How to bypass Dryer switch on W140 A/C system?
 

How do I bypass the Pressure Switch on the Dryer, to activate the compressor so I can charge the system? Its a 3 pin switch variable resistor (which you can't jumper).

Anyone know how to active the compressor manually?

1992 400SE

You dry charge the system first and that will give the system enough refrigerant for switch cut-in for compressor activation.

That sw should be a pressure sw , not a thermistor [ temp sw]

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU24Y0TB6T4&year=1992&make=MB&model=400-SE-001&category=R&part=A%2FC+Pressure+Switch

At this thread it was said that it cannot be jumped:

http://www.peachparts.com/shopforum/tech-help/59565-pins-jumper-w140-c-charge.html?highlight=w140+compressor

How would I dry charge, after removing my vacuum pump, I attached a can of Freon through low-side and the pressure in my guages indicated 80psi, but the compressor didn't kick on. Everything worked fine before changing a cracked A/C line and dryer.

Warm up the cans. With a good vacuum, you should be able to get the entire required weight on board without running the compressor. Do it in the shade when the engine is cold.

If you have a vac on the system you and hooked up a can , it should have sucked that can dry in no time..I think you have not opened the charge valve all the way..I believe the pressure reading ou are seeing is CAN pressure.

As far as the drier sensor, I have not seen that type and only have schematic/info on the E class. ..if that is a variable resistor , it would be an electonic pressure sensor [ as opposed to a mechanical]..I am sure MB Doc knows , so that should be correct info.
You can tell for sure with ohm meter. but I am sure he is correct.

I don't think you have transferred any refrig from can to system..is can EMPTY ????????

http://i179.photobucket.com/albums/w...sureswitch.jpg

If you can't jumper the high pressure switch connector, could you activate the compressor clutch instead? Find the connector going to the clutch and supply the 12v. Just a thought. It's easy to jump the 2 prong switches.

Thanks everyone, I was able to put in 3.5 cans OR 42oz.=2.6lbs. as specified. I watched the sight glass go from White/Foggy to Clear w/o Bubbles. I hope this charge holds. A/C is sooooo coooollllld! :D

Anyone have info on the type of high and low pressure readings I should be receiving? Should I measure with system OFF, LOW SPEED, HIGH SPEED, OR WITH THE VEHICLE TOTALY OFF?

Another note: The very first can I put into the system, became so ICE cold, you could literly scrape the ice off it, while the rest were somewhat cool. Is this normal? I held the cans upright, but sometimes the system would stop taking it, I would have to shake it, drop the throttle back down to idle, then up to about 1500RPM's and off it went. (Never did the Compressor stop spinning, just the Aux. Fans were changing speeds.) To add the Freon it took me at least 25 min. Are there any special procedures to adding FREON that anyone can advise on?

I read the label of a different type of Freon can that had UV Dye and it said to hold the can up side down??? So whats the verdicts on FREON applications for my future A/C wooes?

Thanks to all.

So, what were you doing wrong????

I agree...give us some details!

I wish I knew...after fidling with my guages, restarting the car many times, the compressor finally kicked in.

I didn't add any vacuum, or change pressures, so I don't know. I fidled with many things before starting the car on each time, (disconnecting the compressor, the switch, reconnecting my hoses, etc.)

In the end, make sure you tripple check everything.

<<How would I dry charge, after removing my vacuum pump, I attached a can of Freon through low-side and the pressure in my guages indicated 80psi, but the compressor didn't kick on. >>

This is the question we were concerned with.
If you had an 80psi reading with the first can of refrig., you did not have a
valve open and your gauge was showing can pressure. The comp did not come on b/c there was never any refrig in the system yet. With a fully vac system, one can would not give you a system pressure of 80psi.
But when you did it the second time you posted , you noticed the can got cold..it got cold b/c you now had refrig. going into the system. So, the logical conclusion is you did not have the correct charge ports open the first time around.
That was our question.
But, you now have the system charged, so..nevermind.

Check out MENU#15 and use the Operational Checks to get the readings in the system. The 'best' two are #5 (evap temp) and #7 (compressor pressure in bar).

#5 s/b around 40F and #7 will vary alot from about 7-8 bar on the highway on a cool day to 15 -17 bar in traffic on a hot day. (1 bar = 14.7 psi).

Jim, the bar reading is of the high pressure, what is an acceptable pressure for the low side? That is where my hose originaly cracked (the rubber portion near the compressor) and I'm curious to know why? Maybe too high of a pressure?

Thanks for the info.



Arthur, if I think about, I agree, it must have been a valve on the manifold or on the quick-couplers.

<Arthur, if I think about, I agree, it must have been a valve on the manifold or on the quick-couplers>

That would be the deduction..which is why we like a follow-up ..
Just a common courtesy thing.
Tnx

There's no reading 'spot' for that in the Operational Checks. That reading will vary also depending on temperature; cooler day around 25 or so, and hot day w/ a full charge 35. But you need a 'gauge' to read the low side.

The hose probably died of 'old-age' or acid buildup???