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  #1  
Old 08-10-2005, 07:13 AM
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No power to A/C low pressure switch, this is driving me craZy.

On the 85 300 D I have here is where I am at:

When I turn on the A/C at the climate control I have NO power at the 2 wires that go to the low pressure switch on the drier.

I have verified that my push button climate control unit is good. I swapped mine to another 85 that has working air and it works on his car.

I also swapped my Klima relay on the same car and it is good also.

I checked the micro cutout switch and it is not stuck or showing contact via multimeter when not pressed.

I disconnected the temp sensor in the head and it has not effect on the A/C.

Independent power to the compressor clutch activates it just fine.

What am I missing????

When the climate control unit calls for A/C is there some other relay or switch that it must pass through before it gets to the low pressure switch??
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1981 300D 147k
1998 VW Jetta Tdi 320k
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  #2  
Old 08-10-2005, 08:26 AM
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Question

Do you have power at the evaporator temp regulator?? located next to the expansion under the passenger under-dash panel.

The pushbutton sends power to the ETR & from there it goes to the low-pressure switch.
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  #3  
Old 08-10-2005, 08:31 AM
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Oh...I am not sure, I did not even know to check that!

I will look for it. You say its next to the expansion valve?
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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
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  #4  
Old 08-10-2005, 08:47 AM
LarryBible
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Diesel Giant,

Have you shorted the the connector pins at the low pressure switch located on the filter drier. Don't do it with the engine running. Do it with the key on, engine off and listen for the compressor clutch clicking.

Best of luck,
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  #5  
Old 08-10-2005, 01:10 PM
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I have the same issue on my W201. One day it just stopped working. I have no power at the switch at the Dryer! So shorting the two out no longer engages the compressor. Funny part it that is will work for a few seconds then does not until the next restart of the car. I have no idea of where to look now since there is so much that can affect it. I had high pressure according to a cheapo gauge on the low side line. Someone on another forum said it was blockage. I drained the system and then re-filled. But after a little bit it goes to high again, stays at normal for a few minutes then bam high pressure. I was thinking the expansion valve was shot, but would like some more info. So I am keeping my eye on this thread in hopes that I could have a similar problem. Was fine then all of a sudden one day it just stopped.
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  #6  
Old 08-10-2005, 02:33 PM
LarryBible
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Jamie,

Yours is a different situation than Diesel Giants. Since it is kicking in for a second or two, yours has a slipping belt for whatever reason. There is a sensor on the compressor that can tell if the belt slips. If it detects slippage it shuts it down until engine is started again.

Turn the compressor by hand and ensure that it is free. If it is free then start checking your belt for oil or your belt tension for looseness.

Good luck,
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  #7  
Old 08-10-2005, 05:14 PM
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My belt tension is just fine. Checked that already and the belt does not slip on the compressor pulley either. Could that indicate a bad speed sensor on the compressor?
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1986 190E 2.3 Black, Auto, Mods to come soon.....
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  #8  
Old 08-10-2005, 07:21 PM
LarryBible
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Jamie,

Yes, check it.

Good luck,
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  #9  
Old 08-10-2005, 09:58 PM
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Larry,

Yes I tried shorting the connectors at the low pressure switch and I have used a volt meter, but still no voltage.

I also checked the evaporator temp regulator and had only a little more than 4 volts there. I am assuming it should be 12 volts , not 4.
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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair
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  #10  
Old 08-11-2005, 08:00 AM
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Exclamation

Again, the A/C pushbutton must send the 12V signal to the ETR & thru that(above 40F) to the low pressure switch!
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  #11  
Old 08-11-2005, 08:44 AM
LarryBible
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Sorry I couldn't be more help here, but I expect that MBDoc is sending you down the right trail. He is VERY experienced and if he is suggesting that as the next troubleshooting step it is DEFINITELY worth pursuing.

Do you have a schematic? If not I have not looked in the electrical part of my electronic manual, but I fully expect that I do. I am on a dial up so it might be too large for me to send, but if you need it, PM me and I will try to email it to you.

Good luck,
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  #12  
Old 08-11-2005, 08:52 AM
LarryBible
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Wait a minute! I had a senior moment. I had it in my mind that you are working on a 124 car but when I went back and checked it is a 123. I don't have a schematic electronically that I can send you.

I have some schematics in the back of an old Haynes manual but no way to scan it or copy it. Maybe someone else can help or you can buy a Haynes manual. It is well worth the $18 or so that it costs.

Sorry,
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  #13  
Old 08-11-2005, 09:54 AM
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Well I guess the big question is why do I only have 4 volts in the wiring at the ETR?

I will try and find a Haynes manual per Larry's suggestion as I have no wiring schmatic.
__________________
1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair
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  #14  
Old 08-11-2005, 10:04 AM
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You don't have a schematic?? How do you expect to be able to troubleshoot the system without a wiring diagram??????? How do you know you are supposed to have a 12+ signal where you THINK you should be seeing one????

I don't know about the W123 but in the W124 chassis the signal used to tell the KLIMA relay to engage the compressor is a ground signal fed from the PBU to the KLIMA - there is no 12+ signal used to trip the compressor.
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  #15  
Old 08-11-2005, 12:39 PM
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Exclamation

The 1985 US VERSION 300DT has a unique A/C compressor control!

Please look at this post!! The A/C pushbutton sends 12V to the evaporator temp sensor Passes thru that ETS & to the low switch. IF you don't have power there(12V) then go backwards to the pushbutton control!
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190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
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Old 08-11-2005, 12:39 PM
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