I have done lots of research on the monovalve, and have determined that this is what was causing my intermittant heating problems.

At the beginning of the winter, the heat worked flawlessly, and always has. Then a couple of weeks ago, it would only work every other time or so I turned it on.

Now, it just blows regular outside temperature air. A/C still works fine.

After doing the research on the monovalve, I tested to be sure it was getting the right voltage, which it was. I cleaned the contacts and tryed it.. no go.

So, I just tryed unplugging it. Now, I thought that would allow the hot coolant to run through the heater core full blast and I would have heat. It would only heat up for the first maybe 5 seconds, then go cool again.

Could it be that the monovalve is partially stuck?

Its getting colder, and I would like very much not to waste money..

Should I disassemble the monovalve and see if the diaphragm is busted?

This is for my 85 sd with just over 200k miles on it.

Any and all help is appreciated

Thanks

Yes.

It just takes 5 minutes to pull the monovalve and check it. Could be stuck, could be a ripped diaphragm. If you find just a diaphragm, let me know where. I just bought the repair kit and redid mine. I thought the repair kit was pricey if you only need the diaphragm. But the dealer wanted $96 for the kit. Fastlane was much cheaper.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Hello Andy
Sorry to hear that Sady is sick.
Try reading this information.

ShopForum > Technical Information and Support > Diesel Discussion
Organized MB DIY links post: Climate Control:
http://www.peachparts.com/shopforum/showthread.php?t=135055

Special attention to thread:
Aux. Water Pump takes out CCU
http://www.peachparts.com/shopforum/showthread.php?p=1051832#post1051832

I do hope it is the mono valve, and not the aux pump toasted your CCU.

__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

I will try to find time sometime this weekend to check the monovalve. I have lots to do, but will report back when something happens, THANKS

The Monovalve failures that I've seen have been that the diaphragm failed causing it to stay open which would have the opposite effect.

What is the engine temp showing? If the thermostat fails it opens causing low engine temp, thus no heater.

Good luck,

I'm sure that's true in most cases. Back about 8 years ago, my monovalve failed in a different way. It would not open unless the engine was somewhat warmed up and then it was shut off. If it was restarted, the monovalve would cycle ONCE to full open, but as soon as the current supply was dropped it would shut! I never did figure out it was the monovalve, but actually PAID someone to find it for me.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Shortly after I bought my W123 the monovalve failed after being intermittant for a month......then I had ZERO heat.........made for a cold week adn a half before the Ebay monovalve repair kit showed up.....the diaphram was almost in two pieces.

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

my symtoms have always been all cold or all hot, in this case I would vote for monovalve faluire

__________________
Ron
2015 Porsche Cayman - Elizabeth
2011 Porsche Cayman - Bond,James Bond
Sadly MERCEDESLESS - ALways LOOKING !
99 E320 THE Queen Mary - SOLD
62 220b - Dolly - Finally my Finny! Sadly SOLD
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
19 BMW X5 - Heaven on Wheels
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
73 280 - W114 "Organspende" Rest in Peace
81 380 SL - Rest in Peace

I flatly don't believe this.

Engine temp is 90c

thermostat is new

the monovalve diaphragm is cracked.

I will order a new one tonight.

http://cgi.ebay.com/ebaymotors/Heater-Valve-Kit-Mercedes-Benz-380-420-560-SEL-SL-126_W0QQitemZ8027074045QQcategoryZ33548QQrdZ1QQcmdZViewItem

What is happening?

I have been very busy.

My sister just announced that she is getting married... NEXT WEEK.

We are going to drive to atlanta.

I have not yet ordered the parts. I have a bunch of other little things to get also.

Sorry for not updating.

Nothing like advance notice....

__________________
Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche