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1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
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9 Posts
Discussion Starter #1
I just recently acquired a 1989 420 sel with 80737 miles on it. Odo broke when I first fueled it up and hit the trip reset button...sigh:crybaby2:...

The major problem I am having is no heat! Rolling the climate controller to red max and I can hear a buzz in front of the dash board. Drove the car for 5 miles and it only blows cold air. AC is working fine.

I just drove 1200 miles with it on a round trip to Disney World from Alabama. No heat (41 F on the way back today).

Opening the hood and "shock!!" COOLANT splatter marks right behind the fan. I had to add more coolant to get the warning light off; I have not located the actual point of leak yet.

I am more concerned abut the heating issue.

I've read a lot of threads. Heater core problem? Climate control problem? "Monovalve?" problem? Removing the dash!!?? (I am not mechanically inclined enough to tackle more than simple plug and play items.)

I've checked and changed almost all of the fuses (the vehicle IS a 21 year old car.)
:surrender: I am completely lost on what to do next.

Should I change the heater core? Is it the rectangular thing about 8 x 1.5inch right behind the middle of the firewall?

Help Help Help!!
 

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1984 500SEL
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91 Posts
mono valve you can check easy. Should be 4 screws (on top, easy to see / get to) and is plugged in with a 3 pronged wire (plugs right out). The valve just comes right up. You can see whether the rubber is worn out and if the valve closes right or not. The rings sometimes get bent/rusted and the valve does not open or close which lets cold air in. However, you should still be getting some warm air in unless the valve is completely busted...
 

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Registered
1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
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9 Posts
Discussion Starter #3
@Oprastaj,

Thanks for the quick reply. Is this the rectangular thing that's right behind the middle of the firewall?

I'm going to see if this can be removed when sunlight hits tomorrow, it's 34F outside now in Alabama~

Will keep thing updated.
 

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1984 500SEL
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91 Posts
Okay, tried to take pics, you're pretty spot on. Sorry about quality, hopefully someone can give you diagram since it's 1 AM and 30 degrees in Chicago and this is best I could do with phone picture...
 

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1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
Joined
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9 Posts
Discussion Starter #6
YUP!!

I had to remove the battery and the diagnostic portal to release the mono valve. 4 screws removed and "bam!" The mono valve came up.

The diaphragm was broken. I am ordering a new mono valve, it should arrive within a week.

Will keep all updated. Hopefully, this would fix the problem!!
 

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Registered
1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
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9 Posts
Discussion Starter #7
In the mean time, I am redoing the hood foam. The old one turns to ash when air blows on it.
What would be a good "glue" to use?

The driver front door has a high pitched whistling noise when I hit 70mph. I suspect the weather seal/rubber needs to be replaced. Upon inspection, it looked normal with normal pliability of the rubber; is this a common problem? I don't remember this noise some 20 yrs ago when my parents owned a 420 sel.

Man, you guys are great!!!
 

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1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
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9 Posts
Discussion Starter #8
I've just received the mono valve.
Took the car to Firestone for a look at the coolant leak problem...and *shock* $1490 worth of labor and parts to replace the water pump and its associated parts.....

Is this a fair price, or should I just go to the dealer and have it done with OEM parts??
 

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1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
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9 Posts
Discussion Starter #10
@capescore: Thanks. Enterpirse, AL is about 4 hours from me...sigh.
Called around, all about the same price with MB dealer costing an extra $400 = $1850.
I may just let the dealer fix them.

About the heating problem; mono valve replaced. Still no heat. What to do next?
 

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32 Posts
You "might" have electrical problems

Was the diaphram on the old valve ripped?

I've replaced one valve so far and the old one was ripped so I knew there was a problem with it. You "might" have electrical problems.....

Does your air come out of the vents that the control is set to?

Regards,

Dan
 

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1989 alpine white 420 sel; 2002 silver ml 500; 2008 subaru wrx sti; 1957 porsche 356 speedster (kit)
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9 Posts
Discussion Starter #12
Was the diaphram on the old valve ripped?

I've replaced one valve so far and the old one was ripped so I knew there was a problem with it. You "might" have electrical problems.....

Does your air come out of the vents that the control is set to?

Regards,

Dan
The rubber on the mono valve was torn. I've installed a new mono valve.
Air that comes out is outside ambient air; climate control set to max "hot" and still blows ambient air.
 

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Had the same issue with mine when I bought it. The issue was the "porcupine" resistor that controls the system. Pulled one from a junker, and had it installed for $200.
 

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hi there ,
I have a 1990 mercedes 300se
-Putting the heat on max : hot air come out. IF i lower the temp cool air come out but putting back to max hot ,it will not come back hot.
Sometimes leaving it on max hot,
Hot air come out normaly but sometimes wont heat anymore just cool air comming out
I have to shut down the car restart it and hot air is back. So far i have changed the thermostat, water pump, mono valvle . What else you it be ?
Thank you
 

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Outstanding Contributor
350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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3,250 Posts
Where did you get the monovalve? Your issues sound like a bad monovalve. Search this forum for "monovalve" and see thousands of similar posts.
 

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Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,516 Posts
If you installed an aftermarket monovalve insert such as MTC, URO brand.. They do not work. The classic symptom would be loss of heat at road/highway speeds and OK when restarted, then lose again when the tach gets over 1500RPM or so.
Some fellow members have had positive results changing out the MTC diaphragm onto the original MB insert & reassembling.

I just looked the part number up again on an MB site and I got the 'Part Discontinued' message.

The factory BMW insert is plug n play.

MBL
 

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Moderator
1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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4,516 Posts
It is junk

If you still have tho previous part, you can try to salvage the MTC diaphragm and replace the torn one on the MB part...

M.
 

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Outstanding Contributor
350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
Joined
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3,250 Posts
MTC ...
Dooest it mean is no good?
Yup, junk! If you still have the old part, remove the snap ring that holds the screen/diaphragm thingy in and swap that on to your old spring. that could very well fix your problem. the issue seems to be that the spring in the aftermarket part is too weak, and the higher water flow rates at highway speeds push the valve shut.

If you search, you'll find lots of explanation, and also a discussion of how to swap the parts around.

I have no manners... ;)
 
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