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#1
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a/c recirc switch/ how to keep on?
in my car i like to have the a/c air recirculation switch on because it keeps the car cooler, uses the a/c less and saves fuel.
problem is, the switch is actually a button and the recirculation turns off when you turn the car off, so you have to hit the switch every time you start the car. besides being annoying and a pain, the little picture on my recirc button is rubbing off! i was wondering if anyone knows a way to keep it turned on. ive heard of people jamming something in the flap that it moves so that it stays closed to the outside, but i dont really want to do this and i dont even know how if i did. i was wondering if it was possible to reverse the switch so that instead of starting out open, it starts out closed. maybe its possible to flip around some wiring to reverse the polarity to the servo that moves the flap. has anyone done anything like this? please tell me if you know anything about how this flap works. thanks
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1991 300 D 2.5 Turbo, 220k also in the family: 1981 240 D 185k 1991 350 SD 185k 2006 S 500 2005 SLK 350 |
#2
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No cure that I know of - sorry. It sends a signal to the ACC computer, I believe. You could manually force the recirc pods closed with some vacuum tube modifications behind the glovebox, but I wouldn't recommend that. I think you're stuck pushing the button all the time. Hey, it's winter now, so you're off the hook for another few months!
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#3
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Oh, and to keep this on topic, replacement switches are avilable from Performance Products. Try Fastlane first, but I don't think they carry them.
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Rick Miley 2014 Tesla Model S 2018 Tesla Model 3 2017 Nissan LEAF Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro Chain Elongation References |
#4
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I don't really think it's worth messing around with. The system has three modes - fresh air, 80% recirc, and 100% recirc. It automatically runs in 100% recirc mode if the interior of the car is warm enough. I forget the exact value, but in Texas it isn't a problem getting there. Most of the time it operates in 80% recirc mode. Only when the demand for cooling is minimal does it operate in fresh air mode.
The recirc switch forces the system into 100% recirc mode. Rotating the temperature dial to full cold also locks the system in 100% recirc mode. Based on my (admittedly limited) observations, it's common for the 80% recirc mode to fail (due to a ruptured vacuum diaphram), while the 100% mode still works. This is where my '87 124 is at. In this case manually switching to recirc mode makes a huge difference in how effective the a/c is. I would expect this difference to be amplified in Houston, where humidity removal is a key component of achieving comfort. You don't want to run in recirc mode all the time - you'll have problems with the windows fogging up in cool weather. Guess what I'm saying is live with it. If you're really concerned, have an MB-savvy mechanic check the vacuum diaphrams. It's not too difficult, just requires pulling the glove box to access the vacuum tubing/switchover valve and using a small hand vacuum pump (ala Mighty Vac). Replacing the vacuum elements is whole 'nother thing, but testing and diagnosis is quite easy. - JimY |
#5
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Good point. You may have failed vacuum pods and you're trying to force the recirc on, when the proper fix MAY be to replace the pods and make the system operate as MB intended. It's getting more and more common for the pods to fail as the cars get over 10 years old. It's easy to test the pods with a MityVac at the 7-port manifold behind the glovebox. Now, replacing the bad ones can be a tedious job, but first find out if you need to. Here's a photo of what you're looking for (dash removed for better visibility: )
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#6
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wow, thanks for all of the help
i had all of the vacuum elements checked out and replaced a couple years ago so i know the a/c works the way it should. i didnt know it automatically adjusts the amount of fresh air for what is needed, i figured it was always 100% fresh air unless you pushed the button. so i really dont need to use the button, ill still have 80 recirc air, which sounds pretty good to me. so now i know, and knowing is half the battle... thanks alot also for the great picture, ill have to save that one
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1991 300 D 2.5 Turbo, 220k also in the family: 1981 240 D 185k 1991 350 SD 185k 2006 S 500 2005 SLK 350 |
#7
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The recirculating function is automatically activated whenever the Air Conditioning is running. Its just that the light does not indicate it.
So the good news is that you don't need to do anything to achieve your wish. |
#8
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I can't find a W126 ACC wiring diagram that shows the recirc button so I don't know if the timer is in the button or in the temp control unit. I doubt it's in the button but if it is then convert the button to a simple on/off rocker. Sixto 95 S420 91 300SE 87 300SDL 83 300SD |
#9
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Thanks to a forum member poking my brain offline, I'm pretty sure the timing mechanism is built into the recirc switch. It might be possible to replace the recirc switch with a simple toggle so it stays on as long as the switch is on as well right from start up. I don't know if the temp control unit will miss the other connections. If so, maybe you can leave the recirc switch behind the console and splice in a simple toggle that sits in the console. Anyone know if the traction control switch is a simple toggle like the rear dome light switch?
Sixto 95 S420 91 300SE 87 300SDL 83 300SD |
#10
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None of the switches have timing circuitry built in - they're usually momentary contacts that trigger a relay and/or timing circuit elsewhere (ACC brain, etc). The dome light & hazards are normal rocker switches, most others are momentary (recirc, rear defog, traction control I think, etc). I replaced my fader in the console with an extra dome light switch to trigger my V-1 up above the mirror. Works great!
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#11
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Dave, as usual, is right. The attached image shows the wiring of the S24 switch and the wonderful quality of MB CD manuals. Seems like a lot of wires for a simple momentary switch. Such things are not my forte so I often have to eat crow. Pass the ketchup.
But the concept should work. Leave this switch behind the console and use another switch to send a switchable signal to the recirc switching valve downstream of the pushbutton unit. Anyone know how to wire it so there isn't a signal back to the pushbutton unit when when the override circuit is engaged? Sixto 95 S420 91 300SE 87 300SDL 83 300SD |
#12
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I think it would be tricky without knowing the circuitry of the control unit, particularly the output driver for the valve. I imagine it just switches 12v power to engage the switchover valve. But you have to know the "power supply" impedence of the control box. Some sort of diode arangment in the current switchover line then connect your switch between the valve and the diode might work and protect the control box. It would depend on how sensitive the circuitry is in the box. Even a diode will bleed some current in the reverse direction. Sounds like tinkering with an old control box would be in order first. The folks at General Development Labs might know too.
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D. Blake 86 300SDL 340K (for sale) 83 300SD 205K (gone) 06 Chevy K3500 LBZ CCLB 50K 94 Chevy K2500 6.5L TD 250K (gone) |
#13
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I forget all that SPDT business but I think what's called for is a switch that toggles between poles with a momentary break as opposed to an on-off switch. I don't think there's an MB switch with such function that'll match the W126 console switches but it might be a matter of creativity.
Sixto 95 S420 91 300SE 87 300SDL 83 300SD |
#14
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Quote:
I don't think it's possible to have the 80% diaphragm fail and still be able to get the 100% function. Is that correct?
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2012 E350 2006 Callaway SC560 |
#15
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I just replaced all the pods in my '87. Some of them were "partially" failed. They would work part way but not all the way. I think that you are right that all the diaphragms must be good for the pod to deliver 100%.
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"Buster" in the '95 Our all-Diesel family 1996 E300D (W210) . .338,000 miles Wife's car 2005 E320 CDI . . 113,000 miles My car Santa Rosa population 176,762 (2022) Total. . . . . . . . . . . . 627,762 "Oh lord won't you buy me a Mercedes Benz." -- Janis Joplin, October 1, 1970 |
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