A/C Oops! - PeachParts Mercedes-Benz Forum

VA-Merc · #1 05-18-2015, 10:15 AM

So my wife was driving the 87 300D the other day while I was changing the tranny conductor plate on her S500, and it was a hot day so she used the a/c. She called me to say she heard a weird noise and had a freon smell and she had pulled over. When I went and checked the a/c manifold line on the back of the compressor had popped off and all the R134a came out.

I've had a slow leak for years (had to add about a can of r134a annually) and didn't realize the bolt that holds that line was not torqued properly all of this time (it had completely backed out which is why the line fell off).

I put the line back on within an hour with a new bolt, but I haven't pulled a vacuum/charged it up yet because I wanted to ask a couple questions:

1) What is the torque spec for the bolt that holds the lines to the back of the a/c compressor (don't want it to back out again, but I also don't want to strip the aluminum threads in the compressor)?

2) Should I replace the receiver/dryer if the system was open for approx an hour but worked perfectly fine before? (This is not original a/c stuff...it was all replaced by the PO about 10 years ago...I'm just not sure how long it takes a system to use up all of the decadent if a line pops open like that)

3) How would I guesstimate how much oil to add back in?

Maxbumpo · #2 05-18-2015, 10:27 AM

Quote:

Originally Posted by VA-Merc

1) What is the torque spec for the bolt that holds the lines to the back of the a/c compressor (don't want it to back out again, but I also don't want to strip the aluminum threads in the compressor)?

From the FSM 83-530: 23 +/- 2.3 Nm

Quote:

Originally Posted by VA-Merc

2) Should I replace the receiver/dryer if the system was open for approx an hour but worked perfectly fine before? (This is not original a/c stuff...it was all replaced by the PO about 10 years ago...I'm just not sure how long it takes a system to use up all of the decadent if a line pops open like that)

Yes, always replace the receiver/dryer.

Quote:

Originally Posted by VA-Merc

3) How would I guesstimate how much oil to add back in?

I'd add in all the possible oil that could be in the compressor and the old receiver/drier, plus 10% of system capacity (what was carried in the refrigerant gas).

FSM 83-525 says the compressor can hold 160 cubic cm of oil, and in the case of complete system leak-down, an additional 20 cubic cm is needed to replace what was carried out by the gas. In addition, 10 cubic cm is needed for the new receiver/drier.

At this point you could remove the compressor, and completely empty it to make sure all has drained out, and then fill in 190 cubic cm.

Make sure you use the correct viscosity oil, I believe it is 40 weight? Typical FLAPS AC oil is 90 weight I think.

VA-Merc · #3 05-18-2015, 10:56 AM

Thanks! This is exactly what I needed. Luckily a friend of mine who owns a shop is going to let me use his a/c machine. I don't know why I didn't think of turning the compressor sideways and dumping the old oil...good idea. I really appreciate the help! I will do some research on the oil for that compressor.

BillGrissom · #4 05-18-2015, 03:10 PM

Does yours have the Harrison R4 compressor (pancake shape)? I just re-installed that fitting on 85 300D (hose rebuild), but now have a Sanden w/ same "GM Pad" inlet/outlet for original hoses. I coated the bolt w/ anti-sieze, as I always do for steel into aluminum. I tightened it "by feel". No way to get a torque wrench in there, just a box wrench.

Since it was open for just a short time, the PAG oil shouldn't have absorbed much moisture, but wouldn't hurt to change the filter/drier (assuming you don't snap off a fitting), since the dessicant bags sometimes degrade over time. Why go to a shop, my engine's vac pump pulls down to 20" Hg quick, then I finish off w/ a hand pump to outgas for hours and check for leaks. I did have a leak, but turned out to a fitting on my charging setup.

Maxbumpo · #5 05-18-2015, 03:51 PM

20" Hg is not sufficient to remove all the water from the system. A 29" vacuum is required to be pulled and held for a bit. If you've got a fancy micron vacuum gauge, you can really make sure the system is totally dry.

vstech · #6 05-18-2015, 04:41 PM

Only way to make sure the system is dry with an r4 compressor is flushing pure N2 through everything then pressurize to check for leaks THEN evacuate prior to charging.

