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

sleepstar · #1 06-19-2015, 08:40 PM

25psi seems low. are there bubbles in the sight glass?

otto huber · #2 06-19-2015, 08:43 PM

Quote:

Originally Posted by sleepstar

25psi seems low. are there bubbles in the sight glass?

I haven't checked the sight glass because according to the numerous threads that I've been reading, the sight glass is not a reliable indicator when diagnosing an MB AC system.

I thought that those static numbers seemed pretty high compared to other numbers I've seen people post.

eatont9999 · #3 06-20-2015, 02:55 PM

Quote:

Originally Posted by otto huber

I haven't checked the sight glass because according to the numerous threads that I've been reading, the sight glass is not a reliable indicator when diagnosing an MB AC system.

I thought that those static numbers seemed pretty high compared to other numbers I've seen people post.

It's not any less accurate than guessing if the system is charged by looking at the pressures. If you really want the system accurately charged, you do it by weight. You would have to capture the R12 and recharge by the weight specified for the vehicle. Maybe the superheat method would work but I have not tried it.

If topping off an R12 system, I go by the sight glass. It absolutely does not work for R134a, though; which might be where you are getting confused.

otto huber · #4 06-20-2015, 09:23 PM

I saw lots of bubbles in the sight glass, so I went ahead and add 8 oz. of R12 to the system. After adding the R12 the vent air wasn't much colder and there were still a lot of bubble activity in the sight glass. I put my hand to the low pressure hose and it wasn't cold, more like ambient temp. The high side was very hot and I then noticed that the auxiliary fan wasn't running, but it has worked in the past. After shutting down the engine, I put a jumper on the aux fan and it worked, could it be a relay?

Does the aux fan affect vent temps?

Does anyone know the book time to discharge a system, do a leak test and then refill? Can the recovered R12 be used again, or does it have to be fresh?
I have 14 oz. of R12 left, and I'm not sure if I want to continue the guessing game.

otto huber · #5 06-20-2015, 10:10 PM

I just took the car for a spin and the AC is now noticeably cooler. I guess that getting some air flowing on the condenser helped. The ambient temp is 68*F ,so not much of a test, but I'll see how it does when the temp is closer to 80*.

ROLLGUY · #6 06-20-2015, 11:01 PM

Quote:

Originally Posted by otto huber

I saw lots of bubbles in the sight glass, so I went ahead and add 8 oz. of R12 to the system. After adding the R12 the vent air wasn't much colder and there were still a lot of bubble activity in the sight glass. I put my hand to the low pressure hose and it wasn't cold, more like ambient temp. The high side was very hot and I then noticed that the auxiliary fan wasn't running, but it has worked in the past. After shutting down the engine, I put a jumper on the aux fan and it worked, could it be a relay?

Does the aux fan affect vent temps?

Does anyone know the book time to discharge a system, do a leak test and then refill? Can the recovered R12 be used again, or does it have to be fresh?
I have 14 oz. of R12 left, and I'm not sure if I want to continue the guessing game.

Absolutely the aux fan needs to run. A common problem is the thermal switch on the drier goes bad, and the fan won't run. You could either connect the 2 wires together so the fan runs whenever the ignition is on, or replace the switch. No need to discharge the system for this. The other option is to purchase one of my relay mods that runs the fan whenever the compressor clutch is powered up. As far as recovering the R12, you need to take it to a shop. They will recover the refrigerant, and credit it back to you when you have it recharged. You can then do a leak check and the necessary repairs.

otto huber · #7 06-20-2015, 11:51 PM

Quote:

Originally Posted by ROLLGUY

Absolutely the aux fan needs to run. A common problem is the thermal switch on the drier goes bad, and the fan won't run. You could either connect the 2 wires together so the fan runs whenever the ignition is on, or replace the switch. No need to discharge the system for this. The other option is to purchase one of my relay mods that runs the fan whenever the compressor clutch is powered up. As far as recovering the R12, you need to take it to a shop. They will recover the refrigerant, and credit it back to you when you have it recharged. You can then do a leak check and the necessary repairs.

Thanks Rollguy. I'll see if the fan runs when I connect the two wires, and if so, I'll replace that switch.

One other thing; I've now added about 36oz. of R12 since I purchased the car 5 years ago. I don't know how much leaked out of the leaky schraeder valve in the last year. Isn't there supposed to be a lubricating oil in the system along with the R12? If so, I'm thinking that I would be low on that by now.

ROLLGUY · #8 06-21-2015, 12:27 AM

Quote:

Originally Posted by otto huber

Thanks Rollguy. I'll see if the fan runs when I connect the two wires, and if so, I'll replace that switch.

One other thing; I've now added about 36oz. of R12 since I purchased the car 5 years ago. I don't know how much leaked out of the leaky schraeder valve in the last year. Isn't there supposed to be a lubricating oil in the system along with the R12? If so, I'm thinking that I would be low on that by now.

YES! The problem with the R4 is that they have no oil sump (unlike the Sanden), and when the refrigerant leaks out, so does the oil. The absolute best way to know how much oil to add, is to start from scratch and flush the system (not the compressor, but the evaporator, condenser and hoses). Then you can add the proper amount of oil to the system during the charge process. When adding oil during a recharge, you are just guessing at how much might be left in the system. Too little oil will hasten the demise of the compressor, and too much oil is not good either. Common leak points are at the schrader valve, pressure switch, compressor seals, Oring fittings, and old hoses. When leak checking with dry gas and soapy water, spray all the hoses. The large (low side) hose going into the cabin is the usual suspect on 123's, as well as the hose going under the engine. The hoses on a 126 are less often bad. If running 134a, it is best to change all the hoses, as all new hose is "barrier" hose. The molecules of 134a are smaller than those of R12, and will seep through the old hoses made for R12. The smaller hose (usually blue) going into the cabin is made of a different material, and I have never seen one go bad or had the need to be replaced.....Rich

otto huber · #9 06-24-2015, 06:24 PM

Quote:

Originally Posted by ROLLGUY

Absolutely the aux fan needs to run. A common problem is the thermal switch on the drier goes bad, and the fan won't run. You could either connect the 2 wires together so the fan runs whenever the ignition is on, or replace the switch. No need to discharge the system for this. The other option is to purchase one of my relay mods that runs the fan whenever the compressor clutch is powered up. As far as recovering the R12, you need to take it to a shop. They will recover the refrigerant, and credit it back to you when you have it recharged. You can then do a leak check and the necessary repairs.

Do you have experience replacing the Aux fan temp switch on a charged system or was it second hand information that the switch can be changed without discharging the system? The reason I ask is because I was at the pick n pull today and figured that I might as well pull a switch off of a 560sel since that car uses the same switch as mine. After about a full turn freon started coming out of the drier. The part is sold with an o-ring, so unless the switch is inside the red tab connected to the brass fitting, I don't see how it's possible to change that switch without discharging the system.

Heads up to folks looking for AC parts at the Oakland pick n pull, they don't discharge the AC on their cars before placing them in the lot.

ROLLGUY · #10 06-24-2015, 06:41 PM

Quote:

Originally Posted by otto huber

Do you have experience replacing the Aux fan temp switch on a charged system or was it second hand information that the switch can be changed without discharging the system? The reason I ask is because I was at the pick n pull today and figured that I might as well pull a switch off of a 560sel since that car uses the same switch as mine. After about a full turn freon started coming out of the drier. The part is sold with an o-ring, so unless the switch is inside the red tab connected to the brass fitting, I don't see how it's possible to change that switch without discharging the system.

Heads up to folks looking for AC parts at the Oakland pick n pull, they don't discharge the AC on their cars before placing them in the lot.

Sorry, I forgot about the fact that you have a W126. I thought the earlier models had the thermal switch (like a W123) that could be changed without recovering the refrigerant, but yours may not be that way. Your only choice is to recover the refrigerant to replace the switch, or hook the fan up in such a way to use the compressor clutch power to energize a relay that turns on the fan (like the relay mod I sell). That way the fan switch and wiring can be abandoned. Not sure if you have done this yet, but have you checked to see if the fan switch is even working? As soon as it sees pressure, the contacts should close. If you connect the two wires, the fan should come on.

Also, it is against the law (especially in CA) to have cars in a salvage yard that have not had the refrigerant recovered. What you experienced was residual pressure in the system (unless they forgot one).