Defrost vents PARTIALLY open on AC in w126 300SD

[ykobayashi]

I’m having a subtle problem in my SD.

My defrost flap stays partially open when I’m on MIN temp AC. So I’m getting half of my cold air blowing out my center vents and half blowing on the wind screen.

I have changed my center vent pod in the past (2010 ) and I thought it had developed a leak. I checked it out and it seals. It’s a double diaphragm and both are good. The solenoid valves are also good.

I pulled the glove box and watched the pod activate the defrost. I can get the defrost flap to close on EC. Fully. But on AC MIN temp or anywhere on the blue section of the temperature dial the defrost flap stays partially open.

I know the double diaphragm is designed to do this. The diaphragm has two positions by design. Fully closed and half closed. Looking at the FSM it looks like on heat it’ll stay half open. But on MIN temp it should fully close on AC and EC.

The fact I can get it to reliably close on EC means all the vacuum pods and solenoids are working properly. I just cannot figure out why I cannot get 100% cold air blowing out the center vents. It’s like the CCU is telling the solenoids to open the center flap but simultaneously open the defrost.

I have checked defrost pod. Checked the solenoids’s resistance. Checked the vacuum lines. I cleaned the temperature wheel contacts. I resoldered all the contacts in the PBU. No luck.

Tomorrow I’ll measure the voltages to the solenoids. Im thinking it’s doing as the CCU is commanding and not activating the full diaphragm on the pod and only doing the partial diaphragm. I will confirm this with a voltmeter tomorrow.

Really confusing. Im going to read my owners manual. Maybe the Gen 1 w126 just does this? I don’t remember my AC blowing so weak in front. I feel this just started happening.

I should be happy. It does work and the car is comfortable but I don’t like the anemic breeze blowing when I command max AC.

I’m about ready to hardwire the solenoid to fire with a switch. Likely my air temperature controller (silver box) is defective. I opened it and took a look and it is a complex analog circuit. I believe the half open full open control for the defrost flap originates in there.

Summer is here bring on the AC threads!