AS glmoy says loosen the big bolt on the tensioner pulley assembley I think its an 18mm. Then you must loosen the top hollow bolt near the water pump to releive the tension. Now that the easy part is over. Take off the fan shroud assembly. Examine Carefully how the belt threads through the pulley system and make sure the belt diagram is still on the car or is available. Be patient, as it will take you a few attempts to get it right. I have done this a few times and start at the crankshaft and thread the belt on the power steering side first and the alternater side second. Once I have it about right pull hard on the belt to stretch it a bit and pop it over the top pulley on the alternater side. Good luck. Have some cold ones on hand for after the job is done.

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1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T