OMG! More Stupid A/C questions... - PeachParts Mercedes-Benz Forum

pdrayton · #1 07-12-2007, 11:38 AM

OMG, not AMG, becuase these questions have probably been answered before, but the 20 posts I read didn't have the answers. Maybe you know the answer and save the hair I have left on the ol' noggin.

We're talking about a 1992 300D 2.5 (W124). A/C has been acting up lately. Thought it was the Klima making the compressor cut out after a short time, but after putting in the icecube bypass it still did it. Shorting 5 and 7 on the Klima connector did work though - the compressor stayed on.

However, with the original Klima in and the pressure switch S31/1 shorted, the compressor stays on. The low side pressure reads around 30psi with the compressor running at idle, so there is enough refrigerant right? So the switch must be bad? Is that correct? I couldn't make sense of the FSM procedure to test it

Quote:

check positive connection to battery and ground one after the other at the two terminals on the pressure switch (S31/1). If ground is present only at one terminal, the air-conditioner is either not charged sufficiently or the pressure switch (S31/1) is defective. If ground is present at both terminals and the refrigerant compressor does not switch on, check compressor shut-off

Does that mean there should be ground and 12v at the switch? That doesn't make sense to me.

Anyway, the switch is about $6 at advance auto. So I just drain the R134a put the new switch in , vacuum it down really well and put back 2.2 lbs of refrigerant?

As they say, there are no stupid questions, just stupid questioners, here's mine -

Do I have to replace the drier?

Do I have to add any oil?

Am I barking up the wrong tree - the switch is possibly fine and I don't have enough refrigerant? (the low side pressure sits at about 80 psi when the compressor is not running. Is that OK?)

Sorry for the ramble. I'm having knee surgery tomorrow and I'm a bit scattered. But I want to get the A/c fixed in this car - it is my wifes and she'll be driving me around and looking after me so any brownie points are worth double at this stage.

Last Stupid question - is shorting that pressure switch going to cuase any problems if there is enough refrigerant in the system? If there isn't enough refrigerant, how long til it causes damage to the compressor?

Thanks

James L · #2 07-12-2007, 11:48 AM

What is the high side reading along with ambient temp? I would'nt be so quick to think it is a refrigerant problem. Your car is like my 93 and has a compressor cut off computer system to protect it. The fsm has a good cut off troubleshooting procedure for testing each, things that make the compressor cut off; slipping belt, engine temp sensor, aux fans, micro switch, ovp, and N6 relay (kilma).

If you post your low and high side readings at 1500RPM and ambient that can help to see if it is a refrigerant problem. Does the sight glass stay foamy and bubbly all the time or does it go to clear at some point. When it works, does it cool? (vent temps 40-60 degrees) Factory 134a or R12?

rrgrassi · #3 07-12-2007, 11:52 AM

It is not a good idea to keep the compressor jumpered. If pressures get way off, either too much or too little, you are asking for trouble.

Did you look at the ackits web page? They have pressures listed according to ambient temps. Very helpful.

If the system is open for a few minutes, pumping down and refilling without a new drier should be ok. If you remove the freon yourself, oil does come out along with it, since the oil flows with the freon. Not all of the oil will come out though. You will need to replace some oil, about 2-4 ounces. The oil shots in a can should suffice, but you need to know what oil is currently in it, since not all oils are compatible. You would be better off going to a shop and have them remove the freon for you.

pdrayton · #4 07-12-2007, 07:31 PM

Thanks for the replies chaps. The forum didn't email me so I didn't' know there were replies until now.

To answer the questions:

It is R134a
When the compressor is on, it cools really well 40-60F.

I don't know the high side pressures - I only have the low side gauge. Maybe I can hook something up?

With the AC running, I can't see anything moving in the sight glass on the drier. There is a green dye in the refrig, but I still can't see anything moving in there when the compressor is on.

Looks like refrig leaked from the suspect pressure switch today while the car was driven with it jumpered. Makes me nervous, I think I should unjumper it.

Isn't the Ester oil compatible with all R134a oils? I know one of them is PAG or whatever. Any idea? It's only about $15 for the R134a and $6 for the pressure switch so maybe it's worth just doing it.

I'll try get some high side readings.

pdrayton · #5 07-12-2007, 08:04 PM

Grassi, I looked at ackits, but couldn't find the pressures/temperatures. What part is it in ? The forum?

The gauge I have goes up to 250psi. Is that enough to measure the high side?

James L · #6 07-13-2007, 10:10 AM

If you have dye in the system then you should see it leaking around the pressure switch, look for oil residue also. The ackits pressure table is at the top in info section. You will need two gauges because as high side increases you need to see the low side drop. High side with R134A could get as high as 275 psi or more on a hot day, so your 250 psi gauge will not be enough. Go to harbour freight tools or the like and get a R134A manifold gauge set for $50+/-.

rrgrassi · #7 07-13-2007, 10:37 AM

Quote:

Originally Posted by pdrayton

Grassi, I looked at ackits, but couldn't find the pressures/temperatures. What part is it in ? The forum?

The gauge I have goes up to 250psi. Is that enough to measure the high side?

Careful with cheap guages. They have been known to leak, hoses separate, etc. Go to AutoZone or the like. There are guages that cost about $90. I have some and have had no issues with them.

Like posted befor, look in the INFO section of the ackits site. Also, look into getting a high/low pressure switch. Was your car converted to 134a or did it come from the factory w/ 134a?

pdrayton · #8 07-13-2007, 05:54 PM

I got some manifold gauges from Harbor freight ($25 with coupons and all, couldn't pass up a deal like that! )

I don't know if it was 134a from the shop or not. I think it was converted, but I'm not sure. LAid up in bed with someone elses ligement in the my knee at the moment - ie just got back from surgery. Some might say I have a problem - playing on the internet just after surgery, but lying in bed can be boring and frustrating when you've got new tools and a problem to figure out!

I'll check the pressures when the leg is a bit better - will be a few days, but I'll let you know as soon as I do.

Regarding the leaking dye and oil - it definately is oily. Does that indicate anything? Don't say compressor going out !

rrgrassi · #9 07-13-2007, 05:58 PM

It depends on where the lines or fittings are oily. My car had a leak...it was the front seal on the compressor and a spliced high pressure line. My wifes Chevy Cavalier (thankfully gone) had leak at the accumulator, loose fitting was the cause. My current truck had one due to a bad schrader valve.

Not all leaks mean a bad compressor.

Hope you are up and running soon!