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I have a 2018 GL 450 with only 10K miles. The car was parked in my garage with the main battery disconnected for 18 months during the Covid. When I started the car for the first time in 18 months, I noticed the Auxiliary battery needed replacement and also the AC was was not cooling. I got the Auxiliary battery replaced but the AC problem persisted. According to my mechanics, the compressor is not receiving signal/power and and therefore needs to be replaced. I am not convinced this is the correct diagnosis for the AC problem and need help!
 

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I have a 2018 GL 450 with only 10K miles. The car was parked in my garage with the main battery disconnected for 18 months during the Covid. When I started the car for the first time in 18 months, I noticed the Auxiliary battery needed replacement and also the AC was was not cooling. I got the Auxiliary battery replaced but the AC problem persisted. According to my mechanics, the compressor is not receiving signal/power and and therefore needs to be replaced. I am not convinced this is the correct diagnosis for the AC problem and need help!
Have you tried seeing whether the switch/assembly that activates the A/C on the dash is bad?
 

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1. Have a specialist check refrigerant charge and proper function at hi/lo pressure hoses. Must comply w/MB Sticker installed in engine bay. Test should also detect clogged filter (usually located driver side, under dash, by steering column) or issues w/AC coolant section (I forget its tech name) by radiator. You also have an aspirator motor for sampling air temperature - identified by small opening by ignition key or by overhead reading lights above dash. Opening s/b unobstructed.
2. Drive around a while. Many of today's electronic modules rely on drive cycles. Once that requirement is satisfied there is automatic system reset; applies to power windows, sunroof, CEL caused by loose fuel tank cap, etc., etc. I am not sure - simply 'guessing' here, w.r.t. your A/C issue. When batteries were 18-month non-functional, all memory was erased.
 
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