300E Monovalve DIY instructions - PeachParts Mercedes-Benz Forum

#1 06-07-2003, 07:54 PM

Guys,
After doing an extensive search, I've come across tons of posts telling to "service' the monovalve for the 124 since it's said that a replacement is not made by mercedes. Only a kit is available from fastlane etc. Well that's all great. But, I havent found any detailed notes, perhaps pics, for a 300E, on where to find the monovalve and step by step instructions on how to remove / check it.
I've seen numerous posts detailing this for the W123 however. Would someone like to share their experience with us on this post on how they got the whole "monovalve" job done ?
I ask because I too have been told to check it and I'm looking for DIY instructions.
thanks.

#2 06-08-2003, 08:00 AM

The 124 cars have a completely different heater control valve than the earlier cars with the monovalve. The valve seems to be quite trouble free and from what I can see is not made to be repaired, it is a discard and replace item.

If your valve is causing excessive heat, the problem is probably related to the controls for the valve, not the valve itself.

There is a very good article in the DIY section on this site that explains this subsystem.

Have a great day,

stevebfl · #3 06-08-2003, 10:04 AM

To test the water control valve, one doesn't need or want to disaasemble it. What are you going to do then.

To test it one needs to evaluate its performance or by=pass it so to speek. Do you need more heat, pull the thing out and place a straight pipe in. Did you get more heat?

Do you want less heat? Block it!. THe easiest way is to clamp off either the inlet or the outlet hose with a clamp (I use a visegrip with an extra piece of hose material to protect the cars hose from the vicegrip teeth).

#4 06-08-2003, 10:11 AM

Steve,
the reason i'm trying to tackle the monovalve is, because I'm having problem with the cold air while the air conditioner is ON. The A/C works fine for about 15 minutes as if it is brand new and then starts blowing luke warm air. After posting the problem I was told that the monovalve could be the problem. That's the reason I'm asking this question.
thanks.

stevebfl · #5 06-08-2003, 10:56 AM

So. the hypothesis is that an open water valve is causing your problem. Could be, but unlikely, so lets not waste too much time or money. Do the visegrip test I described. Five minutes, most of it driving, proves or disproves the hypothesis.

This is really an exact science.

The valve is in the same pocket under the hood as the battery.

stevebfl · #6 06-08-2003, 11:01 AM

And remember that if you find that this does prove the hypothesis, you are only just starting. That valve is rather bulletproof, it is just as likely a failure in that area is a control problem than a valve problem,

But, first verify the hypothesis.

MRPerformance · #7 06-08-2003, 04:06 PM

[QUOTE]Originally posted by Benz300
The A/C works fine for about 15 minutes as if it is brand new and then starts blowing luke warm air.

Have you checked the refrigerant (R-12 or R-134)? Low refrigerant can cause the evaporator core to freeze up after a while, preventing air from passing through it. You sometimes notice some white ice fog from the air vents prior to the evaporator totally freezing up. Another symptom is reduced air output from the vents. It's at least worth a look since checking the refrigerant is really easy.

HTH,

///MikeR
'93 180E 2.6 Sportline

stevebfl · #8 06-08-2003, 04:28 PM

I would really like a thermodynamic explanation of that statement:
"Low refrigerant can cause the evaporator core to freeze up after a while".

Since the icing of an evaporator is controlled in most MBs by turning off the compressor electrically when the temp gets too low. It is pretty hard to imagine how any refrigerant or thermodynamic condition would cause icing; especially one that causes reduced cooling. Thats sort of like saying you got frostbite from standing too close to the fire.

I might be willing to believe it but I had real trouble believing the thermodynamics of propane fired refrigerators also. They do work even if I don't believe it. I passed the course but still didn't believe (bg).

#9 06-08-2003, 04:56 PM

MRPerformance,
I just had the shop check for the refrigerant level and they said it's full since i had it filled up last year. I asked them to check the A/C system and they said they couldn't find the problem. I've taken it to the mercedes dealer as well as private shops and none of them seem to figure this problem out. However searching the posts I've noticed that quite a few w124 owners have had this problem.
Any other ideas ?
thanks for everyone's help here.

MRPerformance · #10 06-08-2003, 05:24 PM

Steve,

I don't recall exactly what effect causes evaporator icing but I can assure you it does happen. I have experienced it a number of times-- including on my old Euro 126 (and even my vintage Coke machine). I'll try to remember to ask my HVAC tech friend why it happens. Considering your statement that the 124 has a coil temp switch I'm guessing it doesn't happen in those cars.

///MikeR
'93 190 2.6 Sportline

jsrmb · #11 06-08-2003, 08:06 PM

I had similar symptoms on my '83 300D. The problem was the fan on the AC radiator. It was frozen. Now that the electric fan works so does the AC.

On the '83 removal of the monovalve put the heat on full. Not good except winter in Canada.

MRPerformance · #12 06-10-2003, 06:31 AM

[QUOTE]Originally posted by stevebfl
[B]I would really like a thermodynamic explanation of that statement:
"Low refrigerant can cause the evaporator core to freeze up after a while".

Steve,

My HVAC savvy buddy confirmed my suspicion that the reason low refrigerant causes evaporator coils to freeze is simply low suction side pressure. As he showed me both on manifold gauges and a refrigerant spec sheet, anything below 30.1 psi (in the case of R-12) means evaporator coil temps fall below freezing and thusly begin to freeze the water condensation formed on the outside of the coils. This eventually causes the airflow to diminish and the output air temp to rise (since air is no longer passing over the evaporator core).

Obviously there comes a point where the amount of refrigerant is no longer sufficient to absorb enough heat to allow the coils to freeze but it is certainly possible (and even likely) for an evaporator without a low-temp shut off switch to freeze up due to a somewhat low refrigerant charge. Practical experience tells me this usually happens when a typical R-12 based car system is low by 1/2-3/4 of a pound.

I hope this answers your question.

Regards,

///MikeR
'93 190 2.6 Sportline

stevebfl · #13 06-10-2003, 07:40 AM

While that might happen in home A/C units it sure doesn't in cars.

There is a difference in whether the low side pressure is a result of freon boiling or the suction capabilities of the compressor. The logic of the pressure argument must place the temp near absolute zero when a restricted expansion valve leaves a vacuum on the low side.

While there might be a very small area which conforms to the boiling temp analysis the total cooling is obviously down and the whole matter is occuring inside a large heat exchanger. Heat would quickly move to that localized cooling and to keep any area below 32deg F the rest of the heat exchanger (evaporator) would be within a few degrees (in other words - very cold). AND if that happened it would cycle.

MRPerformance · #14 06-11-2003, 02:53 AM

Steve,

I've been repairing European cars for over 20 years and I can assure you without any doubt that it does indeed happen in cars. It doesn't matter where the pressure drop comes from, if the low side of an R-12 system falls below 30.1 psi and the compressor is not disengaged the evaporator WILL freeze up, assuming there is sufficient refrigerant to absorb the heat in the evaporator.

A quick search of the bulletin board at www.aircondition.com reveals 4 pages of posts/replies regarding "evaporator freezing". Several replies mention low refrigerant charge as the likely cause. I'm sure I could find more specific references in the tech manuals at the shop.

It's no skin off my nose if you don't believe me but this is a well documented phenomenon. I've personally witnessed it many times in my own cars as well as customers' cars.

Regards,

///MikeR
'93 190E 2.6 Sportline

Techie9 · #15 06-11-2003, 12:24 PM

Regarding your original question, I see we are both still chasing that elusive DIY for monovalve replacement. I haven't found it either. Maybe after this weekend I can help. I bought a monovalve from FastLane and now know what one looks like. My son and I will attempt a swap this weekend and try to take and post a few pictures.

On my 1988 260E (which apparently is really the same as a 300E) the monovalve is located under the hood on the passenger side within inches of the battery. It's a small inverted canister over two inches high and over an inch across (from memory but not measured). It has two hoses connected to it, and an electrical connection on top. The whole unit seems to be held in place by two rubber pointy things (I think that's the right technical term). I haven't figured out how to remove or unsnap those yet. I'm sure it's very simple once you know how to do it. Also, I don't know if I have to clamp off the hoses to reduce massive fluid loss of if I can just disconnect them. We'll see. If anyone has input...what about the two rubber pointy things and the hoses?

Thanks,

Techie9