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1994 S320swb
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Discussion Starter #1
Hi all from Oz!

In Oz we have a problem with accessing refrigerant gas 134a. So the recommended local "enviro friendly" replavement is 'HyChill Minus30'. This gas works just fine, however, it operates at a different temperature and pressure to that of 134a. So the recommendation is for the aux fans to start (first stage) as soon as the AC is switched on.

Can anybody tell me what the terminals on the pressure switch on the dryer are for (stage 1 or 2 etc) and what signal (voltage etc) they receive so that I can 'rearrange' the signal to suit:confused:
 

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It isn't that simple. You car doesn't use pressure switch to control fans. That device on the dryer is pressure sensor. It sends pressure signal to the push button controller and it turns fans on and off.
 

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Discussion Starter #3
Part of the answer

Thanks for the reply MYARMAR.

You have provided some of the information. All I need now is which connections on the pressure switch are for the first stage fans and if the switch is a simple on off switch or an analogue value (ie a variable resistance). If it is an analogue signal what is the value for switching?
 

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W220 S320 CDI 2005; W140 S280 1994 (Sold Nov. 2012)
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Thanks for the reply MYARMAR.

You have provided some of the information. All I need now is which connections on the pressure switch are for the first stage fans and if the switch is a simple on off switch or an analogue value (ie a variable resistance). If it is an analogue signal what is the value for switching?
Have a look at this link to Jim Forgione's site: K6JRF Auto Page

It gives details of the switching pressures which I have also seen published in some M-B documentation. However, the relay circuitry doesn't seem to agree with the Mitchell data and I am never quite sure whether there are 3 stages or just 2. But this is the best info I have. The switching is definitely done by the a/c control unit, based on an analogue value from the pressure sensor, which I guess - but don't know - is a strain-gauged diaphragm type device.

Mike
 

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W220 S320 CDI 2005; W140 S280 1994 (Sold Nov. 2012)
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Thanks for the reply MYARMAR.

You have provided some of the information. All I need now is which connections on the pressure switch are for the first stage fans and if the switch is a simple on off switch or an analogue value (ie a variable resistance). If it is an analogue signal what is the value for switching?
Here are the links to the Mitchell data cct diagrams of the a/c:

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1994/S320/SYSTEM WIRING DIAGRAMS/5062.pdf

http://www.autolib.diakom.ru/CAR/Mercedes-Benz/1994/S320/SYSTEM WIRING DIAGRAMS/5063.pdf

As you can see, the fan relay wiring in the second link doesn't agree with Jim Forgione's.

The pressure sensor shown in the first link has a 3-wire connection. From other data that I have, I think that the a/c controller provides 4.75 to 5.25 V across pins 2 and 1 on the sensor: this is the dc energisation for the sensor. The sensor ouput voltage to the controller is between pins 2 and 1 on the sensor. The output voltage should be:

2 bar: 0.5 to 0.75 V
10 bar: 1.4 to 1.8 V
18.7 bar: 2.4 to 2.8 V
29.3 bar: 3.5 to 4.0 V

I can't guarantee this data, so you would need to do some measurements - unless somebody else comes up with solid info.

What I certainly don't know is what type of sensor is used. The above suggests to me that it is a full-integrated pressure sender (strain gauge bridge or variable capacitance) so that you won't be able to get away with parallel or series resistors to simulate a higher pressure.

You could obviously arrange for the stage 1 fan speed to be activated when the a/c is on by modifying the wiring to the fan relays but I don't see how you can get the other stage(s) switched at lower than normal values of pressure.

Mike
 

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Discussion Starter #6
Thanks MikeJT, the info is just what I need. I can now modify the circuits to start the fans when the AC is switched on.:):thumbsup:
 
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