Why is the AC on older MB cars "weak"? - PeachParts Mercedes-Benz Forum

compu_85 · #1 05-26-2011, 06:22 PM

As I was stuck in traffic coming home today I was thinking... it's 100*F out according to the temp sensor on the bumper of my TDI, and the AC is spitting out glorious 41*F air with the fan on maximum and the resirc door closed. Turn off resirc and it would blow 60*. A 40*F drop at idle is quite impressive IMHO, moreso when you look at the 260,000 mile condenser and notice how many fins are bent over. I could feel the radiator fans kicking on and off of high speed. VW has used variable displacement compressors since 1993, so the clutch never disengages when the AC is un unless the evaporator starts to ice up.

From what I've heard others say I could not expect this kind of performance from my SDL, even if it still had R12. What about the Benz's system makes it have worse performance? Size of the evaporator or condensor too small? Compressor doesn't flow enough volume? In a perfect world could you install different parts to increase capacity? I'd love to find a way to install a variable displacement compressor so the clutch wouldn't have to cycle all the time.

I guess this delves a bit into AC theory, but that's something I'd like to know more about

-Jason

1980sd · #2 05-26-2011, 06:45 PM

I have a friend (Always starts like that huh

) who's done a few AC retrofits on US cars. He's a big fan of the GM V5 which is a variable displacement compressor. He's used it with success on systems which used the Delphi C4 compressor.

No experience with MB but he seems to think it would be a good replacement for the C4 in my W116. No low pressure switch required because it's variable displacement and will just stop pumping if the pressure gets low.

The V5 is also pretty compact and light plus you can find them with V belt pulleys.

Not sure if it would be better than a properly functioning C4 but it could be a decent replacement.

pj67coll · #3 05-26-2011, 06:53 PM

Quote:

Originally Posted by compu_85

I guess this delves a bit into AC theory, but that's something I'd like to know more about

-Jason

Me too because I live in Phoenix and have concluded that if I ever acquire another Mercedes which will be of 60's thru 80's vintage I will essentially try to completely rebuild the AC from scratch.

- Peter.

zeke · #4 05-26-2011, 07:51 PM

Just finished redoing my AC in the 300TD. I went ahead and installed a new condensor b/c the old one was pretty beat up after 30 years of sitting out in front of the car.

I reloaded with R12, and I would not call the system weak.

We had a heat index of 102 yesterday with high humidity, and I had to turn it down to the middle of the white part of the wheel b/c it was so cold.

I think old bent or dirty fins on evaporators and condensors, leaky old hoses, and bad r134a retrofits lead to that complaint.

Yak · #5 05-26-2011, 07:56 PM

If you read enough threads, some will say their factory OE AC with R-12 is ice cold, low 30's for vent temps and turns their hands blue. Maybe true, but most likely AC performance in the older cars is dependent on the condition and cleanliness of the components and the charge.

Thoroughly clean all the heat exchangers (radiator, condensor, evaporator, heater core), make sure there's no weirdness going on with the heater adding heat due to malfunctioning valves, then see what happens. That's probably easier than re-plumbing for a new compressor design, particularly since the refrigerant and air still has to use those same components regardless of the source of the compressed R-12.

jimandsuzy · #6 05-26-2011, 09:48 PM

My experience on my 85 300 SD is that it only got down to 60 degrees when I bought it with R134. About a year or so later the compressor started making noise so I took it to an ac shop for new compressor, exp valve, and dryer. While it was there I changed to a new parallel condenser since I had read of good results (lower temp) over the stock unit. Still only blew 60 degrees at the center vent. A few years later I changed refrigerant to propane/butane mix cause I read of lower temps. Still only gets 60 degrees so I gave up throwing money at it. For some unkown reason my 300SD ac is junk and extreme disappointment. 2 ac shops don't know anything else to do to it. Also, I know that the heat is off. New mono with 12v applied.
My 190E blows 48 to 50 degrees with stock parts and R134. Go figure.
I haven't fixed my SEC ac because of the results on the 300SD.
Your results may vary.

leathermang · #7 05-27-2011, 12:42 AM

Yak gets the prize.... but I am going to focus the cause a little sharper because he only listed the main one in a list of other important ones... ("Thoroughly clean all the heat exchangers (radiator, condensor, evaporator, heater core)" -Yak
When you have read enough threads on THIS forum and studied the physics of AC one of these stands out most.
While the condensor is by definition the bottle neck of all AC systems... meaning that with enough air flow across it many less than ideal performing components will not be noticeable .....
The one other item most neglected even in what people think are ' total ' renewing of our systems is THE EVAPORATOR cleanliness.
Here is the eye opening thread by Dmorrison ... the first the short one with the picture... and the second one why most people do not address getting it clean.... ( access )....

evaporator out of the 300TD

Order for installing evap box in a W123.

