A/C Compressor cycling turning on and off
 

So I have my A/C system back together....
I have 2 cans or 24oz of refrigerant in her and static pressure is ~230psi but the compressor just clicks on, then off again 1 second later. Over and over...

Belt is tight and everything was working prior. All I did was remove compressor to replace manifold seals and replaced condenser.

Any ideas why its doing this?

3 Lbs seems like a bit much but I haven't charged a 124 lately and don't know the spec off hand. Cycling on and off though is a typical overpressure scenario. There are no normal operating conditions where it should do that. Static pressure is useless info, though it seems quite high. Need high and low side pressures to figure out what its doing.

EDIT: just noticed it is 2 can OR 24oz, not 2 cans OF 24 oz.:rolleyes: 1.5 LBS seemss closer to right but it still sounds like an overpressure, possible high side restriction.

2x12oz cans for 24oz total.

I watched the high side pressures and it was not rising above the 220-230psi.
It also started doing the cycling about 1/2 way through the first can at around 120psi.

I can't give high and low since it wont engage long enough.

I am wondering if the speed sensor went kaput after the removal. Its the first time its been off the car.

You know what, I think somethings blocked.

I rehooked my gauges to the low side (I was using a separate charger hooked up to my can) and the low side is showing 25psi with the compressor off.

THEN, I shut everything down and the high side is reading 150psi and the low side 70psi and I hear gurgling/bubbling noises coming from the condenser line.

Could there be a blockage or something?

You know what, I think somethings blocked.

I rehooked my gauges to the low side (I was using a separate charger hooked up to my can) and the low side is showing 25psi with the compressor off.

THEN, I shut everything down and the high side is reading 150psi and the low side 70psi and I hear gurgling/bubbling noises coming from the condenser line.

Could there be a blockage or something?

possible air introducedd into the system? Try the simple approach first, re-evacuate system and rechargeuntil the compressor stays on (only if it is slightly below or at the recommended amount of freon) and then stop. I don't think if its slightly below the recommended amount you will have a problem, but if it is over, you may have a problem.

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.

Gotcha. I will inspect the expansion valve and condenser for blockage.

I flushed the condenser, evaporator and expansion valve again...put it back together charged it and same issue.

:confused:

When I watch the gauges, the high side is ~200psi and the low side is 10psi, it creeps up to ~25psi, and that is when the compressor kicks on, and it shoots straight down to 10psi again, compressor shuts off....then it takes a good 5 seconds to get back up to ~25psi and the process repeats.

It seems like the compressor shuts itself off when the low side drops (aka as soon as the compressor turns on)...Its not undercharged since BEFORE the compressor kicks on at ~8oz and stays on while more 134a gets sucked in.

Where else could a blockage be or any other ideas? The compressor itself? Drier?

Check your connections at the condenser. Maybe you kinked a line? Sounds like classic clogged expansion valve but if it all flushed well then I would be looking for some twisted metal.

Ok its definitely a blockage...

I swapped the drier, no change. So I then jumped the pressure switch and sure enough it went straight into a vacuum on the low side.

I checked condenser output lines and no kinks or anything. Is there a specific way to find where a blockage could be without just throwing parts at it? (I did flush the expansion valve, but maybe I should get another JUST in case whatever is lodged in there is jammed in good and the carb cleaner didn't get it out).

At this point I'd put another exp. valve in it. That's what the compressor is sucking against.

If you jump the switch what do the pressures go up to? You'll know if the high side or tx vlave has a problem, the high side will shoot up to 300+ psi in a hurry.

-J

Not recommended to flush the expansion valve---you definitely need to replace it. Mark

Well I only had about ~15oz in and it was 200psi. I didn't see a need to fully charge and stress the system. I just put enough in so that the compressor could turn on. The fact that its going into vacuum was all I needed to know.

I have ordered a new expansion valve and will report back (hopefully saturday) as soon as it is in. Fingers crossed.

Its supposed to 105F starting this weekend, so I need this done ASAP!!! :eek:

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?

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.

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.

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

What the hell?

How can I find out where the blockage is?

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.

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.

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.

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.

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

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

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.

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.

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

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??

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.

You're the man!

I will report back as soon as I get the hose. Getting Genuine MB from the stealership to get this wrapped up.

I wonder if this hose was the reason for the high system pressures for the past few years that I never solved other than passing it off as limitations of 134a....We will find out. Fingers crossed.

Awww shucks, I just try to look out for my fellow 124 wagon owners.

Anything on the high side of the expansion valve should be hot. If the condenser is hot but the hose from condenser to drier is cold, then the hose is the culprit. This is consistent with the rapid cycling, as the pressure switch on the drier is effectively on the low side of the system due to the blockage in the condenser-drier hose.

The stealership at my house wants above list price for the hose ($120!!!)...outrageous...
Ordered it online for $63 (Genuine MB) and should be here tomorrow so will put it in and will report back.

I will also work on replacing the rest of the rubber in the system too. I just need some cold air right now :)

I know the feeling well. I got tired enough of dealing with my '87 wagon that I replaced the entire a/c system all at once. New evap, new vacuum pods, new compressor, new manifold hose assembly, new txv, new condenser. It was so nice to switch on the a/c and just get cold air.

With new everything there wasn't much performance difference between R134 and R12. I collected some data points on each and wrote up a thread. This was back in 2002 or 2003, but the thread is out there somewhere...

Couldn't even blow through the hose with it in my hand it was so blocked. I cut the hose open and hose was fine, but the metal inside was bent...no idea how that would happen, but it was. Maybe it took a rock or something...

A/C is now functional....pressures are still high as before though. 18oz R134a in the system and its 55psi low 380psi high at idle garage 110F ambient (I know its extreme temperatures for a converted system to handle). As I said, ONLY 18oz of r134a in the system so its technically according to MB about ~8-10oz short of the recommended amount.

I already have a new evaporator, 10pa15c compressor, new blower motor and cages and drier in the garage ready, so this winter, I will get a new A/C manifold hose, pull the heater box and basically start from scratch with R12.

Those pressures are high on both sides. Does the aux fan cut in around 250 PSI? How much oil in the system? How deep of a vacuum before charging?

28" vacuum. Aux fan is going on full at 300psi. No idea about oil quantity in system.

When I look at this chart, at 110F ambient, the low side is right, but the high side is much higher...

http://www.aerocousa.com/UploadFile/...3144012361.jpg

All my 124 cars ran higher on the high side than generic temperature/pressure tables predict. I think it has to do with the location of the high side attachment point for the gauges. I wouldn't sweat it. Do you have the MB 124 shop manual? Of course it would not have a spec for 134 in your car, we would still be guessing...

http://www.aerocousa.com/UploadFile/...3144012361.jpg

What about the idle RPM, does it matter when taken pressure measurements?

Should the engine be rev up to say 1K, 1.5K or even 2k or idle rpm of 750 is ok?