A/C recommendations for the MB vehicles - Page 32 - PeachParts Mercedes-Benz Forum

Maxbumpo · #**466** 08-31-2018, 09:32 AM

Yes that helps. I've been thinking about the recirc flap and how well that is doing its job. The vacuum pod for the center vent has failed on this car, I finally got around to hacking that (wired the door open) so I've got good airflow over the evap coil. How far behind are the other flap vacuum pods from failing? That is to be determined. I need to break out the HVAC section of the FSM and go through all the test procedures to make sure all the senors and the other vacuum pods are still doing their thing.

My vent temperatures only climb about 5 degrees at idle vs. moving, but the longer I sit and idle, the higher they go above that initial 5 deg.

Maxbumpo · #**467** 08-31-2018, 10:27 AM

Here's a nice video explaining how to do some basic system diagnosis using temperature readings.

https://youtu.be/wF37ihNBXq8

Diseasel300 · #**468** 08-31-2018, 10:30 AM

Quote:

Originally Posted by Maxbumpo

My vent temperatures only climb about 5 degrees at idle vs. moving, but the longer I sit and idle, the higher they go above that initial 5 deg.

That kind of behavior is to be expected, especially on a hot day, and double especially if the cabin hasn't completely pulled down. Remember that an A/C does not "make cold", it "removes heat". When you're moving, the compressor RPM is elevated which increases the capacity of the system due to mass flow of refrigerant. The engine fan blows harder, and the ram air effect of the car moving through the air dramatically increases the amount of heat that the condenser can reject. Airflow over the condenser is only part of the equation, the other part is the amount of refrigerant flowing through the system at any given time.

Idle is the lowest capacity that the system has. If the heat gain of the vehicle is more than it can handle, the vent temp will rise. Once the cabin pulls down completely (10-15 mins at least on a hot day), the vent rise will be significantly less at idle.

Window tint helps a lot too. Both the SDL and the Honda are dark colors (Midnight blue and black, respectively). The SDL has clear windows, the Honda has tinted windows. Even with the windows and sunroof cracked and a reflector in the windshield, it's not uncommon for the SDL to be 145-150˚ in the cabin at the end of the day after being parked in the sun. The Honda will easily be 15-20˚ cooler from the tint blocking out all that radiant heat.

Diseasel300 · #**469** 08-31-2018, 10:52 AM

Quote:

Originally Posted by Maxbumpo

Here's a nice video explaining how to do some basic system diagnosis using temperature readings.

https://youtu.be/wF37ihNBXq8

That's a very good video explaining how the temperatures relate to efficiency of the system. MVAC is very similar to HVAC, but often operates at much lower superheat settings and much larger ∆T.

It's important to note when testing MVAC systems that the doors/windows need to be open so that the system is cooling ambient air, not recirculating the cabin air.

As a comparison to that video using my own car earlier this summer:

With an ambient temp of 102˚F, I had a difference of 41˚ across the condenser (139˚ discharge temp), 40˚ across the evaporator (62˚ discharge temp), and a superheat of around 4˚ at the return line to the compressor (65-66˚ line temp). I took all those measurements for a friend trying to get the charge right on their own car with an unknown quantity of refrigerant already in the system, but suspected to be low. I don't have a fancy CPS tester like the video creator did, I used a pair of Type K thermocouples attached to BK meters at work.

JHZR2 · #**470** 09-03-2018, 12:41 PM

Quote:

Originally Posted by Yak

The A/C lines and fittings, even though they're on an otherwise metric car, are SAE sized. Having the right tools is also very important.

You really cannot assume anything when you're working on the A/C.

I’m primarily concerned about the W123, but other vehicles are relevant in this thread.

Is this everybodys’ observation on all Daimler vehicles of general interest here (115/123/124/126) that may have run R12?

I will have my system open and therefore will plan to replace all o-rings with new green ones and the appropriate nylog for R-12. But want to be double sure that I’m using the right sized ones. I assume the fittings and o-ring sizes are associated with each other?

JHZR2 · #**471** 09-03-2018, 12:45 PM

Quote:

Originally Posted by leathermang

The more usual way to check for leaks is to pressurize with Nitrogen and four ounces of R22 ( which the EPA allows you to vent legally )..
This would negate any potential oil compatibility issues .
And would usually cost less than having the R134a evacuated by a pro with the proper equipment charging for their time...

