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  #1  
Old 05-28-2003, 03:13 AM
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91 300E AC: Why would this happen?

Today it was warm, 90+ degree. Turned on the AC and it was slow to cool down. Drove about 50 miles and it slowly cooled the car down. I then stopped for about 20 minutes, then started off again and the AC started cooling like it was brand new. Why?

What would cause this?

Haasman
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  #2  
Old 05-28-2003, 11:06 AM
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Our 190E did something similar the other day, but I noticed at the time that HEAT was coming out the driver's vent. I fiddled with various knobs - mostly to stop the heat, and a moment later it was cooling fine. So, maybe your compressor wasn't the problem either.

Steve
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  #3  
Old 05-28-2003, 11:06 AM
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That's a weird one. I suppose it's possible you had a partial blockage on the high side of the a/c system which limited the flow of liquid refrigerant to the expansion valve. I recently had this happend - the receiver/dryer clogged up. But I would never expect something like this to fix itself.

Most likely explanation is the heater valve (AKA monovalve) was leaking hot water through the heater core. The a/c and heater were fighting one another tooth and nail. When you shut the car down, the monovalve opens. When you restart the car, the monovalve closes again. Perhaps it got a good seating the second time?

The 124 monovalve is not rebuildable. However, it only takes a few minutes to disassemble and examine for any obvious problems. I was having some problems with sudden blasts of hot air on my 124. Taking apart the monovalve, staring at it for a while, and reassembling it seems to have cured the problem for now.

- JimY
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  #4  
Old 05-28-2003, 11:18 AM
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The monovalves on these cars are flaky as pie crust. I would agree with Jim Y. I too have had the same problem with my 126 car, even after replacing the diaphragm insert.
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  #5  
Old 05-28-2003, 11:31 AM
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Thanks for the responses.

It was weird. When I first started out, I figured time for an AC recharge. By the end of the drive I wasn't so sure.

Where is the mono valve located?

Haasman
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  #6  
Old 05-28-2003, 03:07 PM
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It's right in front of the battery, can't miss it. Has two coolant hoses attached - one from the fire wall, one from the front of the engine. Remove the four phillips head screws and you can disassemble the whole thing to examine it. Might take five minutes.

- JimY
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  #7  
Old 05-29-2003, 03:46 AM
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A Burst of Cold Air in the Winter Time

I had just the opposite happen around Christmas time, had the heater on and all of a sudden a blast of cold air, it lasted a couple of minutes and the heat came back. This happened only a couple of times and has been good ever since.....I thought it might have been the "Monovalve" sticking closed....A couple of weeks ago I was having trouble w/ my A/C, (luckily it was not related) so after looking at posting on the Forum I looked at the Monovalve, I took the 4 screws out and pulled the coil and plunger assembly off from the top...the plunger was kind of crusty (reminded me of a piston from a brake calipier after about 100k miles) and the plunger did not want to move up and down thru the coil freely so.....I cleaned it up w/ WD40 and a 3M scrubbie pad and reassembled the monovalve...I then turned the PBC unit to max heat and then took the electricial wire off the monovale and retouched the wire to the monovalve a couple times and I heard a "click" each time..(the monovalve was working)....It doesn't leak and appears to be working fine......
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  #8  
Old 05-29-2003, 08:39 AM
LarryBible
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There is a little fan that draws sample air through the grill that is near the rear view mirror. The fan is located in various places on different 124 models, but I think on yours it is behind and to the right of the glove box. It should run any time the ignition is on.

To test it, put smoke such as from a cigarette (yuk!) near the grill and see if it is drawn in. If not, search for the fan and fix or replace.

Good luck,
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  #9  
Old 05-29-2003, 10:32 AM
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Again the resources of th is Forum are remarkable. Thanks for the replies. I will follow your suggestions and report back the results.

By the way, what is the purpose of the Mono Valve?

Haasman
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  #10  
Old 05-29-2003, 10:56 AM
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The monovalve controls the flow of hot water through the heater core. Most modern cars always have hot water flowing through the heater core, and use a door to direct some or all of the air around (or through) the heater core. The 124 takes a different tact - it regulates the quantity of hot water admitted to the heater core, allowing the push button unit to control the temperature of the heater core. Is this better than other designs? I dunno, but it seems to work OK. Each have their problems.

When the monovalve becomes leaky or sticky, it may fail to completely close off the flow of hot water to the heater core when commanded to do so. Hence you get lukewarm to hot air conditioning. (The heater can completely overpower the a/c.)

- JimY
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  #11  
Old 05-29-2003, 11:31 AM
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JimY

Thanks for the explanation.

Could one simply disconnect the electrical connector to the Mono Valve as a short-term way to defeat hot water going into the heater core?

Haasman
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'91 300E-Went to Ex
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  #12  
Old 05-29-2003, 12:39 PM
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The water valves are powered closed so disconnecting them sends them to full heat.
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