I have been looking for a thread that fixes this issue but I haven't been able to find it yet.

It's summer and the heat that is coming out of the defrost vents in my 1987 300td wagon is getting a little too hot.

The PBU has been replaced and works but there is always a bit of hot air that blows out of the defrost vents whenever the car is moving. This happens even if the PBU is set to off.

I think it must be the Mono Valve. Last fall I took the mono valve apart and cleaned it, it was making a ticking sound whenever I would engage the PBU so it must have been working.

What other options do I have? Can I just block the flow of coolant? Any other problems that could be causing this?

My A/C does not work and has been disconnected since I purchased the car.

Thanks for your advice

The monovalve is not closing. With the PBU set to off, the defrost flap is open and the monovalve should be closed.

If you don't need heat, the cheap way out is to install a valve and stop coolant flow through the heater core. If you want to fix it properly, diagnose if you have 12V at the monovalve. If you do, time for a new cartridge. If you don't, time to troubleshoot the PBU.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I put two different monovalve rebuild kits into my car. Both were the MTC brand since OEM is either hard to find or way pricey. I found the klima design works kit online and put it in. I was seriously happy with it.

https://www.klimakit.com/product/monovalve-and-coolant-circulator-pump-eliminator-upgrade-kit/

* I am not affiliated with this company. Just surprised I haven't seen it or something similar on the forums much.