Hi all. I have a 1989 420SEL with no heat. I just bought the car and here's what it does. Blower does work on all speeds, but sometimes I have heat when on DEF, and sometimes not. when i switch to EC, same issue. Now, I do have another 420SEL, an 87, and on that car, when I turn the car on, I can hear the vacuum elements "come to life" inside the dash. But I have not heard this on my new 1989. I'm thinking vacuum leak, but everything looks good as far as I can see. What would a defective climate control switch unit act like? what would a defective mono-valve act like? Any help would be greatly appreciated.

1. Drag out the old Mighty Vac and start testing the Vacuum Pods.
(you brought it up?)
2.Carefully take the screws out of the Top Cover of the Monovalve and remove it...You're looking for a torn or punctured rubber diaphragm.
You can check the two incoming wires to the monovalve...BUT it'll drive
a DMM (Digital Multimeter) crazy BECAUSE the Voltage is a fluctuating
signal (I.E. More "On" time than "Off" gives you a closed valve and no heat.)
3.The CCU (Climate Control Unit) may be Ill (Let's hope not!)
4.If the Auxiliary Water Pump is no good, when you replace it PUT A ONE AMP
FUSE INLINE between it and the CCU to protect the CCU.
5.An occluded Heater Core will prevent Heat to the passenger cabin.
6. How's the Engine's Thermostat? If it's "Locked Open" You'll freeze in
downtown Havana.


A Bad Monovalve always defaults to Wide Open Hot (So you will have DeFrost)
('Could it be only "Partially" bad?... I'm Clueless)

'Ave a look here,the first Pictogram shows an Auxiliary Water Pump #26:

Phil (at the top of the Pages,FASTLANE) stocks a Monovalve repair kit.
They also stock an Auxiliary Water Pump.

http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=126035_568&M=116.965&GA=722.324355&L=765.706&CT=F&cat=568&SID=83&SGR=180&SGN=05

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'84 300SD sold
124.128

Thanks a bunch! I'm planning on checking it our tomorrow.

My 560 SEL did something similar. Bad monovalve.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

I have replaced the insert several times and once it was not "full-heat" - it was blowing nothing but cold air.

Same thing happened on my 1990 420SEL- bad monovalve.

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"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

Same thing on my 380SEL, bad monovalve, but that did not fix it!! On to the MityVac!!

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered