AC pressure switches, leaks, high/low which - PeachParts Mercedes-Benz Forum

samboyellowsub · #1 07-28-2010, 10:23 PM

I've been shopping around for a replacement pressure switch for the 240D. I would think that there would be two switches, a high side and a low side, but I only see one on the high side at the R/D. Does that mean that I have the binary high/low pressure switch, or is there a second switch somewhere?

BTW, the only reason I am replacing it is because I'm chasing a leak. Last time I charged the system with R- 12 and oil, about 4 years ago, it worked great for 3 weeks, and then the compressor stopped kicking in, then I pushed on the low side shrader valve for an instant, and nothing, it was all gone! Do these switches even leak - or do I just need to go through and replace all the O-rings (a set of which I recently bought)?

Like I said, I never had any compressor problems or cooling problems, I just obviously have a pretty good leak(s) somewhere. Also, I know for sure my switch at the R/D is a male fitting, and the R/D has female threads. I took it out to check today.

I have plenty of R-12 and oil to get it going, i just don't want to waste any of it again!

vstech · #2 07-28-2010, 10:33 PM

I'd pressurize it with nitrogen or co2 or helium, and start searching for the leak with bubbles. if not found, add in some R22 and have it sniffed for the leak.

samboyellowsub · #3 07-28-2010, 11:19 PM

Quote:

Originally Posted by vstech

I'd pressurize it with nitrogen or co2 or helium, and start searching for the leak with bubbles. if not found, add in some R22 and have it sniffed for the leak.

i was going to do something like that if it doesn't hold a vacuum after replacing the RD and expansion valve and associated o-rings. What I'd really like to figure out is the switch situation that I mentioned above. Do you know about that?

will a sniffer detect a leak that takes 3 weeks to deplete the refrigerant?

vstech · #4 07-29-2010, 06:32 AM

yes. a sniffer will find the leak.
however, putting a new dryer in a system with unknown tightness under vacuum, is unwise. the best and only effective way to test for leaks is with pressure.
you should pressurize the system below vapor pressure of the gas, and either verify the pressure does not drop for 24 hours, or search out the leak with a detector, either bubbles or electronic detector.

samboyellowsub · #5 07-29-2010, 06:32 PM

Quote:

Originally Posted by vstech

yes. a sniffer will find the leak.
however, putting a new dryer in a system with unknown tightness under vacuum, is unwise. the best and only effective way to test for leaks is with pressure.
you should pressurize the system below vapor pressure of the gas, and either verify the pressure does not drop for 24 hours, or search out the leak with a detector, either bubbles or electronic detector.

Good point about the new dryer in a potentially leaking system. I'll test it with the old dryer in. I'm going to do all that with whatever gas my father in law uses at his shop.
What I'd like to know, if it happens to be the switch on the dryer, is if its a uniary switch or binary switch? Again, its a 1982 240D type one AC and the switch definitely has male threads. I'd like to know which one to get before hand so I can order it quickly if I need it because I'm kind of on a time budget ... trying to get the AC working for my wife's birthday next saturday.

Yak · #6 08-02-2010, 07:58 PM

I'm getting out of my experience with A/C by commenting on the switches, but my understanding is that the binary (or dual low- and hi-) switch was added later and is HIGHLY recommended for 134a conversions. Since 134a runs higher pressures, the binary switch is a safety precaution. It shouldn't hurt for an R-12 system.

What kind of switch you're currently using may not be as important as what kind you need to use with new receiver-dryers.

A 4x4 bumper? Really? Is that european wood?

funola · #7 08-02-2010, 08:53 PM

Yak, do you have a Autozone p/n? If not, then year make model for a binary pressure switch that will fir a W123 dryero the OP, how do you know your switch is leaking? Have you tested it? It is sealed with an o-ring.

Yak · #8 08-03-2010, 07:20 PM

I don't have the AZ p/n. They're listed on the site here: http://catalog.peachparts.com/ShopByVehicle.epc?q=1983-Mercedes--Benz-300cd-Climate--Control&yearid=1983%40%401983&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6169%3AED%7C10000135%3AMBC%7C1496%40%40300CD&catid=242213%40%40Climate+Control&subcatid=242239@@A%2FC+Pressure+Switch&mode=PA

A google search points points out other sources.

I'm sure it's possible for the switch to leak, but I think the failure modes would normally either be a bad pressure/electrical failure OR a bad O-ring. A "leaky switch" that dumps refrigerant is probably a distant third.

samboyellowsub · #9 08-05-2010, 01:36 PM

Quote:

Originally Posted by Yak

I don't have the AZ p/n. They're listed on the site here: http://catalog.peachparts.com/ShopByVehicle.epc?q=1983-Mercedes--Benz-300cd-Climate--Control&yearid=1983%40%401983&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6169%3AED%7C10000135%3AMBC%7C1496%40%40300CD&catid=242213%40%40Climate+Control&subcatid=242239@@A%2FC+Pressure+Switch&mode=PA

A google search points points out other sources.

I'm sure it's possible for the switch to leak, but I think the failure modes would normally either be a bad pressure/electrical failure OR a bad O-ring. A "leaky switch" that dumps refrigerant is probably a distant third.

well, the system is empty so I hooked up the hose to the low side port and blew into it. lo and behold air was coming out the top of the switch

funola · #10 08-05-2010, 07:37 PM

Quote:

Originally Posted by Yak

I don't have the AZ p/n. They're listed on the site here: http://catalog.peachparts.com/ShopByVehicle.epc?q=1983-Mercedes--Benz-300cd-Climate--Control&yearid=1983%40%401983&makeid=63%40%40MERCEDES+BENZ%40%40X&modelid=6169%3AED%7C10000135%3AMBC%7C1496%40%40300CD&catid=242213%40%40Climate+Control&subcatid=242239@@A%2FC+Pressure+Switch&mode=PA

The pics of the binary switch shows it has 2 terminals. Is the pic wrong? It has to have 3 terminals for a binary switch no? (trigger on low or high pressure. Or am I missing something?

Yak · #11 08-05-2010, 08:07 PM

Quote:

Originally Posted by funola

The pics of the binary switch shows it has 2 terminals. Is the pic wrong? It has to have 3 terminals for a binary switch no? (trigger on low or high pressure. Or am I missing something?

It's just a pass through or a relay, I wouldn't overthink and use any binary-electronics logic here.

If the pressure is above the min and below the max, voltage is carried to the compressor clutch. Two pins should suffice. Any "thinking" by the switch on whether to let the clutch engage goes on internally. If there was some requirement to provide an output as to WHY the compressor wasn't engaging, then you might need another pin. The compressor is dumb and doesn't care: on or off, +12 on the output side or not.

Yak · #12 08-05-2010, 08:18 PM

Quote:

Originally Posted by samboyellowsub

well, the system is empty so I hooked up the hose to the low side port and blew into it. lo and behold air was coming out the top of the switch

Fair enough. My uneducated assumption on leaking was off. The link for a new switch (of multiple varieties) should still be good.

I'll stick with the assessment that a new binary switch won't hurt/stop an R-12 system but may not provide much, either.

Get a new switch, put some good o-rings on there (note: the two new R/D's I've bought recently came with o-rings) and put more pressure on the system to look for any other leaks.

You're probably not going to make it for her birthday - and it's damn hot in Texas today, forecast is hotter on Friday/Sat. I'd say skip the A/C repair and get a steak and wine of choice for the birthday (or equivalent - a pitcher of sweet tea make work wonders...)

funola · #13 08-05-2010, 09:12 PM

Quote:

Originally Posted by Yak

It's just a pass through or a relay, I wouldn't overthink and use any binary-electronics logic here.

If the pressure is above the min and below the max, voltage is carried to the compressor clutch. Two pins should suffice. Any "thinking" by the switch on whether to let the clutch engage goes on internally. If there was some requirement to provide an output as to WHY the compressor wasn't engaging, then you might need another pin. The compressor is dumb and doesn't care: on or off, +12 on the output side or not.

I pressure tested the uniary switch. It is normally open at atmospheric pressure. Switch closed at around 45 psi (may be off by 10 psi because gauge not very accurate). I'll have to look at the Mercedes AC wiring diagram to see how the pressure switch controls the compressor. Hard to imagine but the binary switch has to somehow work like this:

Switch is normally open When the compressor turns on and reaches normal pressure, the switch stays open. If the pressure drops below or above a certain pressure, then the switch closes and cuts power to the compressor?

I also tested the temp switch. It is normally open, it closes at approx 135 F. Does that sound about right? Do you have specs?

funola · #14 08-05-2010, 09:16 PM

Sammboyellowsub, Did you use soap bubbles to pinpoint the leak? Using your hand to feel where air leaks are coming from is not very reliable.

Yak · #15 08-05-2010, 09:39 PM

Quote:

Originally Posted by funola

I pressure tested the uniary switch. It is normally open at atmospheric pressure. Switch closed at around 45 psi (may be off by 10 psi because gauge not very accurate). I'll have to look at the Mercedes AC wiring diagram to see how the pressure switch controls the compressor. Hard to imagine but the switch has to work like this:

Switch is normally open When the compressor turns on and reaches normal pressure, the switch stays open. If the pressure drops below or above a certain pressure, then the switch closes and cuts power to the compressor?

I also tested the temp switch. It is normally open, it closes at approx 135 F. Does that sound about right? Do you have specs?

The pressure sounds mostly right. The FSM says ON at 2.6 bar (1 bar = 14.7 psi) and OFF at 2.0 bar. This makes sense because you don't want the system cycling if the pressure moves slightly above/below a specific value. I would re-phrase the statement to say "in a normally charged system, the switch is closed". With a uniary (?) switch, it should only be open if you've had a leak and the refrigerant has leaked out and depressurized your system.

Go through the system logically: the CCU says "time to turn on the compressor" and outputs +12. That signal goes to the pressure switch and it gets a veto authority: not enough refrigerant = no +12 on the output = no clutch engage signal. 3 bar (or 45 psi) = +12 on the output = clutch gets to engage. The switch isn't "interactive" with the compressor, it's just a pass-through - if there's enough pressure then the compressor (or clutch) can do what the CCU asks.

This is one reason there's so much advice that you can't troubleshoot a compressor without gauges. If the resting pressure (no compressor running) in an A/C system isn't greater than the low pressure cut off (LPCO) switch then the compressor can't engage because the clutch shouldn't see +12v because the LPCO says so.

Think of the LPCO as an "It ain't gonna work so don't even try" switch and the HPCO as "It's gonna blow so we're shutting you down" switch.

The FSM says the aux fan temp on the R/D is 62 C or 142 F.