Generally I spare no expense to do a job right, but just thinking out loud... I have a Co2 extinguisher, a regulator and a good deal of r134a.

Would the pressurized leak test be viable with co2 and r134?

My understanding is that this would be done statically without the system operating, so oil miscibility and flow would not be a concern.

Just saves buying stuff I’ll hopefully never use again, and buying r22 (for which I have a license so I can procure, just like r12) merely to vent.

Thanks!!

ROLLGUY · #**472** 09-03-2018, 05:49 PM

Quote:

Originally Posted by JHZR2

Generally I spare no expense to do a job right, but just thinking out loud... I have a Co2 extinguisher, a regulator and a good deal of r134a.

Would the pressurized leak test be viable with co2 and r134?

My understanding is that this would be done statically without the system operating, so oil miscibility and flow would not be a concern.

Just saves buying stuff I’ll hopefully never use again, and buying r22 (for which I have a license so I can procure, just like r12) merely to vent.

Thanks!!

It is against the law to vent r134a, so I would just use the cO2 at around 250 psi to test for leaks.

Concerning O rings (above post), there are #6, #8, #10, and #12 used in these systems. Cars with the Denso compressor have a special manifold seal. Other than that, standard size O rings are used everywhere else.

Diseasel300 · #**473** 09-03-2018, 08:11 PM

Quote:

Originally Posted by ROLLGUY

Cars with the Denso compressor have a special manifold seal. Other than that, standard size O rings are used everywhere else.

Possibly later models. My '86 SDL has the factory manifold on it and uses standard O-rings.

JHZR2 · #**474** 09-03-2018, 11:58 PM

Quote:

Originally Posted by ROLLGUY

It is against the law to vent r134a, so I would just use the cO2 at around 250 psi to test for leaks.

Concerning O rings (above post), there are #6, #8, #10, and #12 used in these systems. Cars with the Denso compressor have a special manifold seal. Other than that, standard size O rings are used everywhere else.

Thanks very much!

It seems to be very difficult to find any set that has #6 size o rings. Do you recall where this size is used?

Thanks again!!

ROLLGUY · #**475** 09-04-2018, 12:05 AM

Quote:

Originally Posted by JHZR2

Thanks very much!

It seems to be very difficult to find any set that has #6 size o rings. Do you recall where this size is used?

Thanks again!!

On a W123 the drier has #6, Condenser a #6 & #10, and #12's for the suction line. The short hose from the hard line mounted on the motor mount arm is either a #8 or #10. The TXV uses #6, #10 & #8 (I may be wrong on the TXV, someone correct if needed). Either way, a complete HBNR O ring assortment is available from Harbor Freight. It has all the sizes needed and more.

Zulfiqar · #**476** 09-04-2018, 11:36 AM

Quote:

Originally Posted by JHZR2

Thanks very much!

It seems to be very difficult to find any set that has #6 size o rings. Do you recall where this size is used?

Thanks again!!

you can buy a correct vehicle specific O ring set from autozone etc, they are branded as santech and come in little baggies for about 5 dollars. They have all the rings including any special manifold seals.

I use these seal sets now on every car I do, no more hunting for correct sizes and the rings I get have an expiry date so I always get fresh rings for a fresh job.

JHZR2 · #**477** 09-04-2018, 03:29 PM

Quote:

Originally Posted by ROLLGUY

On a W123 the drier has #6, Condenser a #6 & #10, and #12's for the suction line. The short hose from the hard line mounted on the motor mount arm is either a #8 or #10. The TXV uses #6, #10 & #8 (I may be wrong on the TXV, someone correct if needed). Either way, a complete HBNR O ring assortment is available from Harbor Freight. It has all the sizes needed and more.

I can’t find an HNBR set that has #6. The other sizes, yes. Can get standard nitrile and viton sets there that have #6 size.

Maybe HF on your coast carries different models than mine?
Thanks!

ROLLGUY · #**478** 09-04-2018, 10:05 PM

Quote:

Originally Posted by JHZR2

I can’t find an HNBR set that has #6. The other sizes, yes. Can get standard nitrile and viton sets there that have #6 size.

Maybe HF on your coast carries different models than mine?
Thanks!

That is odd. I am sure the set available in your store is the same one in mine. I used up all the #6 rings in the kit, and bought 100 of them to re-stock my kit. I am sure Amazon will have a kit that will have all you need. Try them......Rich

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