The a/c compressor on my wife's 98 ML320 is cycling on and off about every two seconds. Thinking it was low on refrigerant, I added about half a can (6 oz) and it continues to do the same thing. I don't have my guages with me, so I'm at a bit of a disadvantage on checking operating pressures. I'll be home in a week but it's pretty warm in south FL right now. Any ideas? Should I add additional refrigerant? Thanks....

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99 E300 133K
98 ML320 99K
87 Saab 900 Turbo Conv 123K