You can not be effective in cooling a car if the exact opposite end of the system ( and equally able to nix the cool air ) from the condensor is caked with stuff so that air can not flow across its fins...
All of the things Yak said are true and important. But from now on when reading what people have done to their systems and are complaining that it is not like new... think about the evaporator fins.
Greg

Quote:

Originally Posted by Yak

If you read enough threads, some will say their factory OE AC with R-12 is ice cold, low 30's for vent temps and turns their hands blue. Maybe true, but most likely AC performance in the older cars is dependent on the condition and cleanliness of the components and the charge.

Thoroughly clean all the heat exchangers (radiator, condensor, evaporator, heater core), make sure there's no weirdness going on with the heater adding heat due to malfunctioning valves, then see what happens. That's probably easier than re-plumbing for a new compressor design, particularly since the refrigerant and air still has to use those same components regardless of the source of the compressed R-12.

Hit Man X · #8 05-27-2011, 01:25 AM

http://www.ackits.com/merchant.mvc?Screen=PROD&Store_Code=01&Product_Code=6984TSI&Category_Code=coretreatment

I have been using that with good luck on my cars with higher miles. Some pretty nasty stuff drips out from under the car as the foaming action works.

I just figured clean condenser/evap core was a given. I should remember we are not all up to speed on mobile HVAC... I know I sure was not 10 years ago.

FWIW my SD puts out about 34°F from the vents. Not bad for the R4 on R12.

sd300td · #9 05-27-2011, 02:06 AM

I think they're a little weak in slow moving traffic or at a standstill (w123). But, like on a recent trip to Las Vegas, I had to turn the A/C off and/or turn the temp wheel up in order to keep from getting frozen. Oh, it's r134 too...

But yeah, my 05 does regulate temps better...

DeliveryValve · #10 05-27-2011, 02:58 AM

Well I agree that cleanliness is the main factor in getting an efficient a/c operation, I also think an upgrade to a parallel condenser and tinting the windows will do wonders.

You have to remember the TDI’s cabin square footage is much smaller then an SDL. So that mean the a/c has to remove heat from the seats, panels and etc. This is where having tinted windows will aid in keeping the interior from absorbing the sun’s heat while car is sitting in the sun and while driving.
Also having a clean evaporator as leathermang pointed out in the dmorrison writeups aids in the air flow to help remove heat from the interior parts. Adapting a cabin air filter for keeping dirt away from the evaporator is a good idea.

.

compu_85 · #11 05-27-2011, 07:54 AM

Quote:

Originally Posted by Hit Man X

FWIW my SD puts out about 34°F from the vents. Not bad for the R4 on R12.

This is my goal

-J

babymog · #12 05-26-2011, 09:54 PM

My 124s with R-12 when new had ice-cold A/C even idling outside a store waiting for the wife to "dash in and out" (usually took longer than that) or idling in traffic, they worked great.

The '94 and '95 with their native R-134a systems were also ice cold.

If yours isn't, it should be and needs an expert and probably an open checkbook to get it back to original performance.

vstech · #13 05-26-2011, 09:57 PM

MY 87 has been converted to 134 in the past, and it works just fine. it was upper 90's with near 100% humidity, I have to turn down the temp to drive... glass foggs up on the outside, or hands turn blue.
it's all about cleanliness, and components.

H-townbenzoboy · #14 05-26-2011, 09:58 PM

The truth is German cars simply didn't have the best AC systems back then. Sufficient? Yes, but not the best. A salesman at a MB dealer told me a story about when he worked at a BMW dealership. Some BMW executives came to Houston in the summertime and drove a rented Lincoln or Cadillac from the airport to the dealership with the AC on full blast. At the BMW dealership, they got into a new BMW and asked if the AC was on full blast because they were feeling a little hot. They were told "it is at full blast!"

I know my 82 300D's R12 system doesn't cool the car down as fast or as powerful as my dad's 02 Caddy, but it keeps me from breaking into a sweat in the summer and that's enough.

ashedd · #15 05-26-2011, 11:25 PM

Quote:

Originally Posted by H-townbenzoboy

The truth is German cars simply didn't have the best AC systems back then. Sufficient? Yes, but not the best. A salesman at a MB dealer told me a story about when he worked at a BMW dealership. Some BMW executives came to Houston in the summertime and drove a rented Lincoln or Cadillac from the airport to the dealership with the AC on full blast. At the BMW dealership, they got into a new BMW and asked if the AC was on full blast because they were feeling a little hot. They were told "it is at full blast!"

I know my 82 300D's R12 system doesn't cool the car down as fast or as powerful as my dad's 02 Caddy, but it keeps me from breaking into a sweat in the summer and that's enough.

Kinda hard to beat GM/Ford a/c.. Sometimes I wish I had a 4.6 crown vic sitting around JUST for the ice cold a/c.

On my TD I removed/cleaned/flushed/etc, new compressor(read: NEW), expansion valve, dryer, o-rings, aux fan. Confirmed no mono valve leaks... then charged with R12 after an appropriate vacuum. Didn't like the results so I re-purged and filled again with R12. I still think the a/c sucks.

My 400E had very cold a/c with the factory R134. I need another 400E