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Aux water pump, monovalve 86 SDL
I've just got this 86 SDL back on the road and having a slight problem finding one of the problems.
I have replaced the CCU and also the klima, the A/C system is fully charged and working, however, it appears that the monovalve is full open. Now, when we installed this engine, we did the acid wash and thoroughly flushed the system, then rebuilt the monovalve, and everything seemed fine for a short while. Then came the heat. At idle, (with the A/C not working), the heat would be like a blast furnace. When moving down the road, it was not quite as hot, but still hot. When the A/C was hooked back up, the heat would equal or slightly best the cooling temperature. The blower motor seems to work at various speeds, and the aux heater pump feels like it is working. A clamp on the heater hose lets the A/C blow really cold air. Suggestions where to start?
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87 300SDL - 215K Miles !! 99 F-350CC Dually PSD - 190K 86 300SDL - 189K All on B-100 |
#2
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With the CCU set to maximum cooling, check the voltage at the monovalve. It should be close to 12 volts, which closes the valve so no hot coolant gets into the heater core. If voltage is present but you still get hot air, then the monovalve solenoid is hung up or the rubber diaphragm is torn.
If you don't have 12 volts across the monovalve then something is wrong with the CCU or one of its sensors. Jeremy
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"Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#3
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Replace the fuse.The fuse drives the monovalve and the transmission kickdown. I had to do mine as the kickdown shorted out and blew the fuse.
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"I have no convictions ... I blow with the wind, and the prevailing wind happens to be from Vichy" Current Monika '74 450 SL BrownHilda '79 280SL FoxyCleopatra '99 Chevy Suburban Scarlett 2014 Jeep Cherokee Krystal 2004 Volvo S60 Gone '74 Jeep CJ5 '97 Jeep ZJ Laredo Rudolf ‘86 300SDL Bruno '81 300SD Fritzi '84 BMW '92 Subaru '96 Impala SS '71 Buick GS conv '67 GTO conv '63 Corvair conv '57 Nomad |
#4
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That happened to my 300D also, but it was obvious because the same fuse powers the tach.
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#5
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Quote:
Looks like the monovalve is going to be opened up. THX !!
__________________
87 300SDL - 215K Miles !! 99 F-350CC Dually PSD - 190K 86 300SDL - 189K All on B-100 |
#6
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Quote:
Nonetheless, if you are getting heat with the CCU set to max cooling and 12 volts on the monovalve then it is definitely the monovalve. Jeremy
__________________
"Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#7
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Disassembled the monovalve, cleaned it, flushed it, checked the diaphrams and reassembled. Tested it and it was operational. Also check the resistance across the terminals and that also checked out OK.
Started car again, and noticed that once again, hot air from the driver's side vent, BUT cold air from the passenger side vent. Also seemed line the center vents were not to normal flow, sooooo, now checking out the vent layout and vacuum hoses... Any more suggestions?
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87 300SDL - 215K Miles !! 99 F-350CC Dually PSD - 190K 86 300SDL - 189K All on B-100 |
#8
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Mercedes climate control systems have built-in schizophrenia: they are capable of "blowing hot and cold" at the same time. The intent is to run the a/c as a dehumidifier and then warm the air (if needed). To that end, the CCU and the flaps have a complex set of operating variables. I would guess that some of the flaps are not working correctly.
Your '86 SDL is in some ways similar to my '87 D; there should be a manifold behind the center-right of the dash with a lot of hoses running to the various vacuum pods that control the flaps. You can remove one line at a time from the manifold and test the pod with a Mityvac. Any pod that does not hold a vacuum is probably bad. Replacing pods is another problem, often requiring dash removal. If the pods all check out good, the other possibility is a bad CCU. This is easier to get at than the pods but more expensive to replace unless you can find one in your local junkyard. Jeremy
__________________
"Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
#9
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Thanks, but the CCU is a new reman. The last one that I got from them (in my other SDL, works flawlessly), but I will still keep in mind that even new can fail.
The vents seem to be a logical place to look but the confusing part is that at max cool, their is still heat, but the monovalve is suppose to be closed, thus not allowing hot coolant into the heater core, yet, I have heat.
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87 300SDL - 215K Miles !! 99 F-350CC Dually PSD - 190K 86 300SDL - 189K All on B-100 |
#10
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Quote:
Remember that the monovalve is fail-safe: with the power off, it opens and allows hot coolant through. This is so you will always be able to clear a fogged windshield. Anything that interrupts the 12 volts to the monovalve can result in unwanted heat. Check for dirty or misaligned connectors, broken wires, etc. Another experiment: remove the plug and connect 12 volts and ground directly from the battery to the two pins of the monovalve. I don't think that polarity matters but check and see. With a constant 12 volts on the monovalve, it should be closed and the heat should go away. Does it? Now remove the 12 volts. The heat should come back. Does it? Be careful not to leave the 12 volts directly on the monovalve for a long time (more than 15 minutes). I'm not sure it was designed to dissipate that much power (even though it has hot coolant going through it). The CCU produces a variable duty-cycle pulsating DC, which dissipates less energy. This is probably overkill but I don't want to suggest something that would damage your car. The only other thing I can think of is to ask whether your SDL can have different temps set for driver and passenger. My wife's '96 E300D can do that and it has a "duovalve" instead of a "monovalve." But it's a much later car. Jeremy
__________________
"Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
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