The belt tensioner on most GM vehicles has either a 15 or 16 mm nut. Put a socket on a long breaker-bar and use it to back off the tensioner.
Use a wrench and place it on the large nut. Push the wrench like you are loosening the nut. The tensioner will rotate and remove pressure from your belt.
The belt tensioner should have a square hole in it either 3/8" or 1/2". Use the right ratchet and pull the tensioner away from the belt. It's spring loaded. Slip the belt off once you have the tension removed.
Loosen the tensioner bolt on your 1990 Buick LeSabre alternator. Slide the alternator to the left and remove the alternator belt. Remove the wiring harness from the alternator. Remove the alternator retaining bolts. Reverse the process to install the new alternator.
Insert a socket or wrench on the spring-loaded serpentine belt tensioner then pull back; the belt will loosen.
If it is the same 3800 engine as the 1993 Lesabre--very simple. Alternator is right on top of the engine. Loosen the serpentine belt ( tensioner) - remove the mounting bolts that hold the alternator on the bracket- remove the wire connections.
Loosen the tension on the 95 Buick Park Ave belt by moving the belt tensioner to one side. Using a socket and ratchet on the tensioner pulley, pull to the side an remove the belt. Install the new belt an release the tensioner to hold the belt into place.
Put the belt tensioner tool on the tensioner pulley, and push away from you. Then you can slide the belt off. You either need to cut it or unbolt the engine from its mount and slip it out.
The belt tensioner on a 1993 Buick LeSabre is located below and slightly to the left of the alternator if you are facing the passenger side of the car **Note there should be a diagram somewhere in the engine compartment, if not drawing one before removing the alternator belt for any reason will save you a major headache later on. GOOD LUCK
You move the tensioner's arm with a breaker bar. The map for the belt is usually glued to the radiator support, or underneath the hood. The schematic for a 1999 Buick LeSabre fan belt routing is in Section 6 page 66 of the owners manual.
The easiest way to loosen the tensioner is to use an automatic belt tensioner. This can be used on a serpentine belt which is connected to every pulley on the engine.belt.
it has a timing belt
I have used a socket with a ratchet handle. The ratchet will allow you to get more travel to move the pulley and get the belt off.