Hi! New to the forums here and I'd like to preface my problem by saying that this is my second W126 and both have bee the best cars I've ever owned. That said, summer is coming to a close and I thought it would be beneficial to try and figure out why my heat doesn't work.

Mid last winter I had to replace my thermostat housing because it was cracked and I replaced my thermostat while in there. Shortly thereafter, my car started to have overheating problems and my heat started acting funky. I have since fixed the overheating (fan clutch) but I still have the heat problem.

It started with my car blowing ice cold air out of the passenger side vents and the air coming out of of all other vents becoming a little cooler. After I eventually did a coolant flush when it became a little warmer, I lost all heat. Not sure if they are related but they certainly seemed to co-occur.

My situation now is a little odd. When it's a little chilly in the mornings and I run my heat it semi works on low but is useless when the fan is set to high. If it gets too cold outside I lose heat altogether. Months ago I pulled my mono valve and it looked ok but the screen was blocked. I ordered an MTC replacement and it did not fit so I pulled a mono valve and magnet off of a 560SEL at the junkyard. This did not fix the problem.

I also pulled off its ccu, thinking that my solder joints might be the problem, and no change. If I unplug the valve, no change either. I get ice cold ac in this heat so I would like to think that the valve is functioning. With the ac on my center vents also randomly seem to blow air and not, maybe engine vacuum related but not sure.

Another problem that may or may not be related is that my washer fluid boils off and I believe its coolant routing is part of the same system.

Not sure where to go at this point...buy a new mono valve insert? Any help would be appreciated and thanks in advance!

Does your car have an auxiliary water pump? I'd try to verify somehow if there is flow through the heater plumbing.

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Current fleet
2006 E320 CDI
1992 300D - 5speed manual swapped

former members
1984 300D "Blues Mobile"

1978 300CD "El Toro"

I haven't temped the heater core plumbing but the in and out are very hot to the touch. It's possible that my aux pump is broken but that only assists heat at idle and wouldn't explain my loss of heat at all rpms.

What year and model 126 are you working on?

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Cruise Control not working? Send me PM or email (jamesdean59@gmail.com). I might be able to help out.
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(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

It's a 1990 300SE

Sounds like auxiliary water pump, or the 'brain' climate control selector switch assembly needs to be replaced. Neither one is a huge dollar deal. I replaced both of mine on m y W126.

In my W126 it did. I had zero heat at all RPMs when driving. It was the dead of winter in the teens for high temperatures when mine went out.

So my cousin is over tonight and he kicked the heat on and off while I had my hand on the aux pump. I could feel the pump kicking on and off when he did that but I'm not sure if it could still be blocked in some way?

When it comes to the climate control unit, I pulled one off of another W126 at the junkyard and no change but who knows if theat one is a functioning unit.

Doesn't that mean hot water is going through the core? If so then the issue would be on the air/ducting side?

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Yeah that's kind of where I'm leaning at this point but I just don't feel like troubleshooting that...haha. Shouldn't be too difficult to test the vacuum lines and pods. Hopefully I get some time to look into things this weekend in between moving and report back.