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Old 11-28-2007, 06:24 PM
bhatt's Avatar
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Join Date: Apr 2000
Location: Oakville, Ontario, Canada
Posts: 421

Wife complains that her 1990 300SL is acting funny with regards to heat.

The heat is fine, but when she slows down to idle, it starts to blow cold. Fan speed or air volume doesn't change significantly -- just the temperature of the air it is blowing gets colder and colder. Obviously the system still thinks it's blowing hot air!

Haven't driven it yet, but have done the following:
-Checked coolant level (Level is fine. Car has never lost or burned up coolant)
-Checked fuse #7 "Climate control/circulating water pump/etc" (Fine. Replaced just to be safe)

Any ideas on what to check next? Can it be the monovalve? Everything I have read says the torn monovalve problem is the OPPOSITE of this (blows hot all the time). Can it be stuck?

Not sure if this helps, but the heat wasn't used all summer and it only started to get cold recently. Got cold a month ago, but it wasn't freezing so the problem probably existed but wasn't noticed.

Heat and everything worked perfectly last winter and nothing has changed since.
'83 300SD 335,000km (207k) mi SOLD
'87 560SL 163,000km (101k mi) SOLD
'86 300SDL 356,000km (220k mi) SOLD
'92 500SEL 250,000km (155k mi) SOLD
'90 300SL 140,000km (87k mi) SOLD
'01 S430 260,000km (161k mi) SOLD
'03 SL500 167,000km (104k mi)
'07 GL320 CDI 4MATIC 289,000km (162k mi)
'07 S550 4MATIC 170,000km (105k mi)
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Old 11-28-2007, 06:26 PM
TheDon's Avatar
Open Discussion Moderator
Join Date: Oct 2005
Location: Windermere,FL
Posts: 13,206
not a pro on the gasser engines but if the cooling system has an auxiliary pump, to pump coolant when the engine is idling, it might be failing.

|1983 300D| OM617 239k Miles|Red|"Passion"|
|1982 300D Euro|OM617 NA| Thistle Green|"Granny Smith|
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Old 11-28-2007, 06:51 PM
halman2228's Avatar
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Join Date: Jul 2004
Location: Houston TX
Posts: 304
I believe The Don... probably correct. That engine has an aux water pump that keeps circulation up at idle.

Elsewhere in posts here, I think I recall some horror stories about the aux water pump frying the CCU when it goes bad.

"Dangerous with a wrench...and not smart enough to know it"

Stable (in order of acqusition):
'84 500SL, 280SEL (Euros)
'77 280SE. '90 560SEL
'79 450SEL 6.9,
'95 C280 (totalled by daughter 8/07)
'81 280SL (rescued) '88 300CE
'86 560SEC '95 C36
'01 CL55

DD's: '04 S500 4Matic (SWMBO)
'04 F150 Crew
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Old 11-28-2007, 07:37 PM
Registered User
Join Date: Jan 2002
Location: NY
Posts: 500
I had same problem last winter and it was a monovalve. The pump is a usual suspect, but it was working fine. You can feel if the pump motor spinning or not. The monovalve was sticking in closed position and the water pressure at idle was not enough to overcome the friction. At higher RPM the valve would work OK. The new one fixed the problem. You could also try to take it apart and clean armature and inside of the coil. But first check you aux. pump.
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