Vehicle is a 84 W123 300D. It has all heat or all cool out of all the vents.

I have been trying to figure out why I am getting hot air out of my center vents when it is trying to lower the cabin temp. The climate control seems to be doing its part controlling the fan speeds and when the cabin temp gets above the setting the center vents start to blow air, except instead of being cool air it is hot.

If I turn the temp wheel all the way to cool and the switch clicks, the hot water supply is cut off and the air coming out of all the vents will cool off. If I turn the wheel back the air gets hot again.

The vacuum diagrams that I have seen do not show (or call out) the heater core and evaporator doors. I am trying to figure out what is going. I am thinking I can override the center vent controls so I can get cool or hot air out of the center vents. I have the hot, I just need the cool.

Anyone have any idea what is going on?

The coolant flow through the heater is not being stopped. Search for "monovalve" and you'll find lots of information on how to diagnose whats wrong with it.

The center vents are not designed to send heat, just cooling. A P.O. may have reconfigured some of the pod lines to keep them open. You can just spin them closed.
Your temp wheel seems to be working, at least to a degree.
Perhaps the CCU is beginning to fail.
And as FI mentioned, check the mono valve.

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83 SD

84 CD

I don't know if it's a matter year or model, but not all W123 centre vents spin open/closed - just the side defrost vents.

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Don't Chrome them; polish them

All four of mine did/do. It's nice, as you can reroute the vac lines to have heat blow out of there. I prefer to make that decision.

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83 SD

84 CD

I just finished exploring under the dash.

I found that the cabin air temp sensor air pump is missing, but it still seems to work. I cant find the power wires for the pump.

All the dash vents are supplied from the same air box down stream of the AC evaporator and I think behind the heater core. That explains why the air temp is the same regardless whether coming out of the center or outer vents.

I also found that it has the doors to select out side air or recalculate inside air like my 126 will, but there is not a switch to control it.

I agree that the mono valve is most likely the cause of the constant heat because there is not a fresh air source to supply cool air to lower the temp of the air being blown into the cabin. My 78 240D center vents are a separate fresh air source where as I can have the heat on (the outer vents) and have cool air coming out of the center vents. That is not an option on this vehicle.

Unless my car is an odd ball, having heat come out of the center vents is just a matter of turning on the valve to supply vacuum to the door actuator when the heat is on.

All my vents are closeable, which is nice.