A/C Compressor cycling turning on and off - Page 2 - PeachParts Mercedes-Benz Forum

Edmund · #16 05-30-2013, 07:20 PM

Quote:

Originally Posted by duxthe1

Speed sensor will shut it off and keep it off until you cycle the key. It wont cycle it on and off. Too high pressure in the high side is the only thing I've seen cause a rapid on off cycling. Sometimes you can find a restriction in the high side by feeling for a cold spot in the lines and or looking for frost. A partial blockage can act like an expansion valve allowing the downstream side to get cold when it should be hot.

Not to hijack this thread, but duxthe1 you just mentioned the problem on my 1990 W124. Whenever I switch the aircon the compressor engages and disengages about a second later. I can only engage it again after a key cycle. You say this is the speed sensor. I dont have that much experience with aircons, does it measure the speed of the compressor and where is it located roughly?

duxthe1 · #17 05-30-2013, 07:32 PM

Its in the rear of the compressor. If the relay doesn't see the signal it shuts down the compressor. Its purpose is to save the belt drive if the compressor takes a dump. I've heard that it is replaceable, but I've never done it. If a compressor is old enough to have a failed sensor, its pretty much time for a new compressor.

Edmund · #18 05-31-2013, 11:51 AM

This car still uses the old gas thats illegal now so its never time for a new compressor! Thanks for the info, I will look for it and measure what it does and take it from there.

ps2cho · #19 05-31-2013, 11:52 PM

New expansion valve in....STILL going into vacuum.

What the hell?

How can I find out where the blockage is?

lorainfurniture · #20 06-01-2013, 12:04 AM

If the problem started when you replaced the condenser, receiver and ? The problem is with those parts.

Disconnect the condenser and try and push air through with a compressor. It should pass through somewhat easily. Do the same with the drier.

lorainfurniture · #21 06-01-2013, 12:09 AM

Reading again, I would say the issue is the condenser or something that you did when you did the compressor seals. If you Have a small compressor, try and push air through.

ps2cho · #22 06-01-2013, 12:47 AM

I am wondering if there really is a bend and its just out of sight.
I was going to replace the condenser anyway with a new one just because its 25 years old. Its just one of the major components. I will get it replaced tomorrow (shop local has one for me) and I will report back. It has to be blocked there right? Since anything else would get jammed in the condenser before making it to the evaporator...

Money not really an issue with the condenser since like I said, I kind of want to replace it.

jcyuhn · #23 06-01-2013, 09:49 AM

If you can get it to run long enough you can find the restriction by feel. I am sure you know the high side is usually quite hot to the touch. It will be cold after the restriction point. Basically a restriction acts like an orifice or expansion valve.

I think you have a restriction before the dryer and the high/low pressure switch is cycling off on low pressure when the system goes into vacuum.

ps2cho · #24 06-01-2013, 10:00 AM

Got it...so it must be the condenser then. We will find out later today as soon as I can pick the condenser up.

ps2cho · #25 06-02-2013, 07:08 PM

Replaced the condenser....still no go.

Fully charged it....jumped pressure switch, high side 220psi, low side -10psi.

You mention I can find the blockage by feel....The condenser is HOT. The expansion valve is cold and both lines are that go to it are too. Both high side and low side. Is that normal? Or could the evaporator be blocked?

Here is a video of the issue: sorry about the sideways, not sure how to fix that
https://www.youtube.com/watch?v=kjdWjR-uGpQ

lorainfurniture · #26 06-02-2013, 09:46 PM

If you don't have an air compressor, go buy or borrow one. Any small one will do.

Disconnect the low side where the low side port is (the big nut), and disconnect the high side at the receiver drier. Take an air hose and connect it as best you can to the high side. I used the "air cleaning tool" thingy that comes with compressors. You just jam the tip in there and let it rip. If the tool builds pressure and pops out, then you know the blockage is somewhere in the evap or one of the hoses on the way there.

If air flows well, then you know the blockage is somewhere in the front of the car. Start disconnecting stuff one by one until you find the blockage.

You are doing a lot of replacing and not quite enough diagnosing.

Report back.

duxthe1 · #27 06-02-2013, 10:10 PM

The drier and the line from the drier to the evp. valve should be hot. The hose from the condensor to the drier should also be hot and sounds like the problem. Feel where it goes from hot to cold and you find the restriction. I think possibly the interior liner in that hose has seperated and is blocking the refrigerant flow.

ps2cho · #28 06-02-2013, 10:33 PM

The hose that goes from Condenser -> Hose -> Drier is COLD.

ps2cho · #29 06-02-2013, 11:30 PM

Ok so the drier IS supposed to be warm/hot correct?

I am getting conflicting information here a little it seems

This is my understanding so far for flow: H=high L=low
Compressor -> condenser(H) -> drier(H) -> expansion valve -> evaporator(L) -> pass back through open expansion valve(L) -> compressor.

Is the above the correct flow?

If so, then shouldn't the everything AFTER the condenser be HOT until the TXV?? So therefore the drier should certainly NOT be COLD with ambient temperatures at 100F??

duxthe1 · #30 06-03-2013, 01:48 AM

The drier should not be cold unless there is a restriction in it or in front of it. Usually the drier is the restriction but in your case with the hose upstream cold, then the hose is the restriction. Very unusual failure BTW.