Heat problem in 1982 300SD

Hello all,
Very bizzare problem in my 82 300SD. About one month ago, I lost most of the heat from my auto climate control system. Last year, heat
worked 100%!
Checked the following:
Good coolant level Working mono valve Swapped temperature regulator (Silver relay box) Swapped push button panels (3x) Good fuses Good Aux water pump Even used an IR thermometer to check the temperature of the coolant at the hoses (temp good!) EVEN UNPLUGGING THE MONO VALVE DOES NOT HELP!
Heat 'kind of' floats out of the defroster vent, but is ice cold from the side vents.
It seems like something mechanical is the cause, vs. a problem with the automatic climate control. Any suggestions? Winter is around the corner!
Thanks! Matt
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You've done more than the usual investigation. When you say the temp of the hoses is normal, which hoses are those? The heater hoses or the radiator hoses?
What I'm getting at is what is the temp of the heater core's EXIT hose?
That will tell you if the problem is coolant flow or in the vacuum duct motors.
Given your work on the coolant side of the equation I suspect them .... or a plugged heater core.
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On Nov 10, 10:47 pm, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote:

TG,
Both sets of hoses, meaning that the radiator hoses and heater core hoses had a similar temperature reading (not Ice cold heater hoses.)
I am unsure which hose is the 'exit' hose from the heater core.
When you say 'duct' motors, are they the same type of vacuum flaps as in any other 123 series MB?> ( I rebuilt my ENTIRE servo system in my 1980 300TD a few years back. What a chore, but it is still working). I also fixed up my 85 300D's system too, and the systems between the late 123 and early 126 look the same electronically, although I see the 126 has more hoses for coolant to go...
Thanks!
Matt
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What TG was trying to establish is whether coolant is flowing through the heater core. There should be two hoses going through the firewall. One will be hot coolant going in, and other will be somewhat cooler water flowing out. Basicly, if both feel hot, you have water flowing. If one or both are cold, then you don't.
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Have you done a voltage test on the leads that go to the mono valve? There should be about 12 volts there when the heat is on.
This sounds like a defective mono valve. Are you sure the check valve in the mono valve is not stuck? Have you replaced the entire valve or just rebuilt an old one? I rebuilt mine a couple of times and then last winter replaced it with a completely new valve and the heating system works much better now. It seems to be able to adjust so the incoming air is just the right amount warmer than the external air. With the old rebuilt valves it was either bake or freeze, but the new valve has the system working like it did when the car was new.
Paul
wrote:

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I agree the mono valve insert is defective.
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Hi Matt; Mine ducting valves, [I assume] are screwed up. I get a satis heat situation by unplugging the A/C compressor.
If I could just get that thing to put out sub-zero air in the summer!! :) :)
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All,
Thank you for the advise! I am going to pull the while mono valve assemble and have a look this weekend!
Matt
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Hello all
Ever stranger!!! I pulled the mono valve insert (clean coolant running through the lines and a proper temp.) At idle, I have heat, but when I start driving, the heat goes away.
A thought. I know the push button control panels are the same hookup between w123's, 126's, and 107's, BUT I have seen a different part number on the cases. IF I have a w123 panel, could it mess up the vent flap operation and cause a limited heat situation, or am I looking to hard at this?
HELP!
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You imply that a vacuum problem exists. Since a diesel creates vacuum by a pump as opposed to a gas engine's throttle there should be no less vacuum when driving than at engine idle.
That leaves the heat supply (coolant). Does the motor run at 80 degrees C.? When driving? A bad thermostat will let the motor cool off when driving due to the cold airflow through the radiator. Try it with the radiator's airflow blocked by a cardboard (don't overheat it!)
Is the heater plumbing's routing correct and without any kinks in the heater hoses?
The electric water pump runs only within a comfort range: ON at 69 degrees F, OFF at 79 degrees F. Its switch is inside the car, on the firewall. Test the coolant flow by unplugging its electrical connection. If you then have no heat at idle the problem is a coolant flow issue.
(I run my 116 without any pump and notice little difference in heating.)
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On Dec 1, 7:04 pm, "-->> T.G. Lambach <<--" <"T.G. Lambach at NoHamorSpamcomcast.net"> wrote:

Not sure on that specific model, but on the 116 if you pull out the glove box which is very easy to do, you can easily reach at least one of the metal lines for the heater core (maybe both, don't remember). Then you could feel the lines while someone is driving and tell if hot coolant is making it to the heater.

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