Discovery 3 2.7 A/C compressor swap.

**veebs** · 28th February 2025, 09:44 AM

Originally Posted by DiscoJeffster

Don’t sleep. Be with us. We need you.

Haha, I fear for our future :P

**BradC** · 28th February 2025, 10:23 AM

Originally Posted by veebs

Is there a reason to suspect the dryer?

Dryers don't tend to fail anymore, they do saturate with moisture though. R134a tends to hydrolyze into an acid when it spends much time with moisture. Combine that with PAG oil being incredibly hygroscopic the role of the dryer is pretty important. It's always good practice to change the dryer when the system has been open for any length of time. A vacuum can't pull all the moisture from the oil.

**shack** · 28th February 2025, 11:05 AM

Originally Posted by BradC

Dryers don't tend to fail anymore, they do saturate with moisture though. R134a tends to hydrolyze into an acid when it spends much time with moisture. Combine that with PAG oil being incredibly hygroscopic the role of the dryer is pretty important. It's always good practice to change the dryer when the system has been open for any length of time. A vacuum can't pull all the moisture from the oil.

I know LR specIfy PAG oil must be used, but what is the benefit of that over the numerous others available?

Considering a number of newer oils claiming to be "virtually non hygroscopic".

I recently cut apart a D3 dryer to see how they operate, incredibly basic.. They have a filter medium to catch bits and a desiccant like you would find in the suspension compressor on D3.

The desiccant can obviously reach saturation level and would then fail to do anything, leaving moisture going around the system.

Considering the simplicity of working on a D3 or D4 AC, replacing the drier as you mentioned is a no brainer, as it's the only easy bit to do.

**BradC** · 28th February 2025, 12:22 PM

Originally Posted by shack

I know LR specIfy PAG oil must be used, but what is the benefit of that over the numerous others available?

Nothing really. PAG was the first cheap "oil" that they could make R134a actually transport. The original PAGs were awful and would hydrolyse as badly or worse than the refrigerant. The newer "double end capped" PAGs are a bit more stable.

Basically the compressor manufacturer specifies PAG, so that's what you use. Any of the later "ester" based oils are ok provided they can be transported by the refrigerant. What you don't want to do is mix oils, so if you do intend to change you need a thorough flush first.

PAG is no good in proper refrigeration systems as it's problematic around copper, but as there's no copper in a vehicle A/C and it's cheap to make that's what they use.

Thread: Discovery 3 2.7 A/C compressor swap.

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