My book does not give numbers for a 1985 300SDL. It does say that 1984 through 1986 should be running 24 to 34 on the low side and 230 to 275 on the high side. I would recommend getting the correct charge in there. Use caution running at the low pressure you have indicated - you could possibly ruin the compressor. My suggestion would be to evacuate it, replace the dryer and lube/charge to the correct capacities of both.

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Jim