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2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #1
Just when the car is driving its best and looking great, something fails out of the blue...

Last evening, while driving my family to Mecklenburg Gardens for dinner, the cold air stopped coming out of the vents and hot air blasted my wife and I in the face. WTF...? Luckily, we were passing by my office, so I stopped there to check things out.

No blown fuses, all climate control buttons totally unresponsive. Heat, hot heat, coming out of the defrost vents at speed only.

We made to the restaurant and back home only with all windows down and sunroof back. I can't drive it like this with rain in the forecast.

From what I've read previous posts, the climate control unit seems to be at fault. If it could be something else, please tell me - I love this to be an easy fix! Who is a good source for rebuilt units? It's $950 at the dealership :eek:
 

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CH4S Admin , Outstanding Contributor
1985 500SEC, 1991 190E 2.6.
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40,145 Posts
I would check the local wrecking yards for a Gen II (after85) unit. Might be a good time to check the mono valve as well.
Have you tried a forum search?
Cheers
 

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Premium Member
2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #3
I would check the local wrecking yards for a Gen II (after85) unit. Might be a good time to check the mono valve as well.
Have you tried a forum search?
Cheers
Yes, I did a forum search and checked the components that were suggested, with the exception of the monovalve. I don't suppose that a failure of the large Klima relay under the hood would have this kind of effect?
 

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1990 420SEL, smoke silver/tobacco
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31 Posts
when the cc unit fails, it defaults to full defost 'for safety". so I'm told. probably a failed p-b panel?
 

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Two '87 570SECs, one '87 560SEL
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You very likely have a blown monovalve.

Order yourself a monovalve (or get one from the local Stealership for this one time) and then check yours. Having one on hand is a good thing since they fail reasonably often.

With the engine cold, pull the electrical connection from the top of the monovalve, then take out the four little standard screws that hold it in place. Thats all there is to it. (Don't drop one of the little screws - use a magnetic screwdriver!!) You don't have to mess with coolant level and you won't loose any coolant doing this.
 

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Premium Member
2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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508 Posts
Discussion Starter #7
You very likely have a blown monovalve.

Order yourself a monovalve (or get one from the local Stealership for this one time) and then check yours. Having one on hand is a good thing since they fail reasonably often.

With the engine cold, pull the electrical connection from the top of the monovalve, then take out the four little standard screws that hold it in place. Thats all there is to it. (Don't drop one of the little screws - use a magnetic screwdriver!!) You don't have to mess with coolant level and you won't loose any coolant doing this.
I really hope it's the monovalve. Would a failing monovalve cause the entire climate control to go dead? I'll call the dealer tomorrow to order one. I'm doing the "MB parts or nothin'" thing right now, just to say it is a complete Mercedes. The dealer gives me a break on most things so that the cost isn't much more than other brands.
These things always happen on the weekend when the parts counter is closed!
 

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In my experience a bad monovalve will cause you to be stuck with heat only or cooling only. It sounds like you are stuck in heat only, so that may be it. But...do you still get the heat with the pushbutton unit pushed OFF? Like Mike suggests, getting a new monovalve insert is probably good insurance anyhow. Its basically an inexpensive rubber diaphragm that goes bad/tears over time. And like he said, easy enough to switch out. If that doesn't work and you decide to get another pb unit, be careful. Just went through this myself. NOT ANY genII unit will work! If you have an 89, I think you need an 89-91 unit. They ARE different!!! I had an 86 unit in my 89 and the a/c wouldn't work, but heat would. Switched units and everything was fine. Who knew? The electrical diagram shows differences at least in 86 and 87 not sure about 88. Of course, the 89 pb unit is working just fine in my 86, so maybe they are backward compatible, just not forward!

Good luck.
 

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Premium Member
2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #9
The climate control panel is totally unresponsive to all input. No fan, no air moving through the dash or floor vents. Only get hot, hot air coming out of the defrost vents while driving. Compressor does not engage.

There was no warning sign that this may happen. Every button worked perfectly, all vents worked, A/C was cold and the heat was hot. The unit was very responsive to input from the temp wheel. Then in a mere second, it quit and heat started boiling out of the defrost vents.

Like all Mercedes systems, the ACC is complicated.
 

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Two '87 570SECs, one '87 560SEL
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Check the fan fuse strip in the little "coffin" forward and just inboard of the brake booster. It's on the back of the front (second) firewall.

Gently push it down in case it's cracked and holding it's position. Gently.
 

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2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #11
Check the fan fuse strip in the little "coffin" forward and just inboard of the brake booster. It's on the back of the front (second) firewall.

Gently push it down in case it's cracked and holding it's position. Gently.
It is very easily bent, but not cracked as best I can tell.
 

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2001 M-B E320 (W210)
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493 Posts
I have a problem with very little heat on my '87 420SEL, so based on some forum member suggestions, I ordered a monovalve repair kit from Autohaus Arizona for $38.86. While I don't know if that will cure my lack of heat problem, maybe it's the solution to your problem. Here's a couple of informational links:

Monovalve Repair Article #1

Monovalve Parts and Repair Kit

I also have a link to a second DIY momovalve repair but can not post the link in this forum; let me know if you want the web address.
 

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2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #13
Thanks, Richard

I will get a repair kit tomorrow and give it a go.
 

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Premium Member
2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #14
Check the fan fuse strip in the little "coffin" forward and just inboard of the brake booster. It's on the back of the front (second) firewall.

Gently push it down in case it's cracked and holding it's position. Gently.
Well, now that I've looked at it in direct sunlight, the 30A strip fuse is cracked under both retaining screws. For PM I'll replace that too.
 

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1990 SEC
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4,142 Posts
If a new monovalve and fuse fail to solve your problem, contact George Murphy for a replacement push-button panel.

I got a refurbished unit from him about a year and a half ago for $250, which is a lot better than the almost $1,000 the dealer quoted you. :)

George refurbishes old ACC units, so when you buy one from him, you'll send him your old core.

He'll tell you to verify that the aux water pump and the monovalve function properly. The reason is that they both ground through the ACC unit and if either has a short, you'll fry your new ACC unit when you hook it up. :(
 

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2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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Discussion Starter #17
Axel

Thanks as always for being there with advise! I'll need to check the aux water pump this evening. Is the failure rate for that as high as a monovalve and should I go ahead and get one?
 

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1990 SEC
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4,142 Posts
Thanks as always for being there with advise! I'll need to check the aux water pump this evening. Is the failure rate for that as high as a monovalve and should I go ahead and get one?
As far as I know, the aux water pump holds up quite well and there is no need to replace it as a maintenance item.

You want to make sure there isn't an internal short in the pump motor. Apply 12V directly and make sure it works properly.
 

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Premium Member
2004 S500 4Matic, 2018 Ford Expedition Max, 2017 Jaguar F-Pace S, 2016 BMW 750i
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508 Posts
Discussion Starter #19
Problem solved!

Replacing the monovalve cured the sick patient. I still don't see how it would cause the panel to go dead, but hey, it's now working.

Also replaced the 30A blower strip fuse. The old one had the consistency of dead soft tin foil - it was very delicate and could be torn. The new one is stiff.

An aside observance while driving to work today is that the engine ran cooler than before. Right at 82C rather than 90C. Don't know how a monovalve would have an effect on that. I'm wondering if there was an air pocket in the monovalve housing.

Thanks to everyone for the advice!
 

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1982 280 se aut. ecc. - sold
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Thanks for reporting back.
This might help someone else in the future.
Glad it was an easy repair!

gr J
 
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