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

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

A repair kit is available for about $35.

Good luck,


------------------
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..

------------------
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..

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

You got one in before I replied :D. 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.

------------------
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.

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

I have heat only when the the fan is in LO mode, obviously don't care now, but in winter takes a while to get the dew off the windshield.

My tech says is isn't the monovalve, but more likely something electrical, around $250. Is that the push button unit? Why would it only work when fan in lo? weird? This is for the 300.

------------------
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)

Rick00,

I missed that this is a 124 car. Somehow, I originally thought you were working on a 123 with a late Climate Control, I guess because that's what I automatically think about when I hear the term monovalve.

I replaced the monovalve on my 124 car last year thinking that was the problem. A waste of money. When I explored further I found that the pushbutton unit was not turning it off. If you disconnect a properly working valve on the 124, it will continue to flow hot coolant.

There are several ways to determine if the problem is too much heat or not enough cool. One would be to somehow cut off water flow. I believe you could clamp the hose leading to the valve without permanently damaging it. If you clamp it off and you then get cool air, then the problem is the valve or the pushbutton control unit.

Also, is this HOT air coming out? Is it hot when you first start the engine when cold? If it takes a little bit for HOT air to come out it's probably flowing hot coolant and that's your problem.

It also could be a combination of the two problems.

BTW, if the problem turns out to be the pushbutton control unit, make sure you check the auxilliary pump for current draw. It should not exceed 1.3 Amps, if it does replace it, or it will blow your new pushbutton control unit.

Best of luck,

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

I am almost 100% positive at this point that the pushbutton unit and the monovalve are intact. Unscrewing the monovalve and observing it engage/disengage as I press A/C on and off makes me assume the monovalve and pb unit are OK.

Im not so sure about the compressor. Yes it turns on when the low pressure switch is jumped, but does this mean the compressor is ok? Not sure.

I do know that there is no refrigerant in the check window, and I have gone thru the check process of disconnecting the low pressure switch, starting and idling the car, then reconnecting and looking at the window for rising fluid.

So I am going to go with low refrigerant due to a system leak and try and get it vacuum tested at a shop. If it is a leak then I have a feeling I am screwed because the evaporator is the culprit so often, and I hear it is a ridiculous amount to replace.

Thanks all for the help.

The shop can probably do better adding some UV dye to the system and looking for a leak. This way they not only find out that there is a leak, but where it is.

Pulling a vacuum, then standing back to see if it holds the vacuum is only good for a gross leak check.

Good luck,

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

Rick, I checked with my p.m. today and there is no insert available for 124 and 201 chassis as the monovalve has a different design than 123,107, and 126 chassis.Sorry.

Rick, it is best to get the gross leaks ruled out before you start looking for the small, hard to find items. If it won't hold a vacuum, then there's no point in trying to get it to retain 2 or so pounds of R-12. My local A/C guru suggests blowing in some UV dye and adding R-12 until a low side pressure is readable on a gauge (about 5-10 PSI), bypasss low pressure switch (you've already done that). Watch to see if any pressure builds on the high side. (suction side will be low or negative). Continue adding R-12 until 50-60 PSI on high side with compressor off. (read as shut off engine). See if system maintains this level of pressure and start scanning for leaks with black light. If it got cold enough scan the condensate from the evaporator drains to see if the UV is coming from the evaporator.

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

Jeff, yeah I made the decision to hand it over to an A/C tech to have it leaked tested. Its just so hard to find a decent one. I went to one around the corner who said I had a big leak in my Thunderbird. I had it filled somewhere else and it was still cooling 100% 2 years later when I sold the car.

Leitz, thanks for checking. I had already called the dealer and was told it was the whole valve or nothing. Seems like the valve is not the problem and I will keep my old one until a tech tells me to change it.

Thanks all for shaving off my A/C labor bill a bit by letting me do some of the diagnostics myself.

For a 300E. Show us one and tell where to get it.

Stupid question
 

Is the brass plunger head still attached on your monovalve?
I never saw one and didnt know it was missing until I stuck my finger into the antifreeze in the housing, found it and then all of a sudden figured out how thing worked. It had corroded off.