Hey,
Those tensioners are a total pain to replace. Your indicator should be all the way to the left (passenger) side mark with no tension. As you tighten it will slowly move to the right and should stop at the right most mark.
I bet your AC problem will not be solved with this new tensioner. Does your system have enough freon? Is the compressor turning at all? If yes, Do you see any bubbles in the sight glass? If the compressor is not turning, it could be the clutch is bad (this was my case), no freon, or AC Relay out. With the car running, jump the wires on the receiver/dryer --the cylinder with the sight glass. There are 2 sets of wires-one for fans, other set for the low pressure switch, touch them together for a second to see if the clutch engages. If it engages then the clutch is elimated. Has it been converted to 134A?

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1999 E300 TD
1999 E300 TD, WVO conversion
2007 Jaguar XJ8
1993 190E 2.3, Limited Edition, 239,000 miles (sold)
1991 300 SL (sold)
1977 MGB
1969 TR6
1980 TR8
joturbe@comcast.net