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Front seats no heat; Solenoid valve/pump or something else?
The rear seats get plenty of heat when asked, the front seats get marginal heat (and marginal AC?).
I replaced the duo valve pump thing with a used unit, so it's possible it's not good. I did test the solenoids though and they were working at the time.
What's throwing me off: the AC also operates marginally. It operates, but not how it should. The pump/freon/oil/expansion valve/dryer is all brand new. This suggests to me a common fault between the heater and the AC. Where to look?
Possibly the heater core/AC are gunked up and need cleaning for the front seats. Possibly there's an actuator that's not actuating. Possibly a combination of the above, or possibly the pump/duo valve/solenoid isn't working right for the front seats.
The fact that the rear has heat suggests that the duo valve pump is working. The fact that the AC doesn't get as cold as it should, and the heat doens't get as hot as it should suggests an actuator flap is stuck.
Are there schematics or similar for the HVAC function that I can find this flap?
[update]
I did some homework, following these instructions and came up with error code: FRA V ERROR 1426.
Did some diging, found this, he later links to https://m.facebook.com/story.php?story_fbid=1633016540301967&id=1632264070377214, that facebook translates thusly:
This might explain my AC behavior, but it's winter and the time to diagnose that is not now. No other codes that might explain the heating problems.
Last edited by Fried Chicken; Jan 15, 2024 at 09:58 PM.
Flap 5 here could be causing issues if it's not closing appropriately. Flap 4 I believe is the one you can control in the center. Germans apparently like being able to choose the type of air blowing on them, while maintaining a separate ambient cabin air temperature. On BMW you have the choice of hot or cold, on mercedes it seems it's neutral or cold.
The PDF confirms values 44 and 45 of the test are flap #5 on the diagram (there are two, one for the left, the other for the right). I will check in due time.
The PDF has informaiton on the error code I got:
"B1426 Circulation Pump (A31/1m1)", with no further details on that.
Maybe this is the duo valve pump. This wouldn't explain why the heat is working in the rear... maybe it's a solenoid thing?
Thinking about this further. I still don't have an adequate explanation for why the front seats and defrost vents get cold air and the rear get hot air, as I believe the car is not equipped with a rear AC unit. It does have four zones of climate control, but I don't think it has a separate rear HVAC unit. But now I could be wrong, which would also have implications for the AC, as the freon was filled for the car not having that option code. How to verify?
I believe that on some models, there is a circulation pump available that continues to pump coolant through the system to enable residual heating with the engine off.
I'm not sure why that would cause the front heating not to work, but at least it might be a clarification for the code you're having. (https://w220.wiki/Coolant_Circulation_Pump)
I'd suggest to read out the system using DAS and verifying if resetting the system makes the AC operative again
I think it might be the solenoid for the front heater core, since the rear is working, and assuming one solenoid is for the front, one for the rear. I ran the tests for the flap valve actuations, and they all seemed to move (thank god), so I can rule that out.
Barring that, it's possible the heater core and AC evaporator are so dirty/clogged up they don't transfer heat/cool properly. This would explain the poor heat and poor AC functionality.
I think it's the pump. It has a part #:
220-835-00-64
I suspect the car also has the separate rear AC system, which means the freon is probably low as well, since two values were given "with and without rear AC", I went safe and went without. Probably it needs another 100 grams or so.
Look under the trunk floor mat and see if the option codes are listed on the sticker there. If so, find the option code for the rear air conditioning system.
If there are rear air controls in the back of the console, you have the option.
It is possible to disassemble and refurbish the duovalve and pumps. There is a post on BenzWorld with detailed instructions.
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