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.

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.