AC not working when most needed - 1991 W126 - Mercedes-Benz Forum
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post #1 of 20 (permalink) Old 07-19-2015, 08:17 AM Thread Starter
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AC not working when most needed - 1991 W126

The AC will not switch on seemingly when it's very hot and/or humid outside. Are there any typical causes on the W126?

(The condenser was replaced recently, if it matters.)
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post #2 of 20 (permalink) Old 07-19-2015, 09:12 AM
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post #3 of 20 (permalink) Old 07-19-2015, 12:28 PM
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Originally Posted by carlos.l.sanchez View Post
The AC will not switch on seemingly when it's very hot and/or humid outside. Are there any typical causes on the W126?

(The condenser was replaced recently, if it matters.)
(thanks ColoRich)

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The way to help is here Mercedes-Benz Forum - Announcements in Forum : W126 S,SE,SEC,SEL,SD,SDL Class
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post #4 of 20 (permalink) Old 07-19-2015, 03:29 PM
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Quote:
Originally Posted by carlos.l.sanchez View Post
Are there any typical causes on the W126?
No, there aren't typical causes on the w126.
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post #5 of 20 (permalink) Old 07-19-2015, 03:39 PM
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Typical??? Low Freon!
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post #6 of 20 (permalink) Old 07-19-2015, 04:14 PM
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Originally Posted by Rebor View Post
No, there aren't typical causes on the w126.
Um, yes, there are some typical causes:

You said your condenser was replaced recently, can we assume a correct and full charge? And is it R12 or 134?

One 'Typical' problem after freon is the compressor, is it kicking on? Can you see the clutch engage? When it does kick in, if it kicks in, then do you see bubbles in the sight glass? (By the left hand headlight) See if it is bubbling, if it is, then you should be good on freon and the comp should be working.

The next 'Typical' AC problem is the vacuum pods which control the air direction flaps. What happens over time is that the rubber diaphragms in the pods that actuate the flaps deteriorates and cracks and loses the vacuum seal. When this happens the AC system goes to 'default' position which sends all air to the defrost vents to keep it clear. I wrote a whole thread on it with a DIY if you care to read it, search for my name and 'Finally Fixed' in the threads I've authored, If the pics are missing, and you want the service manuals as well, PM me your email and get your reading done and up to speed, and then I will help you with any additional questions you may have.

Another 'Typical' problem is the expansion valve. It goes, and the receiver/Dryer bottle should be replaced by now. The Exp Valve is a bitch to get to. I had my entire dash and the Bendix AC box out when I did mine, and it was easy then, but it's a bitch to get to with everything in place. Not that it's hard to get to per se, but the connections are a bitch to make without possible stripping of the threads for the pipes.

There is not any function of the A/C & Heating system I do not understand, nor can help you fix. It's an easy system to troubleshoot, but greasy and messy to fix, that's why tey charge so much at the Mechs. But if you have the aptitude and time, I can and will guide you through it.

I will also send you the complete manual sets digitally and a lot of other docs I've collected over the years. Look through my threads and you wll see many DIY's I've done. It speaks for itself.
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Last edited by mclare; 07-19-2015 at 04:23 PM.
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post #7 of 20 (permalink) Old 07-19-2015, 05:30 PM
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Carlos, Welcome to the Machine. Lets do a basic, does the ventilation system work at all, blower yes or no, if it does work that is one major point we can pass by. Then like what was mentioned does the compressor kick in with 3rd button pushed and temp wheel all the way in the blue (65 degrees)
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post #8 of 20 (permalink) Old 07-19-2015, 09:22 PM
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Oh I am jumping on this bandwagon. I just had to drive at 10pm at night what was 85-90degree weather with no a/c and I was drenched. I will lurk and follow this thread and check off the list you guys are posting of what it could be.
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post #9 of 20 (permalink) Old 07-20-2015, 06:18 AM
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post #10 of 20 (permalink) Old 07-24-2015, 07:18 PM Thread Starter
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Thanks for all the answers. I have been discussing with the dealer who replaced the condenser. They had noticed the air not coming from the center vents, only from the defroster. This seems to be your "vacuum pods" scenario, @mclare ?

They think that on the two extremely hot days we had earlier this week the air from the defroster was hitting the hot windshield, making cooling slow. But the problem is that I got no cooling at all, even after 1.5 hours of highway driving…

And on the less hot days we’ve had since, the center vents do open occasionally, making the cooling much more efficient.

They also say that “It can be fixed but would be extremely costly. Dash would have to be removed, not recommended, at that age once I start removing things that have not been removed since being built, very high likelihood of parts breaking by default."
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