Hello,

I think I have read just about everything there is to read about the w123's no heat issues and have tried most of the fixes.
Here are my symptoms and fix attempts to date:
1983 300D (not Euro) 213k miles. It's never had heat since I bought it.
The fan blows only from windshield defrost vent, and side vents, nothing from the middle. Assuming bad vacuum pods.
From what I've read, when vacuum fails, the blend door defaults to defrost vents.
I have no heat.
Replaced mono valve with junkyard unit. No heat.
I repaired broken wire on monvalve plug. No heat
I replaced monovalve with aftermarket new unit. No heat
I bypassed the monovalve with a brass ball valve. No heat
I replaced the aux pump with bench tested junkyard unit. No heat
I checked for voltage at aux pump, - 12Volts
I checked flow out of aux pump, appeared poor.
I checked flow out of my roomate's 1983CD aux pump, great flow.
Unplugged the aux pump. No heat.
Bench tested the junkyard aux pump again....several months after installing, and it looks to be frozen.
I replaced aux pump with new, and flushed the heater core. Lots of rust, but was able to flush the core in both directions for several minutes.
All hoses feel warm on both sides of the brass valve, and heater core.
Ensured good flow from the cylinder head.
Coolant level is good. Main water pump was replaced last year due to a bad bearing.
I can get luke warm air for about 10 seconds after I've driven the car for awhile, then it goes cold again.
I've also tried two different push button control panels with no luck.
Let's see, any other relevant info...the AC does not work either, although I'm not looking to fix it here in SoCal. But this El Nino is making it seem like I'm back in Indiana this winter so heat/defrost would be nice.
I doubt there's air in the system. I've pulled enough hoses and reattached while running coolant that it would seem any air would be gone.
The engine gets up to just under 80 C in this weather according to the gage.
Could the heater core need more flushing, or maybe a blend door is stuck?
I would lean heater core since I do get some warm air for a few seconds. What am I missing?

When you had the hoses off the heater core, did you try flowing thru it? You could use a bicycle pump if you don't have an air compressor, or connect a garden hose, or even blow thru a hose w/ your mouth. It is a 3-tube core, w/ one inlet and 2 outlets (or vice-versa), but fairly obvious which is the single side tube (the other 2 connect via a splitter). Of course, also good to "back-flush" the heater core w/ water until it runs clear.

The aux pump gives me no problems in my two 300D's because they are on a garage shelf. A length of 5/8" silicone heater hose replaced them. Since 1986+ mono-valves are cheaper, one guy installed one of those where his aux pump sat and ran a hose where the mono-valve sat. He had to extend the wires.

My two 300D's run at 82 C in the winter, maybe 80 C on a <32 F day, so yours sounds normal. One used to run ~60 C and the heat was marginal. The T-stat tested sluggish in a pot of water (opened ~60 C, then slowly opened w/ temp). A new one tested snappier and fixed the issue.

The center vent is supposed to blow only in "cooling" mode (owner's manual?). It is possible your system is letting outside air in, thus over-whelming the heater. I am not clear on how that works, but it would be coming from the cowl vents around your wipers, so you could try blocking those as a test, perhaps use a bunch of flat refrigerator ad magnets. The climate controls are quite erratic. I have swapped all 3 boxes various times, but in both cars I must press the "max" button (left) then back to "normal button" (2nd left), plus play with the blower high & auto to get air. These are endemic problems. Some here (John Dean?) have fixed the circuit boards.

My son was fussing about the 1984 300D having no heat (heater core bypassed then) when he had it in San Diego. I did give him an electric windshield defroster box. The same week, the Car Talk guys fielded a call from a lady in Santa Cruz w/ no heat and told her she didn't need a heater there, but reversed and said "maybe, but if you lived in San Diego ...". I think they didn't even put heaters in cars sold 20 miles away in Mexico.

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

Thanks for the response!
I did flush the heater core when I had the hoses off and it definitely was dirty for a while. I back flushed it until the rusty color turned clear. It took about a full minute. That's a clever idea about covering the cowl to check for outside air intrusion. I'll give that a try. I have not replaced the thermostat yet, so I'll double check my temp gage readings and see if I'm low.
Believe it or not we've actually gotten some frost out here on a few mornings this year, but mostly rain, so at least having a working defrost would be nice so I can see where I'm going. I'll report back my findings.

When I got home last night I covered the cowl with a thick moving blanket. Suddenly I had heat! It looks like there is a blend door issue letting outside air override that of the heater core. Now, which vacuum pod or actuator should I be looking for? Maybe for the time being I can shove foam under the cowl. I had never heard of this problem in all the heater related threads I'd read through. Thanks again.

Aux Pump is operated from the air conditioning console

Go to the service manual climate control section and at the beginning there is a procedure for checking every function of the system. This includes when the aux pump runs, what door is at what position, etc. Take the glove box out and the knee panel off the passenger side. Search for posts under my name. I think I put the paragraph number for the ops check in one recently.

I grabbed a few HVAC pods at PickNPull from a W123 recently since the dash was off and they were visible and easy. I recall one at the top acts longitudinally on the car's centerline to open a long door w/ horizontal axis running cross-wise (left-right). That door probably goes to the cowl vent. The pod was easy to take off and I recall it looked like one might be able to do so with the dash installed. You would probably have to remove the radio and push-button switch to get to it.

Behind the push-button are ~5 vacuum-electric solenoids that control all the pods. You can find pages from a 1983 manual here if you search that explains how each solenoid actuates in each climate mode. Allow for the translation to English. I recall the solenoids are called "code" 1 thru 5 or some strange term.

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

I have the same issue with my car... I have checked heater core flow and the circulation pump. I have also bypassed the mono valve. Unfortunately, it is -5 F here in New Hampshire, and I have 45 minute commute. Were you able to access the cowl vent actuator? Did that fix your problem?

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1980 300TD with 1985 300TD Turbo motor and 240D 4 speed.

Hopefully I'm not piling on, but...could it be that either the Climate Control Push Button Unit or the Temperature Regulator Relay that sits above (I believe) the glovebox are causing the issue? Reason I thought it could be either (in your case) is because of the age of your ride; likely neither of those have been replaced or refreshed electronically. If the electronics are sideways, you can press any of the Climate Control buttons all ya want, and function likely won't be "correct" for the button you press---aka outside air doors could be staying in an open position, etc.

Just a thought; perhaps others can chime in regarding.

You wont get hot air from centre vents. Read your manual .Its to keep the driver awake .Hot air will make you tired

Ditto on the Pushbutton Controller. Your first statement "the fan only blows from windshield and side defrost vents" indicate a problem with the Controller. This is the default mode if the ACC fails.

It likely has cracked solder joints inside the Controller and needs repaired. If your not handy with a solder iron, contact James Dean here on the forum. I've seen many favorable reports on his electronic repairs.

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Sam

84 300SD 350K+ miles ( Blue Belle )

Agree with SDBlue on the work JamesDean does; fixed my cruise control box. He also works on the Climate Control boxes as well. If it were me, and you can a)afford it and b)drive something else until you get working boxes back from JamesDean, I think it would well be worth the investment. Sure, your problem could lie elsewhere, but even if so, those two refreshed items will likely never have to be touched again while you own it. (see his link below---I didn't realize there are 3 control boxes, not just 2)

https://sites.google.com/site/mbcruisecontrolrepair/home/other-services-repairs/climate-control-systems