DUTCH is right, an aftermarket radiator may not do the trick. I had a factory AC installed on my previous 300GE, the kit included a new BEHR radiator for increased cooling. You can not go by size or rows on these as you can with grandpas Olds 98. For example my Unimog has a tiny radiator, yet it cools a 5.7L diesel in tropical conditions under rediculous loads. Your G needs a factory designed radiator to cool properly, they are of a unique specification.

If you do have a BEHR radiator that seems to fit properly, look into the cooling fan operation. Often times aux fans run constantly when the AC is on to improve engine and cabin cooling.

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1995 G320
1984 280GE
1971 Unimog 416