Ac temperature differential, what should it be? - PeachParts Mercedes-Benz Forum

JamesDean · #1 05-27-2012, 04:28 PM

Hey everyone,

I'm heading home from my cousins in my 420sel from Columbus and its 91-95 outside here according to my outdoor temp sensor.. My dial temp gauge in the center vents reports 60..were doing about 60-70 mph..

Is that a reasonable differential? Or should it be wider?

I added a bit of r12 before leaving as I suspected it was a bit low based on my readings. It was about 85f, aux fan on, low side was about 22, high side was 150-165. This was all at a high idle.

I wonder if the txv could be fault? High side seems a bit low for r12 with those ambients..

Air&Road · #2 05-27-2012, 10:45 PM

What car is this?

The a/c performance table for the W124 cars shows the vent temp for that ambient range to be only slightly below 60 degrees. I don't have the table close at hand, but that is about where it falls.

I don't totally understand why the vent is so high, but that's what they show in the table. When the ambient gets to about 80, you should be seeing only slightly above 40 at the vent. Try to check it at night or something when you have a lower ambient and see where it falls. If the vent doesn't come down then, then you might have a bad pod bringing outside air.

Good luck with it.

JamesDean · #3 05-28-2012, 07:18 PM

Larry, this is on a 89 420SEL.

I decided to check the pressures again today as it was 95F outside. The results put the system on the low side. It was only reading 15-18 Low, ~160 High at high idle condition.

I added more R12 until I achieved about 20-22 Low, 230-250 High at high idle. This gave me about 48-50 F at the vents. So 45-47 degree temperature differential. It should be nice and ice cold when 75-85F ambient temps hit I'd wager.

In my googling I found a book that might be helpful:

Automotive Heating and Air Conditioning - Mark Schnubel - Google Books

Around page 185 it has a nice section on diagnosing various conditions on R12/R134 orifice tube and TXV type systems. Might be helpful to some of you out there.

Air&Road · #4 05-29-2012, 03:47 PM

Good work James. The first vent temp and ambient numbers were a little worse than the performance chart shows for the 124, and the 124 book is all I have to go on. Your pressures sound on the money now, and of course your performance tells the real story.

I am a fan of the Master Cool issued book. I'm not sure of the author. It might be the same book.

Good luck

JamesDean · #5 05-29-2012, 04:59 PM

Quote:

Originally Posted by LarryBible

Good work James. The first vent temp and ambient numbers were a little worse than the performance chart shows for the 124, and the 124 book is all I have to go on. Your pressures sound on the money now, and of course your performance tells the real story.

I am a fan of the Master Cool issued book. I'm not sure of the author. It might be the same book.

Good luck

I did not something odd last night though, it was only 78F outside and I fully expected to achieve nice low temperatures based on the 95F outside, 48-50F inside temp.. At 78F, I only was able to get about 55F inside. It was comfortable but still struck me as odd.

Air&Road · #6 05-29-2012, 06:11 PM

It MIGHT have been that the system had brought the temp down where the temp wheel commanded it to be, so the system started cutting back to prevent it from being too cold. Remember it is an AUTOMATIC climate control system.

If you keep a thermometer in the vent and monitor it all the time, you will see many odd things while remaining comfortable. For me, this comes from monitoring vent temps on manual systems quite alot over the years. On those, turn it to max cool and it will just keep trying to cool until icicles drip off the vent. The ACC OTOH is supposed to keep it at the set temperature.

280EZRider · #7 05-30-2012, 03:57 AM

Not to sound like a broken record, but replacing the automatic CC with the manual CC on my car was the best thing I have ever done for it.

In some cases, human control is better than "high tech."

Air&Road · #8 05-30-2012, 02:25 PM

I understand what you're saying EZ, but the 124 ACC is actually pretty robust and when working correctly is one of the nicest operating climate controls you'll ever use. When it's working right, you just push the center button, the auto fan button, dial the temperature to what is comfortable for you and never have to touch it again Winter or Summer.

That said, it's very easy to understand how someone would consider a manual climate control would be a nice find.

JamesDean · #9 06-03-2012, 08:09 PM

I did end up finding the chart for the 126 a/c system:

Based on a 95F (35C) outdoor temp, one should see between, 55-66 F depending on humidity.

Based on a 77F (25C) outdoor temp, one should see between, 48-53 F depending on humidity.

Does my interpretation sound right to you?

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