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#1
01-25-2011, 07:25 AM
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COLD RIDE FOR DAYS BUT WHEN CAR IDLES IT'S HOT.. 1985 300SD
I have a 1985 300 SD. Been having trouble with heat in the vehicle. I topped up the radiator fluid thinking that was the problem, but for days I drive 40 miles to and from work... Not heat while driving but as soon as I get off the highway and drive at 20 or 25 mph at most, the heat or defrost gets warm/hot. Opened the hood and felt the radiator hoses. Top one was warm, bottom one was not really warm at all.... large hose coming off resivor tank was warm and fluid level in tank was normal.
Temp on gauge is 80 while driving. noticed it dip down a little when sitting in parking lot. Cabin temp was comfortable then. Last week, I did turn on the AC while driving home from Yoga class. The cabin temp was too hot and I decided to circulate the AC. Heard that it was good to turn it on maybe once a month to keep things from freezing up.... Was that the start of my troubles or coincidence? Do I have a thermostat problem or is it something worse?
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1985 300SD 
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#2
01-25-2011, 10:03 AM
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Searching here on the forum, I see some posts about fuses and also about the monovalve as well as ignition switch. Will check the fuses in the main fuse box at lunchtime....
If I turned on the AC on a cold evening, could that have blown a fuse and started the non heat problem?
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1985 300SD
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#3
01-25-2011, 10:07 AM
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Check your fan clutch. Staying normal at road speed and surging upwards when you slow down is classic fan clutch failure.
With the engine hot but NOT running, go out and spin the fan by hand. The pulley-driven one, not the electric one. It SHOULD stop spinning fairly soon (less than 1-2 revolutions) and have some resistance to it when you turn it. If it "free wheels" -- no turning resistance, and takes several spins to stop, then the fan clutch is shot. A/C is likely just a confounding factor with nothing to do with the problem. My $.02
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 ~Michael S.~ Past cars: 1986 300SDL 1987 300SDL 1982 240D 1982 300SD Current: 1987 300SDL
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#4
01-25-2011, 10:07 AM
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Probably a torn diaphragm in the monovalve. Pretty simply fix.
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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#5
01-25-2011, 10:32 AM
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Check your fan clutch. Staying normal at road speed and surging upwards when you slow down is classic fan clutch failure.
Hey, not sure what you mean by surging... The temp gauge does not go "up" when vehicle slows down, or is at rest. Only thing that happens is I start to get some heat... However, I will check the fan as mentioned above, and below.... With the engine hot but NOT running, go out and spin the fan by hand. The pulley-driven one, not the electric one. It SHOULD stop spinning fairly soon (less than 1-2 revolutions) and have some resistance to it when you turn it. If it "free wheels" -- no turning resistance, and takes several spins to stop, then the fan clutch is shot. Is there detailed directions on the diaphram change for the monovalve. Can I do it outdoors in the cold? Whats a safe way to override the monovalve if I want to wait for better conditions for the repair?
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1985 300SD
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#6
01-25-2011, 10:35 AM
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Check the aux water pump and the monovalve. Corrosion on the pump leads will keep it from moving coolant into the heater core. Likewise corrosion on the terminals of the monovalve will keep it closed, it could be pressure buildup from the engine heating up after you get off the freeway that opens the monovalve just enough to move some coolant into it.
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Closing the store Benzbonz.biz on your smart phone or tablet.
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#8
01-25-2011, 10:53 AM
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Not sure where it is on a 126SD but on a 123d it's a five minute repair that could be done in sub-zero temperatures if necessary. My impression is that the monovalve is between the two firewalls in a 126.
The diagnoses I have read on here in the past explain the situation in the reverse of what is described above. A tear will allow the monovalve to work at low pressures, sending hot coolant through the heater core. At highway speeds when the pressure increases, coolant is forced thru the tear inhibiting the proper function of the monovalve and reducing flow thru the heater core. Also, your symptoms are the reverse of a non-functioning auxilliary pump. When the pump fails, you get heat at highway speed but much less heat at idle.
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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#9
01-25-2011, 10:58 AM
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Quote:
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Closing the store Benzbonz.biz on your smart phone or tablet.
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#10
01-25-2011, 11:00 AM
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I agree typicall the pump help move coolant at idle for heat at lower RPMs. But coolant will flow without the pump at highway speeds but have no heat.
Thus monovalve is suspect. Now what happens if the monovalve is closed AND the pump tries to pump coolant through a closed monovalve?
__________________
Closing the store Benzbonz.biz on your smart phone or tablet.
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#11
01-25-2011, 11:17 AM
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look to the right of your battery. see the small square lid with the wire on it? that's the monovalve. camera phone quit just now, or I'd take a pic for ya.
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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread "as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do! My drivers: 1987 190D 2.5Turbo 1987 560SL convertible 1987 190D 2.5-5SPEED!!!  1987 300TD 2005 Dodge Sprinter 2500 158"WB 1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!
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#12
01-25-2011, 11:26 AM
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Thanks folks, in a half hour I will be looking under the hood....
Just to tell you whats happening , one more time. I get no heat in cabin while driving fast. As soon as I slow the car down or when it's sitting idle, I get all the heat I would ever want... reading through some of the input above, sound like some mixups. Maybe it's me, my excuse if it is, I am multitasking.......
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1985 300SD
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#14
01-25-2011, 11:59 AM
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I agree to look at the monovalve and aux water pump. Start with the monovalve (once you find it). If it's anything like the w123, it will be a little box on the passenger side firewall, and should have coolant/AC hoses running to it, and an electrical connection. Unplugging the electrical connection should make it default to full heat. If that doesn't solve your problem, look to the auxilary water pump. I would not be assuming a headgasket, since you aren't overheating or anything funny. This is all assuming your engine temp it up to spec (read: thermostat).
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1989 300E 144K
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#15
01-25-2011, 12:35 PM
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well, i looked under hood. found monovalve. drove about 3 miles to get lunch and back. sat in car while eating sandwich with engine running. top hose on radiator is hot. smaller hose that leads to bottom of radiator (comes off the thermostat housing is much much cooler. do not feel any heat from it....
does that mean the thermostat is not opening..... will unplug the electric wires from monovalve for the ride home....
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1985 300SD
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