1988 300 SEL - on a long drive last week, the temp in EC mode would either be all cold in the MIN position or jump to hot air once I moved the dial to the 18-22 range (coming from the center vents).

Now (a few days later after the trip) it has gone to all hot in any mode, all the time. It will cycle to the proper vent in the modes - as if the climate control is telling the air where to flow from, but it's always hot. For instance, if you crank the temp dial up to the hot range it will start kicking it out the floor vents - cold range will send it through the center vents.

Even if I cold start the car with the climate control in the off mode, it will flow through the center vents at full temp once the car has warmed up. If I turn off all the center vents it will just flow through the defrost vents at full temp (obviously).

Is this a sign of a bad monovalve, bad CCU, bad temp sensor? Please help! I am supposed to be leaving for a 10 hour trip tomorrow and this will be like driving in a sauna

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(2) 1988 300 SEL W126

I'll go for the monovalve. Check the control! One of the two wires should be battery voltage and the other should be ground when in the "min" position. If you are missing either you will get full hot. If you have both and still have heat, the valve is defective. Usually an insert will fix it. The insert is easy to replace, the valve not so easy.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

have you gone thru the standard checking of the freon gas level? sorry I could not totally understand the proceedure you have gone thru, there was too much reprtitions, Just check the freon vele first, then make sure when the aircon is on the compressor has also engaged, if not then the culprit could be the control switch, relays, or just the freon level ( if its the freon level, there might be a leak in the system).

Hi check your duo valve, Ihad same exact problem and that was the culpret

Here's what I've found so far - the air from the center vents in economy mode will run cool for a while and then kick over to full hot after running anywhere from 15 min to 1 hour.

From the climate control module there was a clickity click click click sound as you rolled the temp wheel. Sitting idle at a stop you could hear the clicking shorting sound and then it seemed to quit all together.

So on my long trip it was running cool for a while at first and I thought it was going to be ok... then it kicked into hot mode but luckily it was about 50 F outside so it was bearable with the windows cracked.

However, after many hours into the trip I noticed that the area around the temp wheel was ROASTING hot. I pulled the unit out today and the plastic cover of the climate control unit around the temp wheel side is melted and sunken in. The actual plastic frame around the climate wheel is melted and bent inwards towards the wheel so it rubs against it. What the heck!!

Apparently you can't re-solder these newer W126 (1986+) units like the older ones? I don't see how it would even begin to come apart like the DIY tutorial shows on the older units.

Does it sound like this could be a result of something else failing, or is just a total "meltdown" of the climate control by itself?

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(2) 1988 300 SEL W126

Check to be sure the auxilary water pump isn't shorted. I'd just disconnect it till you were sure.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

ok I will check the Aux pump next. Upon further inspection last night, I found that the clicking is actually a relay back in the console and NOT the climate control unit itself. It sounds like an erratic effort by the climate control to attain the proper temp setting but it becomes confused as you slowly roll the temp dial.

After pulling the unit apart and plugging it back in, the night time display lights are working on it once again on the temp wheel and pushbuttons... strange.

Are these newer 126's still wired up in such a fashion that the Aux pump can take out the CCU?

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(2) 1988 300 SEL W126

Checked the aux pump via the climate control wiring harness pins as shown on Programa's tech support page.

According to their specs:
Resistance should be greater than 6 Ohms. Mine is 4.6.
Current should be less than 1.5 amps. Mine is .75.

Can the resistance cause problems with the pushbutton unit, or should one only be worried about the current?

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(2) 1988 300 SEL W126

Are you expecting the ac to work in the economy mode?

My understanding of the system is that the ac will not come on with the economy button pushed.

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“Fundamentally, sustainable development is a notion of discipline. It means humanity must ensure that meeting present needs does not compromise the ability of future generations to meet their needs.”--- Gro Harlem Brundtland


1989 300se 250,000
1983 BMW 528e 100,000
1999 Olds Alero 80,000

These pretzels are making me thirsty.

No, I expect only outside vent air in the EC mode. But not full max hot from the center vents when I have MIN selected on the temp wheel. When I turned the AC on, it actually tried to override the hot air flowing in, and the center vents were a mix of furnace hot air + AC cool.

Today I stuck a new climate control in and it is modulating the temp just fine... but after a couple minutes of operation, it goes to max fan speed and you can't even turn it off, it just keeps blowing out of the defrost vents if you try. I stuck the old unit back in and the fan speed control was correct. So back to Programa this refurb goes.

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(2) 1988 300 SEL W126