minutes the temp climbs to 250. Twisting the temp knobs and turning on the blower doesnt change anything, and its cold air output. As soon as driving the temp goes down and heat starts to come out the eyeballs.

I have been looking for a system diagram to no avail. Not even certain my car has the aux pump, but if it does;

1. does it run all the time or is it switched?
2. if switched, what turns it on/off?

Any other ideas about no heat when stopped and temp rise?

Thanks

All I can tell you is I removed the aux water pump in my 1984 & 85 M-B and I get plenty of heat, even at 700 rpm idle. Don't know if your 1979 even has an aux pump (why not look for it?). Indeed, the duct temp sensor in the 85 has failed so I get full heat (ignores setpoint dial), which becomes way too hot even when 32 F outside.

Are you talking about your coolant temperature climbing to 250 F? If so, always state units with any measurement. My M-B temperature gage is marked in C, with 120 C being "in the red". I doubt the heater flow appreciably changes engine coolant temperature, though for highway emergency over-heating it is smart to roll down the windows and run the heater on max.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

M82:

There is a rather high probability that the engine water pump, and/or its housing is almost tits-up. At idle speed water ceases to circulate through the radiator, hence high gauge reading, and also ceases to circulate through the heater core, hence cold air. With an increase in engine speed, and pump speed, circulation returns.

I agree that this is a likely cause, but cant figure out a failure mode of the pump that would work at road speed but not at idle. If the impeller vanes were partially worn away that may happen, but I was told these are not plastic thus not likely to wear away. I did have another car that wouldnt flow well at idle and the temp would rise, but it was easily remedied by upping the rpm a little, and the heat still worked.

As far as temps changing with the heat circuit opened, yes I can initially see a temp drop when turning the temp knobs open, as cooler fluid from the heater circuit enters the loop. And I can usually see the thermostat open too.

An attempt to view the FSM only got as far as diagrams of the main pump and something that indicated that some models did not have the aux pump. Unable to view further leaves me uncertain whether I am supposed to have one and the vehicle isnt readily available to check (nor do I know where to look without a system schematic) but I will poke around when I can. The questions wrt the aux pump still stand.

I think your first fix should be to figure out why you are going to 250F at idle, temp shouldn't increase. As stated above you may have a tired water pump. I believe cavitation can, over time, wear the vanes out on a metal impeller.


I don't believe cars that came equipped with the manual HVAC controls have the aux water pump.

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"It's what non-car people don't get. They see all cars as just ton-and-a-half, two tons of wires, glass, metal and rubber. That's all they see. People like you or I know, we have an unshakable belief that cars are living entities..... You can develop a relationship with a car, and that’s just what non-car people don't get."
-Jeremy Clarkson

I agree, a tired water pump could be a cause of your symptoms.


After driving a bit, when you stop, you can get heat soak...there is a sudden lack of adequate coolant flow and of adequate air flow at idle for dissipating the heat that still radiating from the engine.
Another cause of temperature rise when stopped is from the transmission generating heat while in gear, which passes heat to the coolant via the transmission cooler in the radiator.
The FSM recommends putting the car in neutral when stopped due to traffic congestion as a countermeasure.
See the last page of the FSM section 20-005.


Auxiliary heater pumps only came with the automatic climate control option offered on the larger displacement engines.
Your model should not have one.


External blockage (bugs, dirt, etc.) and bent fins can lower the efficiency of your radiator and the heater core (a mini-radiator).
Internal scale and sediment in the coolant system, and old coolant can lower the efficiency of the radiators as well.

It may be time for cooling system preventative maintenance if you haven't done it in a while.
P.S. Your car is on Disc 2 of the W123 FSM

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78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

If you do have this auxhliary pump , its there to circulate coolant They are on the W123 petrol .But i dont know about diesel The reason is that the pump on the engine is not strong enough on its own to circulate the quantity needed in the system .. Remove the plug and put 12 volts to it .This will tell you if the pump is good .

Eh, I also yanked my aux pump and can get a good sweat going if I want. I'm skeptical of it ever needing to be there in the first place.

-Rog

Rog,
Maybe it helps in Northern Germany or Alaska, but not needed where I prefer to live. It can also block the heater flow if stopped or clogged. Even worse, it can leak from the shaft so you slowly lose coolant and only find out when your engine overheats and is possibly damaged. Anyone who keeps it should fuse it so it won't cause a dash fire if the motor stalls (suspect in some incidents).

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans