... going to do the accessory belts on my 240D in the next few days and was wondering about setting the tension ... is it OK to do it by feel, or is there any advantage to using a tool? If you do it by feel, is it pretty much common sense as to what the appropriate tightness is?
Right now it has the wrong belt on the A/C compressor and its squealing. Other two belts look OK but I'm replacing them anyway due to unknown vintage.

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)