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  #1  
Old 07-18-2003, 10:33 AM
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Location: Michigan
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300 D Air

A week or so ago someone mentioned shutting off the flow to the heater core to check a no air problem.
I purchased an in line shut off valve to do so but where should i place it? will it affect the engine cooling system?

I also have some oil type leakage under the steering colum, it was on the cover above the brake pedal
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  #2  
Old 07-19-2003, 01:28 AM
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I once saw a 300D that had a manual shut off valve installed as a means of controlling temperature.

The valve was installed in place of the mono-valve.
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  #3  
Old 07-19-2003, 12:17 PM
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The simplest place is often the outlet hose from the motor. This is the hose that comes from the head on the drivers side about two thirds of the way back.

The hose has no water flow when the heater is off (when working correctly). Since this is the case on this car plugging the hose will not affect the cooling system. In a recent post someone mentioned that the car ran hotter with the hose plugged. This is logical as the heater core is a heat exchanger. I have driven many a failing cooling system to help by engaging the heater full bore and rolling down the windows.

The system on this car and most MBs does not expect the cooling capabilities of the heater core to be used when the heat isn't on.
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  #4  
Old 07-26-2003, 11:59 AM
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300 d air

Thanks for the tip. I finally found time to block the flow to the heater core via the hose described coming from the rear of the head, that seemed to do the trick, now what?
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  #5  
Old 07-26-2003, 12:03 PM
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Your oil leak is from the oil pressure line to the gauge. You will need to pull the cluster and see if you can find/resolve it. I would not let it go for too long because it will make a big mess. Do a search on instrument cluster. It just pushes out but there are a couple/few tricks to it. Pretty simple.
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  #6  
Old 07-26-2003, 02:03 PM
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If blocking the flow through the heater core fixed a no AC condition, either the monovalve or the temp controller is bad.

Monovalve can be checked by applying 12V across it from the battery -- must have no flow.

Or you can pull it out -- if the bellows are wrinkled or punctured, replace.

It it is good, and works from the battery, either the PBCU or the electronics for the temp are bad.

Peter
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1972 220D ?? miles
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1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
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  #7  
Old 07-26-2003, 05:46 PM
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84 300 d air

I just replaced the monovalve 2 weeks ago .....next step?
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  #8  
Old 07-26-2003, 11:13 PM
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Check for 12V at the monovalve wires with the AC on and set for max cold. If you get voltage, the solenoid is shot and you need or replace it (or the valve it binding). If no voltage, the PBCU is bad, replace (or take apart and look for a bad solder connection).

This happened to a friend of mine last year. Had to replace the PBCU.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #9  
Old 07-27-2003, 07:58 PM
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300 d air

I have 12 volt to the monovalve. so the temp sensor is bad?
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  #10  
Old 07-27-2003, 07:59 PM
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air

so where do I find the pcbu?
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  #11  
Old 07-27-2003, 10:47 PM
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No, if you have voltage at the monovalve, the PushButton Control Unit (PBCU) is fine, the solenoid is bad.

The monovalve is held closed by current flowing through the coil, it opens when no current is present. If it is opening with voltage (meaning you have water going through, and hot air coming out the vents), the solenoid is bad or the valve is sticking.

Make sure the spacers are correctly installed, the same way they came out, and that the coil is dry and clean. Sometimes when the monovalve diaphram leaks, you get corrosion in the coil chamber, so the armature can't move properly, so it stays open.

Check for continuity across the pins, too -- if you have an open circuit, you will need to replace the coil.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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