No pressure reading on my AC system - Page 4 - PeachParts Mercedes-Benz Forum

otto huber · #1 07-20-2015, 07:45 PM

I was planning on emptying all three of my cans of R12 into the system, but according to vstech, the system needs to get started with weighed liquid R12 first.

I also just saw that my Egelhof expansion valve has R 134a printed on it. I'm hoping that this means the valve is compatible with R134a, but not solely for use with it.

Maxbumpo · #2 07-22-2015, 09:50 AM

Quote:

Originally Posted by otto huber

I also just saw that my Egelhof expansion valve has R 134a printed on it. I'm hoping that this means the valve is compatible with R134a, but not solely for use with it.

Not an AC expert, but it you are keeping the system R-12, this doesn't sound right to me. Two different gasses with different pressure curves, I doubt a TX valve is made that is designed for both. However, the MB factory guide for replacing R-12 with R-134 does not require replacing the TX valve, so go figure.

vstech · #3 07-19-2015, 09:40 PM

If the system gets all o rings changed, then oil installed, then pressurized with a dry gas and leak tested. Then it needs to be totally evacuated, and liquid weighed in, the high side, then vapor topped off until proper subcooled liquid is in the receiver.

otto huber · #4 07-20-2015, 12:08 PM

Quote:

Originally Posted by vstech

If the system gets all o rings changed, then oil installed, then pressurized with a dry gas and leak tested. Then it needs to be totally evacuated, and liquid weighed in, the high side, then vapor topped off until proper subcooled liquid is in the receiver.

Bummer. I was hoping that I could just use the three 14oz. cans of R12 that I have. I know of a garage that still has R12, but it's $70 a pound. Would you guess that they'll use 2 pounds of their R12, then top off with whatever they can use of mine?

97 SL320 · #5 07-20-2015, 08:00 PM

Systems are never charged through the high side when running. Charging through the low side with vapor while running is typical.

A heated charging station is really needed if pumping liquid through a non running high side. Even then that is more for large scale charging / at the factory charging.

What is the capacity of your AC system? What you do is divide can contents into AC capacity. If part of a can is needed, weigh the can / valve / hose as hooked to the car, watch the weight change then shut off the can when done.

otto huber · #6 07-20-2015, 09:41 PM

The R12 capacity for my car is 3.3 lbs.(52.8 oz.) . I have 42 ounces of R12 in cans, so I was just going to empty all of them into the low side port and top off at the shop if the vent temp wasn't satisfactory.

otto huber · #7 07-21-2015, 02:27 PM

I got started on the job last night, and threading the bottom two fittings back into the expansion valve is a nightmare! I threaded the top two fittings in no problem and left them very loose so that I would have enough play to get the bottom ones started. Since that didn't work, I torqued up the top fittings thinking that raising the valve up might help line up the bottom, no luck.I then checked the engine bay to see of I could create a little more slack in the bottom hoses by disconnecting them an pushing them towards the firewall. The plastic sound buffer prevents either of those lines from moving back to give me the slack that I need. Any tips from guys who have done this before? Thanks

97 SL320 · #8 07-21-2015, 07:04 PM

Quote:

Originally Posted by otto huber

The R12 capacity for my car is 3.3 lbs.(52.8 oz.) . I have 42 ounces of R12 in cans, so I was just going to empty all of them into the low side port and top off at the shop if the vent temp wasn't satisfactory.

OK got it, I was thinking the system took less than that. I'd still have the shop pressure test before you recharge in case there is a major leak. You will need them to vacuum the system before you recharge anyway.

Maxbumpo · #9 07-22-2015, 09:53 AM

Oil: what weight of oil does your compressor need? When I started doing AC work on my cars, I had no idea there were different weights of oil required by different compressors. My 124 cars need a light weight oil (40? from foggy memory), most of the oil on the shelf at FLAPS is 100.

rrgrassi · #10 07-23-2015, 05:45 PM

It depends on the compressor used. The R4 does not have a sump or drain plug.

otto huber · #11 07-23-2015, 05:48 PM

It's an R4. If I have the car on ramps, maybe the oil will flow out when I remove those lines at the back.

rrgrassi · #12 07-23-2015, 05:58 PM

It depends on the oil level. Turning an R4 ports side down will remove some oil. there is also oil in the dryer, condenser, etc. When I rebuilt my A/C system, I flushed it with Methanol, and replaced the expansion valve, drier, pressure switches and the compressor. Also replaced all the O-rings and used the "Baby snot" o-ring lube, and BG Fridgi-quite oil. That was 10 years ago.

otto huber · #13 08-24-2015, 08:03 PM

It's been a while, but I finally completed the project, or so I thought. After replacing the expansion valve, dryer, aux fan switch, pressure switch, manifold hoses and getting the compressor resealed I charged the system with 3 cans of R12 and got enough cold air to know that the system is working at least adequately. I put in 42oz. of R12 which is 10 oz. below capacity because I only have 2 cans left and the season is almost over. The bad news is that 4 days after charging I can tell that I've already lost some freon

After all of the work I've put in, I'm so frustrated that the thought of selling the car or rolling it off of a cliff would probably feel better than having a fully functioning AC system. There is a hissing sound coming from the expansion valve that started just today and I think that that may be the source of the problem. I ran my finger along the valve and the lines and no trace of oil. Any ideas? Thanks

eatont9999 · #14 08-24-2015, 09:46 PM

Quote:

Originally Posted by otto huber

It's been a while, but I finally completed the project, or so I thought. After replacing the expansion valve, dryer, aux fan switch, pressure switch, manifold hoses and getting the compressor resealed I charged the system with 3 cans of R12 and got enough cold air to know that the system is working at least adequately. I put in 42oz. of R12 which is 10 oz. below capacity because I only have 2 cans left and the season is almost over. The bad news is that 4 days after charging I can tell that I've already lost some freon

After all of the work I've put in, I'm so frustrated that the thought of selling the car or rolling it off of a cliff would probably feel better than having a fully functioning AC system. There is a hissing sound coming from the expansion valve that started just today and I think that that may be the source of the problem. I ran my finger along the valve and the lines and no trace of oil. Any ideas? Thanks

The TxV will hiss if you are low on refrigerant. You already mentioned it was low to start with. Charge the system until the sight glass runs clear on the hottest day possible. If it leaks after that, you need to find the leak. Ideally, you want to charge by weight but if you are adding a charge, go by the sight glass when using R12.

leathermang · #15 08-24-2015, 09:51 PM

Quote:

Originally Posted by eatont9999

The TxV will hiss if you are low on refrigerant. You already mentioned it was low to start with. Charge the system until the sight glass runs clear on the hottest day possible. If it leaks after that, you need to find the leak. Ideally, you want to charge by weight but if you are adding a charge, go by the sight glass when using R12.

THE FACTORY SHOP MANUAL FOR MB AIR CONDITIONING SPECIFICALLY FORBIDS USING THE SIGHT GLASS FOR ANY INDICATIONS DURING THE REFRIGERANT CHARGING PROCESS...
Using it to charge runs the serious chance of Overcharging your system... about the last thing you want to do.. since AC works on the differential between the high and low sides...