No pressure reading on my AC system - Page 3 - PeachParts Mercedes-Benz Forum

otto huber · #31 06-21-2015, 01:40 PM

Thanks for the link, looks like a good site.

BillGrissom · #32 06-21-2015, 04:01 PM

The 120 psig both sides w/ AC off on a 70 F day sounds high to me. In my two 300Ds' (one w/ R4 compressor, other w/ Sanden), I usually measure 70-90 psig and is usually hotter here. 25 psig low side running is not abnormally low on a 70 F day. I usually measure 15 - 45 psig, depending on ambient temperature. I am currently using Duracool, which is similar temps to R-12, and recall similar pressures w/ R-12.

I second that the aux fan not running makes a big difference. We were stuck in a 2 hr traffic jam on I-5 in my 1984 300D on a 115 F day and the AC wasn't blowing very cool, so had to open the windows. Once we started moving, the AC was cold. I later found that the fan temp switch was bad/erratic. I tested 3 temp switches on the stove and found all go close at 86 F => "all summer" here. I don't know if other sensors or actuators get to vote on turning on the rad fan, perhaps the AC clutch also has to be engaged.

Charlie Foxtrot · #33 06-21-2015, 07:52 PM

bubbles still in sight glass>>your system is low; keep adding R-12 until no bubbles plus 1-2 ounces.
adding oil>>Companies such as Enviro Safe (Pekin IL) sell cans of refrigerant oil (mineral for r-12) that are vacuum sealed w/o refrigerant. If you are clever, you can hook this up to your refrigerant gauge manifold, push a very small amont r-12 into the can (tenths of an ounce at very low pressure) then use the suction side of the compressor to pull some/all of the oil into the system
aux fan>>bypass the switch at the receiver drier and jumper the two conductors to make the fan run. In the winter, pull the jumper.
leaks>>R-4 compressors leak...period. they leak through the front shaft seal. the leak pushes out refrigerant & oil. when these things were new, it was a standard practice to add a pound of R-12 & an ounce or two of oil every second year. R-12 was cheap, about $15-$20 a 30# cylinder. A bi-annual recharge was $29, $25 on sale. Not any more, last time I bought R-12 (late 90's) it was $700+/30# (virgin).

Good luck, enjoy the summer!

otto huber · #34 06-21-2015, 08:19 PM

Quote:

Originally Posted by Charlie Foxtrot

bubbles still in sight glass>>your system is low; keep adding R-12 until no bubbles plus 1-2 ounces.
adding oil>>Companies such as Enviro Safe (Pekin IL) sell cans of refrigerant oil (mineral for r-12) that are vacuum sealed w/o refrigerant. If you are clever, you can hook this up to your refrigerant gauge manifold, push a very small amont r-12 into the can (tenths of an ounce at very low pressure) then use the suction side of the compressor to pull some/all of the oil into the system
aux fan>>bypass the switch at the receiver drier and jumper the two conductors to make the fan run. In the winter, pull the jumper.
leaks>>R-4 compressors leak...period. they leak through the front shaft seal. the leak pushes out refrigerant & oil. when these things were new, it was a standard practice to add a pound of R-12 & an ounce or two of oil every second year. R-12 was cheap, about $15-$20 a 30# cylinder. A bi-annual recharge was $29, $25 on sale. Not any more, last time I bought R-12 (late 90's) it was $700+/30# (virgin).

Good luck, enjoy the summer!

I've already decided to purchase oil from enviro-safe, just need to hear back from them about which of their hoses to use. I'm hoping that they don't require a license for the oil.

I'd rather replace the switch or relay for the Aux fan so that I don't wear out the bearings by running the fan unnecessarily.

Now that I'm making some progress with my vent temp, the PBU is starting to crap out on me. Three times in the past 2 days it has become completely non functional. If I'm having problems with the AC, I'd expect the Economy setting to still work. If the blower motor is on it's way out, will it shut down the PBU? Thanks

ROLLGUY · #35 06-22-2015, 12:45 AM

A common problem with the climate control (PBU) is that the solder joints crack and need to be re-flowed. It is easy to do. Just remove the PBU and take the cover off. Then just put your soldering iron tip on each solder joint, thereby re-melting the solder. This should restore the unit to it's proper function....Rich

otto huber · #36 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 · #37 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).

otto huber · #38 06-24-2015, 06:50 PM

Well, I think that they forgot one because I had to walk away as this drier was spewing green liquid into the air, probably leak detector mixed with freon.

