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  #1  
Old 04-16-2004, 11:23 PM
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Location: Sarasota, FL
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heater coming on while a/c running

I got my system filled with R-12 a little while ago, then resoldered the ACCU so the blower would work, and it worked great until the other day. Then it started blowing heat randomly.
I figured it was the compressor cutting off due to no pressure, so I had it checked for leaks. Mechanic said there were no leaks, and that the heater is probably cutting on. He said it's probably an electrical problem, probably something to do with the monovalve not being grounded. It's late now, and I'm gonna check that tomorrow with a voltmeter, but I wanted to see if you guys agree with him, and also what else it could be.

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  #2  
Old 04-17-2004, 04:08 AM
Benzcrusher's Avatar
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Check you connection at the monovalve and check your fuses...
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  #3  
Old 04-17-2004, 06:05 AM
LarryBible
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The monovalve usually fails and stays failed until rebuilt. If it is intermittent it could be the temp sensor. I'm not familiar with the temp sensor in the 126 chassis, but make sure that the plumbing that feeds air to it is intact.

Good luck,
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  #4  
Old 04-18-2004, 02:18 AM
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I still haven't gotten a chance to look at that ground my mechanic was talking about, but another thing I noticed is that the air blows harder that it used to when the a/c worked. That makes me think the mechanic was right, because i'm probably getting air from both the compressor and the heater.

Since the mechanic looked at it, it hasn't been blowing hot, but it hasn't been blowing cold either. It's just luke warm, maybe a little colder than the outside temperature. I know that the way you get like medium temp coming out of the vent is that the heater vents intermittently come on and mix the hot and cool air. Is it possible that mine are stuck in that setting?
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Old 04-18-2004, 08:48 AM
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Quote:
Originally posted by jivadent
I still haven't gotten a chance to look at that ground my mechanic was talking about, but another thing I noticed is that the air blows harder that it used to when the a/c worked. That makes me think the mechanic was right, because i'm probably getting air from both the compressor and the heater.

SNIP
????
The Compressor doesn't compress air, only refrigerant.
So any additional airflow may be the result of a higher voltage getting to the blower motor, either due to better earth, or less resistance in the 12v feed from the switch .
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  #6  
Old 04-18-2004, 09:39 AM
LarryBible
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AGAIN! Before you go any further, CHECK THE PLUMBING TO THE TEMP SENSOR!

Good luck,
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  #7  
Old 04-18-2004, 09:42 AM
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Does your horn work? If not, check fuse #14...no power to the horn, no power to the monovalve.
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  #8  
Old 04-19-2004, 04:51 AM
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Leo, The horn works.

Larry, what do you mean the plumbing to the temperature sensor? And just to be sure, you mean the sensor that's at the front of the roof, right?
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  #9  
Old 04-19-2004, 08:21 AM
LarryBible
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I'm sorry that I'm not intimately familiar with the 126. On the 124, that grill at the roof is not wear the sensor is. The sensor is down low and there is a fan that draws air through that grill. To check it you can put some smoke at the grill with the ignition is on and make sure the smoke is drawn through the grill. If so, the plumbing is intact and the fan is working.

Maybe a 126 efficienado can chime in and tell us if the 126 is the same way, I expect that it is. If it is and you're not drawing smoke you need to fix that. It will cause very eratic climate control operation as you describe.

Good luck,
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  #10  
Old 04-20-2004, 08:30 PM
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Larry,

I checked the Benz manual (I have the CD), and it say that that's temperature sensor at the top. There's something else, apparently behind the radio, called a temperature sensor for heat exchanger. Is that what you meant?

Also, I finally checked what my mechanic told me to, which was the 2-point plug on the monovalve. He said that once the key is in, one pin should be 12V and the other should be ground. He suspected the ground one wouldn't be ground, and that's what would cause the heater to come on. When I checked it though, both of them were grounded. I have no idea what that means.
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'91 350SD 162k
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'84 300SD 210k - totalled
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  #11  
Old 04-22-2004, 03:46 PM
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I read somewhere that if the heater comes on while you're at low speeds, and then it stops when you get up around near 60mph, that your problem is the monovalve. So I went out and turned the A/C on, and it was coming out hot as it had been, and the air wasn't coming in through the center vents, just the sides and the top. I get on the interstate, and it stops coming out so hot (not cold, but not hot either), and the air starts coming out of the center vents and quits coming out of the top.

So I think, "ok, it's the monovalve". I take it out, and sure enough, that rubber ring (the big one, not the small one), is cracked. So I get a new one install it, and IT DOES THE SAME EXACT THING.

So now I'm really at a loss. The A/C was working excellently two weeks ago. It got cold for a couple of nights, so I turned on the heater (for the first time since I got the A/C fixed), and ever since then I'm having this problem. I'm not sure if my turning on of the heater prior to this was a coincidence or not, but I find it very unlikely for the monovalve to have been damaged, AND for whatever is causing the heater to still not work properly to happen -- for both of them to have happened at once.

One last thing. Previously when I checked the 2-point plug on the monovalve, I was stupid and just checked it without the wires plugged in. So now I took the cap off, left the wires plugged in, and checked the voltages: the red wire is a little above 12V, and the white wire is 0.3V, with the A/C off. With it on, it's at 1V. I'm not sure if that's a high enough voltage to do anything weird.

If you guys have any suggestions on all this, I'd love some.

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'91 350SD 162k
'83 300SD 210k - rear-ended and scrapped
'84 300SD 210k - totalled
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