w126 1988 300sel

I'm getting a lot of heat thru the vents while driving around with the heater and AC turned off.

The heat coming out the vents only gets worse the longer you drive.

When turning on the AC the heat sort of diminishes but still seems to be affecting the AC performance (blowing cold air).
It's almost like I'm getting both heat and cold air out of the vents at the same time.

The last shop (MB Independent repair shop) I took it to said that it was the Heater Control Valve. He pretty much explained to me that it was a small easy to get to part that is not seating properly. He said it was located next to the battery.

He also said that such valve was not worth buying at the dealer because it was so expensive for such a small part. He continued on to tell me that he would be on the look out for a used valve and explained that all we needed was a valve diagram/seal that was not deformed. He said that when the valves are not used for long periods of time they deform and the seal conforms to a permanent shape that then does not allow it to seat properly when you get it to work again.

Well its been a year and the shop repair owner still has not found a good valve with a good seat, so he says.

This is a back up car and the MB shop owner knows that so i think if it was up to him he will wait another year.

I can't really find any reliable or trustworthy MB shop that knows how to work on HVAC for w126's.

Does anyone know a good shop in southern CA or does anyone here have experience with this symptom?

I replaced the front shocks yesterday and when I finished I went to look for the heater control valve. Looks like an easy fix if thats the real problem.

Any advice is greatly appreciated.

go to the dealer and buy the valve-no other solution to the all heat problem. good luck, chuck.

Does your valve look like this?

Pelican Parts - European Automotive Parts and Accessories - Porsche • BMW • Mercedes • Volkswagen • Audi • Saab • Volvo • MINI

If you don't need heat, you could always install a shut-off valve on the line going into the heater valve.

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1993 190E 2.3
2001 SLK230
1971 LS5 (454) Corvette Convertible

The monovalve is bad or the CCU isn't sending voltage to the monovalve coil. It's as easy as that.

Start with the monovalve cartridge. Buy OEM only, Aftermarket will leave you disappointed. The OEM part is less than $40 and commonly fails. If the cooling system isn't maintained, it'll rust and seize in place. If $40, 4 screws, and 30 minutes of your time is too much to ask, drive with the windows down.

__________________
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!)

Removed it a few minutes ago. Rusty deposit and gunk observed on what looks like a screen.

Just ordered the Genuine MB part from Pelican.

Will see how it goes

Thanks

Rusty gunk is enough to make it "stick". Mine would randomly operate, then get stuck on full heat (not nice). New valve cartridge has worked fine since replacing it.

Make sure the coolant is nice and clean before you install the new cartridge. If it's rusty or in poor shape, use the opportunity to flush and refill the cooling system. It'll be appreciated by your engine and your climate control system.

__________________
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!)

Will do

Apparently, there are shims on the valve cartridge/plunger that need to be positioned or oriented correctly when replacing it.

In the following link, there is mention that the shims were marked when it was replaced.

Is this information accurate?

W126 Monovalve Photos - Heater Problems HVAC Mercedes-Benz - Auto-Trend Repairs and Service Information Forums

There aren't any shims, at least there weren't in mine or in the kit I received from Mercedes. There is a wavy washer and a large washer though. Put the cartridge in the hole, drop the wavy washer over the spindle, then the large washer on top of that. Chances are good that the new cartridge will come with them already fitted (mine did).

__________________
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!)

Yup, came with wavy washer.

Just going to swap and install.