Max heat no matter what button is pressed. I was pointed in the direction of the monovalve by this site.

OK I removed the monovalve and the rubber insert looks good as far as I can tell.

I checked the power coming into it. There are 2 leads. In Defrost mode both leads had power, while in all other modes only one lead had power. This is with temp wheel max cold and max hot. I am going to drive around today with the power disconnected just to see.

Anyone know what these readings point to? Are they normal signals sent from pushbutton unit? I dont want to replace the monovalve unless necessary ($145 for monovalve, no repair kit or insert listed).

Its for a 87 300E.

The mono valve should have power from fuse at all times & pushbutton sends ground. The mono valve must be energized to shut coolant flow off. When you saw only 1 wire with power, the other side must have been grounded. Remove the 4 screws & w/engine not running watch insert move in & out by switching a/c on & off.

I am guessing it is the monvalve coil, one poster repair his by finding that one of the coil wires had broken.

To check if the coil is working, ground to the engine the pin on the monovalve that you saw did NOT have the voltage, then connect lead. This should cause the monovalve to close and allow you continued summer coolness.

If valve is working, witht he pusbutton on EC, move the temp control to MIN, the vlave should be closed. The push button contols closes the circuit to ground. Moving the temp controller off MIN, the grounding will be still trhough the push button controller but is controlled by the "temperature controller" located behind the glove compartment. Setting the temp control to MAX leaves the circuti fully open.

Note, the Haynes manual says that you must rem,ove the centrol console to access the push button control. It is suffecient to remove the radio, then pry off the wooden panelling ....

Michael

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82 300TD Turbodiesal +250, 000
86 Volvo 740 GLE 220000
85 Honda Civic 120000

A repair kit is available for about $35.

Good luck,


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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

OK I feel as if Im getting closer, thanx again guys.

Larry, the dealer said there was no repair kit or insert for the 87 300E, I would just have to pony up the $145.

MBDoc, I took the monovalve off and watched it. When Defrost on the valve pulled in, when A/C, half A/C, economy, or Off pushed in the valve pushed out. This sounds like the pushbutton unit and the monovalve are both good, right? So what next?

I also read that maybe my compressor wasnt coming on, maybe due to low refrigerant. I know there are 2 wires I must cross on the drier to check this. But I have no idea where the compressor or drier are. Im assuming the compressor is one of the pulleys on the belt system, so I can figure that out. But where is the drier so I can see if its low refrigerant?

Help!! I think I am almost there.

Rick, the wires are on the low pressure switch which is located on the top of the receiver/dryer on the right front end of the engine compartment just under the forward edge of the ABS unit. There are two spade clips encased in a black rubber insulating material. The spades on the switch are pointing upward. Start it up, select max A/C, remove the two leads from the switch and connect them together. If the compressor comes on then you are either low on R-12 or your switch is bad. Hopes this helps..

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE

[This message has been edited by jeffsr (edited 08-17-2000).]

Jeff, I found the connectors, and the compressor. There is a window there, which the Haynes manual says to watch for fluid while idling, as you reconnect the connectors, no fluid with that.

I will try your method next, but how do I know if the compressor is engaged? Will I feel cold air? Or do I need to hook up a voltmeter to the compressor somewhere?

Thanks all again.

OK Jeff, bear with me. I idled the car, put it on max A/C, and connected those connectors. I heard a distinct whirring of the compressor. I assume this means it works?

So my next step is to have it checked for leaks I assume? Which switch may be bad that you referred to? Can I test it, or is it cheap enough that I can just pop another one in?

I am almost positive there is low, or no refrigerant in the system. At least the monovalve and pushbutton assembly seem to be OK, as well as the compressor.

You will hear a distinct clicking sound as the clutch engages. From the side of the car look beyond the receiver/dryer below the P/S pump and you will see the compressor pulley. Watch the front section of the clutch to see if it starts to turn. You should be able to hear it engage though..

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE

You got one in before I replied . You have found your problem. You will need to get the system checked for leaks. You're suffering from low refrigerant pressure, hence the low pressure switch cutting off the compressor. See, that wasn't too bad. BTW the W124 is noted for leaking evaporator cores. So have your tech start there and work his way back to the engine as these are sometimes the most difficult leaks to pinpoint.

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE

Back to heat, very common for mono valve to hydro-lock. Remove insert again & pull diaphram off of the lip on metal piston. Move the piston back & forth, if water comes out you need a new insert.

MBDOC is it possible to purchase just the insert or repair kit for a 87 300E? My local dealership said they only listed a new monovalve, at a steep $145. Does partshop list either of these?

Also I force grounded the monovalve, by grounding the connector to the engine. It made no change. Im assuming, based on all my tests, that it is indeed a problem with low refrigerant.

I can have the MB part number for the insert tomorrow afternoon if no one comes through for you sooner.

thanks leitz, and a price also would be super.

Looks like my next stop is a shop to get it vacuum checked, or to do a trial run of a pound or 2 of R-12.

Rick, before you toss away $$$ on R-12. Have the shop pull a vacuum on your system. This is the least expensive way to start. If it doesn't hold a vacuum, you haven't contaminated the atmosphere or watched you $$ float away to the stratosphere. If it does hold vacuum, then go with a partial charge leak test.

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Jeff Lawrence
1987 300e
1989 300e
2000 Dodge Grand Caravan SE