Hello Can anyone tell me how the heater operates? My 300d just has the supply pipe connected together with no heater I would like to fit a heater but not sure how the water flow should go or be switched?

W115, W123, or W124 300D? It matters.

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Its a 300d engine some one has put in a 1964 Land Rover
We think it may be from a W123?

It comes out of the rear end of the head and goes back in down by the water pump. Can put the valve where ever you like, MB had it on the return side of the heater core though

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

Thanks so when the heater is off is flow stopped completely?

That is correct, if the valve is closing the way. On my 240D, I don't think the valves are closing completely, I can always feel some warm air coming out the vents.

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Milan Brown 1979 240D, rebuilt OM617.952 turbo diesel, rebuilt 722.315 transmission (my only daily driver)

Instagram: @maximed93

Thanks, so could I just block them off as I have no heater?

Yes.

Best to block it off close to the cylinder head and as close to the engine on the return side as is practical (I am not visualizing what is on the return side).

I don't think it is a good Idea to have coolant just sitting in the heater core say for years and never get circulated. But, I have not read any info on that.

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84 300D, 82 Volvo 244Gl Diesel

thanks here are my two connections
One is on the right hand side of the engine viewed from the radiator near the rear cylinder, the other is below the thermostat housing currently both are connected together very poorly and it has lots of leaks, but as we have no heater yet could I not just remove and blank them?

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Not entirely sure what I am looking at.

On the pic that has top of the radiator written on the Hose it looks like the large hose is where the leak really is. My Bypass Hoses was like that and to a lesser extent one of the radiator hoses.

It turned out that the aluminum nipples were heavy cratered where the aluminum corroded out from under the hoses.

I took a tooth brush sized stainless steel brush and got out all of the white corrosion on down to the bare metal and filled it in with the slow curing JB Weld Epoxy and wrapped tape around them so that the JB Weld formed to the contour of the nipples. When that was cured and the tape removed I had to go back and fill in any voids or air bubbles and re-tape it.

I think you know this but cooling system stuff is often plugged by using a section of hose long enough to attaché it to the Nipple+clamp and still leave room for an internal plug+ clamp.
I have used wood dowels before. You need to coat them with gasket sealer so as to water proof on the end and sides of the plug that goes into the hose. Hard wood is best.

But they may make plugs just for that job. I simply have never pugged off cooling system stuff enough to go searching for such plugs.

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84 300D, 82 Volvo 244Gl Diesel

The leak is from the copper pipe joining the heater ports together, the hoses are all showing signs of age with some bad cracks so I want to replace them all. I am wondering when the thermostat is closed does the water travel from the pump via the heater hose to the rare of the block behind the injector pump to avoid dead heading the pump?

All of the Water Pumps I have seen on Engines are Centrifugal Water Pumps including the Mercedes. They are not positive displacement pumps.

That means if the outlet is blocked/dead headed it continues to turn but there is no damage to anything.

The system of how the coolant is moved up front on the Mercedes where the thermostat is different then on other Cars but as far as coolant for the heater goes with the exception of the small electric pump works the same on every car I have ever seen.

On your typical Car Engine the Thermostat in fact blocks off the coolant flow and as you said that dead heads the flow from the Water Pump.

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84 300D, 82 Volvo 244Gl Diesel

Thanks Really I ned to see these engine properly fitted as it should be to fully understand but.
I understand the pump isn't a positive displacement so should be fine, what I am now thinking about is if I blank these ports off when the thermostat is closed the engine will have no flow around it just static water? If I connect them together it would have flow so allow all paths of the engine to warm uniformly?
On the other hand I could be talking complete rubbish?

So on a Mercedes the water comes out at the front near the pump to the heater matrix and returns from the matrix to the side of the block ( Or possibly the opposite direction) is the Matrix hot all the time and air flow is controlled or is the flow to the matrix stopped allowing it to cool? If the flow is blocked can the water circulate back to the engine?

I don't know if this will help as it is complicated. The right side of the drawing is the where the heater comes off of the back of the cylinder head. Not shown is the on off valve. If you follow line B you can see where you can block off without disturbing the rest of the system.

#1 is the water pump.

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84 300D, 82 Volvo 244Gl Diesel

Thanks so it looks like it should be ok if blocked off?