Do you know what kind of refrigerant is in the system? If I am not mistaken this car was designed for r12 refrigerant. I am not familiar with the rules regarding r12 in Singapore but 134 refrigerant does not do a very good job cooling these older cars once the temp gets above about 90dF - especiallly in traffic.

Is it cycling on/off fairly often? It could be low on refrigerant.

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Jim