Hey Guys,

I bought the car and the A/C doesn't work. The ACoff button stays lit. I changed the Pressure sensor and it did absolutelly nothing. Please help me troubleshoot. How do I diagnose this system?

Thanks,


Adam

If you changed the sensor you drained the system. When you drain the system, you need to jump (bypass) the low pressure cut off sw. to get the pump to spin. At that point you need to vacuum and then charge the system...BUT...

Why did it leak out in the first place? is it gradual or is there a leak?
a vacuum test will tell (if it doesnt hold a vacuum). Most likely the system just ran low and the low pressure cutoff sw. did its job. All you may have needed to do is to jump the compressor and add a can of sauce.

One last thing, when you open the system you should immediately cover or plug the hole that is open. If you dont, you may wish to consider replacing the dryer. On the 300E the LP cutoff sw is on the dryer. The dryer has charcoal pellets in there to collect moisture. They get easily used up if the system is left open for several hours or more. Maybe a good vacuum (hr or so) may pull most of the moisture out of the system and maybe the dryer is ok. I made a jumper wire with two male connectors (one on each end) specifically to jump AC switches. It stayes in my AC toolbox.

Which "Pressure Sensor" did you change? A faulty Evaporator Temperature Sensor will cause the problem you're having.

I changed the one that's in the left(drivers) front corner of the engine compartment.

How do I jump it, it seems to have 3 wires, which ones do I need to connect?


I changed the sensor because I've read that it can create the problem I'm having.

The car was previously in a collision and I bought it salvage for a good price.

Is there any steps I can take to determine which area of the system is bad?

ahh ha! now your speaking a language I understand. (I ran a autobody shop for 18 years).

Im guessing now because I dont know your car but perhapse they combined your sensors into one with that model? but thats ok.

Was it hit in the front? Many body shops dont re-charge air in the winter for several reasons. If it was, a) they may have left it dry or b) it may still have a leak. Also, AC systems can leak down if they sit for a few years without being used. They will leak to the point where the low pressure switch will shut down the circut. When you replaced the sensor did 134a leak out? If so, thats a good sign. If 134 didnt leak out I would put a vacuum to the system. Go to the nearest A/C charging shop and ask them to put a vacuum on it to see if it holds. They will probably want an hour labor. Tell them to draw it down to -30lbs and let it sit for 10 min with the valve on the guage >closed. If it doens't move at all (assuming they have good guages) the system needs to be fully charged. (I would bring the 134A with you and maybe they can put it in..assuming you dont decide to replace the dryer/Ex valve).

As for which wires to jump, I wouldnt know. Putting 134A into a leaky system will be throwing money away anyway. Vacuum first, charge second. >>>You need to put a vacuum to the system anyway to put freon/134 into the system. You just can't add it after it was drained.

I've read in one of the other posts that the never mercedes don't use the same type of a compressor/clutch as the older ones.

If anyone has an update on this I am experiencing this same problem with my 2002 S500. The "AC off" light will not go out. My shop can't look at it until Wednesday.

When I look at the pulley/clutch on the compressor it appears to be turning but the compressor and clutch are different from previous years and I suspect the compressor clutch is not really engaged because the A/C hoses are both the same temperature. My gauge set says I have 90psi on the "low" side with the engine running. This would be consistent with a system "at rest".

What would turn that light on and inhibit the A/C system from operating?

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)

I have an update for my problem. My shop read a code that indicated the system was locked out because of a "positioning motor" or something similar to that. They did not provide the exact code, but once they cleared it the A/C system began to function just fine.

Time will tell if this is an indication of an ongoing or emerging problem.

__________________
02 S500
92 500SL
92 400E (Sold)
87 300E (Sold)
83 300D Turbo (Sold)
75 300D (Sold)
74 240D (Sold after 20 years)