Diagnosing AC unit grinding noise


Idling at the Lights
First Name
Mitsubishi Galant VR4
Hi all,

I've been trying to solve an annoying issue with my VR4's AC unit, and I suppose I'm searching for some guidance or confirmation of the issue and necessary repairs.

The unit (MSC90C, AKC201A204) continually makes this (very) loud metallic grinding noise after the clutch engages. I'm not particularly knowledgeable of how these units work, but hoping for a cheap fix I re-shimmed the air gap to within factory spec and can visually confirm that the clutch isn't slipping.

The pulley sounds normal when the AC is off, leading me to believe that the issue is internal, not the clutch bearing. If I hand turn the compressor using the clutch plate, I can feel that the rotation is not smooth, instead feeling 'gritty' and sticking in the same locations. (Although sticking may be typical of a scroll compressor?)

I don't have the tools or knowledge to completely disconnect and disassemble the unit to confirm, but assuming the compressor is damaged, what exactly must be replaced? I've found an (expensive) used ac compressor with the same part number, but if compressor damage contaminated the closed system with debris, then the receiver drier and condenser have to be replaced as well?

Can anyone confirm if this is the likely problem and fix? Any alternative causes with cheaper solutions are most welcome ;)


3 AYC Bars
Lifetime Member
New Zealand
First Name
1999 Legnum VR4 Type S
"The unit (MSC90C, AKC201A204) continually makes this (very) loud metallic grinding noise after the clutch engages"

If that doesnt scream the compressor is totally fucked i dont know what does.

Problem is its not as simple as just throwing a new pump at it cos the old pump has probably contaminated the TX valve, evaporator and the receiver drier with metal shavings so its likely a whole system overhaul.

Either way you need to degas the system and remove the compressor and assess it. If your very very lucky it might be a bearing and it hasnt pumped metal shavings throughout the system. But if the clutch isnt slipping causing the noise i dont like your chances.
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