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S65: rear climate control AC not working

5K views 17 replies 4 participants last post by  amstel78 
#1 · (Edited)
Just realized this afternoon that the rear climate control A/C doesn't appear to be functioning. Control panel works along with blower, but air coming out is warm. A/C up front works fine and the air is ice cold.

If memory serves, the M275 has a second compressor and separate circuit for the rear. I'm wondering if it needs a refill of R134. Can anyone please let me know where the A/C schrader valve for the rear is located?

I'm going to do a bit more googling and research, but if I'm also just missing the obvious, I'm all ears.

Edit: I'm mistaken about that 2nd compressor. Don't know why I thought that. Maybe it was the Maybach engine I was thinking of.
 
#6 ·
I asked the same question on MBW about whether or not a temp sensor exists for the rear. Didn't get a definitive answer. I did find the following video while researching last night, but based on my post above, I don't believe I have a sensor issue. Where did you a replacement thermistor btw?



I picked up a set of A/C manifold gauges and 3 cans of DuPont R134A with no additives included. It's an 11 year old car, so I can imagine some refrigerant loss has taken place over time. I'm going to top up the system next weekend. Hopefully that will solve my problem.
 
#5 · (Edited)
Well, I just got back from fiddling some more with the rear climate control. This is what I've discovered:

1. There's not much of a difference when setting temps from 68F to LO in terms of coolness. Outside air temps are presently 82F.

2. Bringing the temps back up from LO to 68F doesn't increase air temps;

3. Increasing one side.. in this case, the right rear passenger temp settings to a higher temperature of 78F does bring the air temperature of that side up accordingly. I kept the left side at 68F and there's a noticeable difference.

4. Reducing the right side temperature to match that of the left side does bring the temps back down but it does so quite slowly.

All of the above leads me to believe that the rear climate duovalve that's located in the front passenger wheel arch is functioning. However, the air coming to the rear just isn't as cold as the front. Additionally, the front air temps while cold (and I did say ice cold before but now that's probably an exaggeration), are really not that cold. They're comfortable and will cool the vehicle down on a hot day, but it's not arctic cold like a system running at its peak.

I will try an R134 recharge first. Hopefully the system is just a little low on refrigerant.

Quick question. The low pressure side for refilling is the one on the left in the picture below? It's also cold to the touch whereas the right side is warm. Usually the low and high pressure ports are differentiated by color coded caps, but in my car, both caps are black.
 
#9 ·
I bought 4 thermistors but each bag contained 10 so I have like 40. PM me your details and I will put a couple in the mail to you. If you prefer email me at Roger@928sRus.com.

That is the tutorial I used plus some additional info. Soldering the new thermistor in needs young as eyes so I had a friend do it. The thermistor is in the overhead light unit and easily accessed.

Follow the instructions on how to put the AC head unit into a diagnostic mode. The first of 100 data points is the thermistor. The thermistor was toast in my S55 and S65. We could only get cold air on the "low" setting. Now we set a temp and the system maintains it automatically.
 
#12 ·
Hi Wally - I have for a number of years now. Mercedes, BMW, Jaguar - mostly European.
I very quickly found out that every 928 owner has another car or two in their stable 8>).
I have not said much as I do not know what the posting etiquette is on this forum as yet.

I am certainly learning a lot about the S55 & S65 and so far pretty straightforward if you can turn a wrench.
 
#16 ·
Ambient temp was about 76F in the garage. Low pressure gauge read about 27 PSI. High side around 135 PSI. Seemed low given the temps but not knowing exactly how much refrigerant was in the system already, decided to add just one 12 oz can of R134a. After that, low pressure read 32 psi, high side about 140. Based on the pressure and temps alone, I could probably add another 12 oz and be safe. Now the rear AC vents did get colder, but I forgot my meat thermometer so I couldn't get an exact reading. But that said, the little refrigerant I had on hand did drop the output temps in the back. The front vents also got a bit colder. Based on my back of the hand readings, the differential between front and rear temps are probably only a few degrees at this point. I think the results are good and confirm my suspicion that after 11 years, the system just needed a top-up.

I'll have to check the system for leaks down the road; hopefully it's just some o-rings going bad.
 
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