Engine Size: 3.0
Refrigerant Type: R12
Ambient Temp: 96
Pressure Low: 40
Pressure High: 275
Country of Origin: Germany
Newbee here so first order of business is to thank the supporters and moderators of the forum. I have learned a great deal from the posts.
Just a bit of history: I'm new to working with A/C systems but over the last couple of years, I have started working on the Mercedes A/C after several attempts at the shop ended with several compressors, a retrofit to 134 then back to R12. When the last compressor seized - I figured I could do about as good with diligence and patience. Had a few oops moments but now finally I am at the point where at least the compressor is compressing and the blower is blowing cold air. I flushed the system several times (back flush and forward flush) until the fluid came out without particles - condenser, evaporator, and hoses. I installed new compressor, TXV, receiver/drier, and compressor. I added 170 ml plus a little of mineral oil, per the FSM, UV dye, pulled a deep vacuum, and added about 43 oz of R12, 3 12 oz cans plus 7 more by weight (42 oz per the Haynes manual plus a little to account for hoses- the decal with the correct charge was removed in the R-134 conversion and not replace when converted back).
The reading above are at 2000 RPM, doors open, blower on max, min temp setting. The vent temperature was 64, dew point was 43 deg F and there were no bubbles observed in the sight glass. These pressures just seem a tad high based on the forum's temp/pressure chart. At idle the pressures are 260 & 63 with vent temperature of 76. Not the best cooling at a stop light but ok at speed. Since I have had trouble with compressors, I think the high pressures are an issue and as I am expecting 110 deg F days in July and don't believe the compressor will hack 300+ psi. Also I have not heard much praise of the R4 compressor nor Mercedes A/C in the 300SD. What I have gleaned from other posts is that perhaps the condenser air side is clogged or the freon side is fouled or blocked. I cleaned the condenser with my garden hose and "simply green" cleaner, straightened any bent fins, but really there was no obvious blockage. No significant change in pressures. I sprayed mist on the condenser and saw pressures of 190 and 48 at idle and jumpered the aux fan saw pressures of 240 and 60 idle.
I think the condenser is fouled on the inside probably with little bits of compressor or caked on desicant (I did note when I started on this journey, the sight glass was black and I wondered why I never could see anything in the sight glass - the shop never did replace the R/D but I paid for it) and I wonder if this seems right or if my observations indicate some other line of trouble shooting.
If "misting" the condenser dropped the high side pressures fast, then airflow may be the problem. Fist check that the aux fan is turning the right way, and I would give the engine fan a serious look too.. Might be time to change it.. Hope this helps..
Freedoms just another word for nothing left to lose
Thanks for the reply - this is stuff I didn't think to check.
I checked on the aux fan switch and the manual indicates the switch should close at 143 deg F or about 231 psia (217 pisg where I live). Originally, this was an actual temperature switch, but the newer receiver/drier has a pressure switch. With the change, I don't know what the pressure set point is and the web site store I bought it at does not specify. I know it kick on a few days ago, but don't know exact temperatures and pressures. Certainly this is a issue to check out since if the switch is set for 217 psig - it did not trip when I was seeing 260-270 psi on the gauges.
The main engine cooling fan is a viscus coupled temperature controlled clutch. I have heard it work, but it might take a little work to figure out if it is broken. Overall, I have not seen engine cooling issues - more or less always at the "normal" point on the temp gauge.
Misting the condenser, showed a fairly quick drop but it took a while, perhaps a minute or and repeated misting to stabilize at 190 indicated below. How fast and how much should I see before I suspect condenser blockage?
I'll go back and do some homework to see what I find out.
Edited: Wed May 27, 2009 at 11:26 PM by Fritz
Hi Fritz; Just curious---When you had a R-134 setup, did you change the condenser to an aluminum one, or did you retain the stock Mercedes unit? I think that you have debris still inside the old unit. If you did not change it, I strongly suggest that you do. Use a parallel flow after market, biggest one you can fit across the front. I did it on my old Chevy and the difference was incredible. It dropped high pressure way down, ran much cooler, hence did not pass hotter air to the radiator, which made the whole system cooler even in the hottest (once at 110 degree, high humidity) weather. Please consider it; it will make a big difference. Sure, you have non-stock inlet and outlet fittings on the parallel flow unit, but the folks here can make you short "jumper" connectors to adapt to your existing hoses. You may have a problem with other items, but this one change will really help! Also, you said the shop never changed the R/drier??? That is MANDATORY when a system has been opened, such as yours. My advice---go back to the dealer, raise hell about parts/labor that you paid for, and have them fix it! That R/D MUST be new. Keep in touch,OK?
Edited: Sat May 30, 2009 at 10:44 PM by fonebone
I think you are hitting on my basic question of condenser health. I have toyed with the idea that if I had to replace the condenser, that I would look a parallel flow - high capacity model. When I took the car into the shop, I was ignorant and innocent in the practical side of A/C and should have made sure the R/D was replaces, system flushed, etc. but alas not. Right now it looks like the aux fan is not kicking on - so I'm running that down. If I have to replace the temp/pressure switch - I'll have to open the system anyway and may just do the condenser and be done with it. Good input thanks.
I re-did the mist test and saw the pressure drop from 220 to 205 in 5 seconds - is this bad.
Also, I confirmed the fan temperature switch should activate at 143 deg F then I put an IR sensor non the fan switch and saw 143 deg F but the fan was not on. The fan runs when I jumper the switch, I think the wiring is good but the switch is bad. I see if I can figure out when it trips and how to work around this without having to open the system.
Hi Fritz; The drop when you mist sprayed the condenser confirms that your condenser is partially plugged/ inadequate. With the hot temps you will be getting you will need ALL the cooling possible from your condenser. The drop in temperature that you will get using this type will cause a corresponding drop in vent temps in the car, a lot less strain(due to less head pressure in the compressor) on the engine, and I'm positive will get you through the coming heat wave nicely. Please look seriously into installing one(the largest you can fit) in you car. And of course, get that receiver/drier replaced! Let us know how you progress, bitter?
Well I think I found the problem. As I was gluing a second temperature switch to the receiver/drier to provide a second input to the aux fan as troubleshoot step, I had remove the head light to get access ( is that weird or what?). This also gave me a good view of the condenser inlet and was then able to see a kink in both tubes. Please see the attached image - the view is looking to the right of the car through the spot where the left head light should be. Not sure when this happened in the 25 year life of this car, but I know that I was always careful to use a backup wrench while working with the connections. Interesting the bend is from torque 90 deg from the torque for the connections. Also a little suspicious is that following a trip to shop, the strain relief bolt for the inlet hose was not present, and I had to add the bolt with multiple washers to standoff the mounting flange to prevent strain on the hose. Regardless, I think I'll be shopping for a new condenser as I think the chances of un-bending the tubes is somewhere between slim and none. I'll see if a parallel flow is available and fits, but a stock one ought to bring back original performance. I will replace the receiver/drier, the temperature switch (since it is suspect), mineral oil (the manual indicates 90 cc for condenser and R/D replacement). Now back to the web to fill up my shopping card - again.
Hi Fritz; Good find! It looks like you are on the right path. Good feeling diagnosing things yourself and locating trouble spots! You are learning quickly. If you were happy with the stock system performance, then replacing you condenser should do the trick. Of course, any cooling fans included are top priority item. Keep in touch. Always a real pleasure to hear success stories.
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