Hello All,
Wondering if anybody out there has rebuilt/repaired the monovalve on a late E420 and wants to share the experience - local dealer wants $396.00 for a new one and it has to be ordered.
I removed the top (1 connector, 4 Torx 10 long screws and 1 heater hose clamp) and saw the deteriorated rubber seal/diaphragm. Solenoid tested good.
It's getting warm in Richmond. Any help will be greatly appreciated!

Hi JC, if you do a search on monovalves, you will find many hits. Unfortunately I don't think there is any magic repair on this one, despite many wishes for this - I don't think you can buy just the diaphragm or plunger with diaphragm. On older BMW's with basically the same monovalve type setup, you can buy just that piece fix things relatively cheaply.

Anyway, have a search and you may get some other inspiration for a bypass, etc. Or maybe I've overlooked an easy fix someone has found, it's been a long time since I looked at this.

Rgds,
Chris W.
'95 E300D, 391K

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Objects in closer are mirror than they appear.

Maybe this is what you are looking for:


How I fixed my duovalve (W210), got my heat back and saved myself $250!

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles

Just replaced the mono heater valve ($300 + for a MB OEM part). Started the engine and set the A/C to 65 degrees (it's 85 outside) and it worked as it should; cold air from all vents.
Turned ignition off, went inside the house, came back and started the engine and drove 4 blocks. Heat is again coming out of side vents Any suggestions?

ive got a mono valve from an 94 e320 wagon if you want to try and rebuild it, i replaced mine because heat was coming from sides when ac was on , the new one fixed that issue , i saved old part

Bit the bullet and bought a new valve. Problem solved!