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Old 06-01-2003, 04:29 AM
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A/C problem 260E

Hi Guys,
Once again, I'm here to seek the advice of experts !
Here's the situation, the a/c seems to work perfectly fine half of the time and then it just doesnt whenever it feels like it. I've had the freon checked, it's full. All the other parts have been checked out to be OK by the dealer. They didnt find anything wrong.
Here's what I've noticed. If I turn off the car completely and turn the car back on, the A/C starts to blow ice cold air. after a while it intermittently works or it doesnt. I do hear the compressor come on everytime so that's not a problem I think. but even when the compressor is running, it starts to send out luke warm air.
Any suggestions ?
it's an 87 260E.
Thanks again guys. !
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Old 06-01-2003, 09:28 AM
J.HIDALGO's Avatar
Registered User
Join Date: Jul 1999
Location: Jax, FL
Posts: 1,785
It sounds like the belt is slipping!

Did you wash the engine bay recently or any foreign material on the belt? (i.e. oil).
On the other hand, this part is not clear for me:
"If I turn off the car completely and turn the car back on, the A/C starts to blow ice cold air. after a while it intermittently works or it doesnt. I do hear the compressor come on everytime so that's not a problem I think. but even when the compressor is running, it starts to send out luke warm air."
Is it blowing cold air or luke warm air?
'86 300E
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Old 06-01-2003, 09:29 AM
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Join Date: Jun 2000
Location: Plano, TX
Posts: 2,476
If you're certain the compressor is running, then you probably have a leaky monovalve. The monovavle controls the flow of hot water into the heater core. If it leaks, you are in effect reheating your air conditioned air.

Some models have rebuild kits available for the monovalve, but this is not true on the 124. However, some folks have had success just dissassembling the valve, cleaning it up with a kitchen pad, and reassembling.

As for identification and location of the monovalve I refer you to the search feature of the bulletin board - this topic gets discussed to death every spring. Good luck.

- JimY
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Old 06-01-2003, 02:34 PM
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well from the problem it seems like that even compressor is working just fine but its getting hot and thats why after a while its getting cut off stop and i guess using it like that over and over again would harm the compressor
i checked myself in the search forum s but i still cant understand whats causing this
with me when the aux fans dont work and the car gets heated all of a sudden AC would stop working too
still trying to figure that one out myself
hope u overcome this issue soon let me knwo
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Old 06-02-2003, 07:02 AM
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There is a small fan that pulls sample air over the temperature sensor. The fan is located in various places on different 124 models, but it is probably behind and to the right of the glovebox. It should run any time the ignition key is on.

To test it, hold a cigarette(yuk!) or other source of smoke near the small grill near the rear view mirror. The smoke should be sucked into the grill when the ignition key is on.

If this fan is not working, you will have cooling at strange times.

Good luck,
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Old 06-02-2003, 06:13 PM
dpetryk's Avatar
Electrons can do anything
Join Date: Sep 2001
Location: Houston, Texas
Posts: 1,071
Its probably the expansion valve. They can get intermittantly clogged up with particles of dirt and debris that are shed by the compressor and hoses particularly in an older system. I have had two cars that did exactly the same thing as yours. It was the exapansion valve clogged by junk.
I got too many cars!! Insurance eats me alive. Dave

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Old 06-04-2003, 12:40 PM
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Join Date: Aug 2002
Posts: 19
Me too, now, how to?

Benz300 - I also have an '88 260E that just started to exhibit the exact same problem. AC works fine, then suddenly only blows warm humid air. If I turn the car off and restart, AC works great again (for awhile).

J.HIDALGO - The belt "seems" tight (to my untrained belt-tension-testing finger).

jcyuhn - As I remove and clean, (or replace for $92.09 from PartsShop/FastLane on this site), the monovalve, should I expect a massive loss of liquid under pressure or should I expect a trickle (maybe just the amount the monovalve itself holds - like about two shotglasses full)? Do I just remove the four screws and remove and clean the cylindrical aluminum colored top cover and its internal components, or do I remove the entire monovalve unit from the two attached hoses? How do I prevent or minimize the fluid loss if it's going to occur?

LarryBible - The small fan (which seems to be behind a slotted grill that's part of the forward overhead interior light) appears to slowly inhale second-hand smoke quite readily.

dpetryk - If it's the expansion valve, is it a difficult task to remove & replace? And is this step a "last resort"?

Everyone else - I bought this car last summer and have been "lurking" and learning on this forum ever since. All the expected problems have occurred and the suggested fixes have been right on target! Maybe now I have a small amount of experience and may actually be able to become a contributor. Thanks again to ALL!!!
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Old 06-04-2003, 02:08 PM
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Don't replace the expansion valve unless the high and low side pressures are telling you that it is bad. If it loses mercury and closes, the high side will be low and the low side will be lower or even into vacuum. If it is too open, you will have a high low side and a low high side because you don't have enough restriction between low and high side.

The heater valve on the 124 cars is not rebuildable like the 123 and 126 cars. It is, however, much more reliable. Look at Steve Brothertons DIY article to see how to test. You can get to it by clicking on DIY at the top of this page.

Good luck,
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Old 06-04-2003, 02:41 PM
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Join Date: Aug 2002
Posts: 19

Thanks for the additional info. I'll read the DIY article and decide my plan of action. I'm no mechanic but I'll attempt every simple repair and the difficult or "special equipment" jobs go to my local mechanic.
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Old 06-05-2003, 01:54 PM
Registered User
Join Date: Aug 2002
Posts: 19
To original poster Benz300


Any update on your '87 260E AC problem? My '88 260E didn't "act up" at all yesterday. AC was fine and outside temps were in the high 80s. Highway, city, and inbetween were all OK. I won't continue troubleshooting until the problem rears its ugly head again...but I have full confidence that it will return, probably very soon. Maybe together, meaning with the continuing help of experts on this forum, we'll get our seemingly identical problems fixed.

If anyone has any experience on that "fluid loss" question I had earlier regarding removing the monovalve, I'd appreciate any input.


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Old 06-07-2003, 07:20 PM
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If monovalve, were, to be the problem in this situation, would it only act up when using the air conditioner. my fresh air conrols work just fine when i'm on the economy mode and i dont feel any warm air being mixed up. I would just like to make sure before I induldge myself in this tedious DIY ! :-D
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