My aux fan works when I jump the wires connected to the switch. I'll send you a PM about your mod. I'd rather not have the aux fan running all the time. The weather is pretty temperate where I live and AC isn't a life or death issue like it is in some other parts of the country.

cfh · #39 06-25-2015, 10:27 AM

On my 123 (with Rollguy's Sanden conversion - which I recommend) I had a devil of a time chasing leaks; turned out both hoses going to the tx valve were bad - even the narrow high pressure hose. It's not too expensive to have the hoses re-done using your original fittings, by a shop that specializes in a/c hoses - costs much less than buying new hoses. There is a place in Dallas that does them for $30 a pop while you wait.

I also have a 126 and the elusive leak was at the shaft seal on the R4 compressor. The seal and the tools to replace it are not expenive and it's a pretty easy job to do.

sleepstar · #40 06-25-2015, 02:16 PM

i wired my aux fan into the cabin with a switch that i can manually control. i usually just turn it on whenever i have the a/c on unless i'm on the freeway. its very easy to do and was maybe $10 in parts.

BillGrissom · #41 06-25-2015, 02:44 PM

Quote:

Originally Posted by otto huber

I've already decided to purchase oil from enviro-safe, just need to hear back from them about which of their hoses to use. I'm hoping that they don't require a license for the oil.
...

No license needed to buy Enviro-safe or the oil. Same deal for Duracool (in all my cars). I think PAO 68 oil is the best one today (any refrigerant) and ideal for our non-sump R4 compressors. See Duracool's site. The fittings for both should be standard R-134A type. If changing from R-12, you will need to install adapters. I vaguely recall that my 300D's had the same-size R-12 port on both low & high sides.

otto huber · #42 06-25-2015, 08:36 PM

I've been getting a short hissing sound from under the driver's side kick plate recently when turning on the AC, so I figured that I should check the expansion valve. Sure enough, there was oil on the plastic kick plate and a little bit of oil on the valve. It's a cheap part, but looks difficult to replace. I was able to get a 17mm tubing wrench on only one of the fittings due to the tight space between the lines. Are there special wrenches that need to be used. Are the lines to the expansion valve just as likely to leak?

cfh · #43 06-25-2015, 11:58 PM

Those fittings are SAE, not metric - I think one is 7/8 and the othe 11/16 but my memory could be off. I've had good results with crow's foot wrenches and also by using a cutoff wheel to make a very short-handled wrench. If these fittings have never been apart they can really be stuck and you can do serious damage to the copper pipe going into the evaporator. Use MAP gas torch (like a yellow butane torch) to apply heat (have a fire extinguisher handy too) and also use striking blows with a hammer on your wrench, rather than steady torque, to break the fittings loose. Steady torque will twist your pipes rather than breaking loose the fittings

otto huber · #44 06-26-2015, 12:11 AM

Quote:

Originally Posted by cfh

Those fittings are SAE, not metric - I think one is 7/8 and the othe 11/16 but my memory could be off. I've had good results with crow's foot wrenches and also by using a cutoff wheel to make a very short-handled wrench. If these fittings have never been apart they can really be stuck and you can do serious damage to the copper pipe going into the evaporator. Use MAP gas torch (like a yellow butane torch) to apply heat (have a fire extinguisher handy too) and also use striking blows with a hammer on your wrench, rather than steady torque, to break the fittings loose. Steady torque will twist your pipes rather than breaking loose the fittings

You just convinced me to take this job to my indie. He's very knowledgeable with old MBs, but pricy. Depending on the estimate, this maybe the death knell of my AC system. The compressor has also been squealing for a good 5 seconds when I turn it on which probably has to do with a shortage of oil in the system. I'm tempted to add some oil to the system just to see if the squealing goes away.

Charlie Foxtrot · #45 06-26-2015, 11:35 AM

Start up squeal usually is caused by the belt, not lack of oil. Check the belt, it may be shiny...the front shaft seal leaks oil/R-12 mix, gets picked up by the belt and flung off (that's the source of the greasy stripe on the engine side of the hood). In the process the belt gets shined up and squeals...same thing used to happen to power steering pump belts. If the AC shaft leak is bad enough, the compressor clutch gets saturated, and slips/squeals on start up. (brake cleaner sometimes helps cleaning the clutch). Good luck w/the indie. Old AC systems are a heart breaker.