VA-Merc · #7 05-18-2015, 05:07 PM

Thanks for the tips! This is not an R4 compressor...it's an OM603 diesel engine so it has the newer style compressor...haven't looked closely enough to see who makes it.

leathermang · #8 05-18-2015, 05:15 PM

Quote:

Originally Posted by vstech

Only way to make sure the system is dry with an r4 compressor is flushing pure N2 through everything then pressurize to check for leaks THEN evacuate prior to charging.

Good, I hate to be the only one suggesting ' over the top ' methods... which are actually just correct physics and prudence.... only seen as ' over the top' because so many people and shops cut corners ....
the N2 is used when people want to dry House AC systems ... and the SOP is a ' triple evacuation' ... .putting the N2 in and letting it sit a little time.... then taking that out...and putting new and obviously dry N2 another two times..
N2 is cheap ... and this really is the way to do it properly....
and why does all that moisture need to be out.... check the thread in my signature.... acids are made when oil and water and refrigerant are mixed which can cause bits of metal to come off the inside of your AC system... and of course the Bottle Neck for those kinds of things is your TX VALVE...
So a little preventative patience can make all the difference ..
and as has been mentioned... ANY time the system is opened up.. the FSM says to replace the receiver dryer.... it has a limited capacity and no way to check at what stage of that capacity it might have gotten to... and you AC system is well worth the cost of the R,D as insurance.....

leathermang · #9 05-18-2015, 05:17 PM

Quote:

Originally Posted by VA-Merc

Thanks for the tips! This is not an R4 compressor...it's an OM603 diesel engine so it has the newer style compressor...haven't looked closely enough to see who makes it.

EVEN SO.... if you are not fully flushing the system.... this is important for places where the oil pools.... see my signature ac thread.... if you want it to last a long time... with minimum cost to keep it going....

Maxbumpo · #10 05-19-2015, 11:26 AM

Quote:

Originally Posted by vstech

Only way to make sure the system is dry with an r4 compressor is flushing pure N2 through everything then pressurize to check for leaks THEN evacuate prior to charging.

x3! I would only add that the vacuum must be held for a bit to draw any water out of the oil. If you observe that the strength of the vacuum doesn't hold (and here is where a micrometer vacuum gauge is required) that is either water coming out of the oil (more vacuum!) or a leak.

This is very similar to what I have borrowed from a friend when needed:

http://www.grainger.com/product/SUPCO-Gauge-1TC70?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/1TC70_AS01?$smthumb$

funola · #11 05-19-2015, 12:24 PM

Does vacuum reclaiming a system remove 100% of the oil? If the vacuuming is done when the engine is off, it would seem the oil that is not mixed with the refrigerant would not be vacuumed out since it is sitting on the bottom of the compressor/ dryer/ evaporator. So it seems the only way to get the oil out is when the oil is mixed with the refrigerant and that is possible only when the compressor is engaged with engine running.

Maxbumpo · #12 05-19-2015, 12:30 PM

I think the fancy machines that an AC shop uses can reclaim some of the oil. I've only had the opportunity to work on systems which suffered total failure / loss of the refrigerant. One I took to a shop for an R-12 fill, the other two I filled myself. I've also worked on the HVAC heat pumps on my house, with help from a friend in the business.

The vacuum pump instructions that I had said to change the vacuum pump oil periodically, which leads me to believe that it was getting contaminated, possibly with oil from the AC system?

vstech · #13 05-19-2015, 12:31 PM

No.
Vacuuming the system doesn't remove ANY oil...

Reclaiming does remove oil that's in suspension with the refrigerant but nowhere near all the oil.

leathermang · #14 05-19-2015, 02:15 PM

Quote:

Originally Posted by funola

Does vacuum reclaiming a system remove 100% of the oil? If the vacuuming is done when the engine is off, it would seem the oil that is not mixed with the refrigerant would not be vacuumed out since it is sitting on the bottom of the compressor/ dryer/ evaporator. So it seems the only way to get the oil out is when the oil is mixed with the refrigerant and that is possible only when the compressor is engaged with engine running.

The only way to get all the oil out is to take the system apart and flush the individual pieces... The MB AC FSM gives very specific directions...

but the receiver dryer can NOT be dried in any fashion due to the properties of the dessicant bag and it being under oil.... it has a limited moisture holding capacity...and MUST be replaced... any time the system is opened up or there is a leak. I know the visual many have is that since the whole system is under pressure.... that a leak can be a one way deal... only letting stuff out...... but the fact is the system is DYNAMIC and variable ... and can suck moisture in where a leak occurs ... replacing receiver dryer is really cheap insurance....