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#1
12-16-2014, 10:19 AM
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Monovalve and ACC info/ troubleshooting thread
To start, I shot a video of a light bulb connected across the Monovalve to show its duty cycle under 3 different temp dial settings. This is my 85 300D where the climate control system is fully functional per descriptions in the owners manual. This is an easy temporary test to set up, however there are risks that is preventable.
Video was taken after the coolant gauge went to normal operating temp. Note the change in duty cycle of the bulb (on time vs off time). Bulb on = no coolant flow Bulb off = coolant flow With temp dial turned to cooler setting (more blue showing vs red), you can see the bulb on time increasing and the off time decreasing. If you rig this test up and it does not behave similarly, it may point to problems in the climate control unit, the push button unit, the temp sensor etc. Click to play video For the bulb, I used fine stranded wire and wrapped/ twisted it around the monovalve terminals then taped the connections to keep them from shorting to each other, then put connector back on. Warning: Make this temporary connection WITH KEY OFF or you will blow a trace in the push button unit.  I will post more tests when I do them. If you have anything to contribute, please post them.
__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked 
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#2
02-19-2015, 01:23 PM
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This test was done on a used OE Mercedes monovalve that came out of a W123 with a working heating system. I have a 25 ft hose between the faucet and monovalve with a loose hose connection at the faucet end to bleed off water pressure (a cloth is wrapped around it so it doesn't spray water all over the place). This video shows normal operation of this monovalve with a certain water pressure (unknown at this time). Applying power = no flow/ removing power= flow. You can hear the pressure buildup when the flow was shut.
CLICK TO PLAY! In the video below, the faucet connection is tight (no bleed off), providing max pressure. As can be seen, the pressure (unknown but greater than in the previous video) is too high for the monovalve spring to overcome and latched itself shut when power is removed. If I release the pressure, it unlatches itself. If a pressure gauge is Tee-ed in, the latching pressure of the monovalve (spring) can be measured. I may do this up at some point but if you are up to it, please do so, take some videos and post here. CLICK TO PLAY
__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked
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#3
02-19-2015, 03:11 PM
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Interesting, a nice visual... I would have suggested using an LED instead of a light bulb as I would have been afraid of drawing too much current through the board...
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2004 F150 4.6L -My Daily 2007 Volvo XC70 -Wife's Daily 1998 Ford F150 -Rear ended 1989 J-spec 420SEL -passed onto its new keeper 1982 BMW 733i -fixed and traded for the 420SEL 2003 Volvo V70 5 Speed -scrapped 1997 E290 Turbo Diesel Wagon -traded for above 1992 BMW 525i -traded in 1990 Silver 300TE -hated the M103 1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one 1979 Silver 300D "The Silver Slug" -Sold
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#4
02-19-2015, 05:31 PM
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Quote:
__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025 83 300 D turbo 297K runs great. SOLD! 83 240D 4 spd manual- parted out then junked
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