This page was originally intended for serpentine belt replacement, but it also applies to idler pulley and tensioner pulley replacements.  This page was completely revamped in early September '06.

My experience says the tensioner pulley should be replaced along with the belt every 30 Kmiles, or every two years or so.  The fixed idler pulley might go 60 K miles or every other belt change, but I don't like to risk it.  If the belt or either of the idler pulleys fails, you really only have one choice between two very unsavory options: Stop the car and shut off the engine immediately, or keep driving the car until it overheats and you blow a head gasket and/or warp the head, or destroy the engine completely.  And oh, by the way, if you decide to keep driving, you'll be trying to do so without power steering and you'll get a real fine appreciation for just how front-heavy the Saab is.  Put simply, you really don't EVER want to experience a belt or pulley failure.  It happened to me once with what I thought was a good pulley.  I was very fortunate in that I was only a mile or so from home, and passing by a Home Depot at that - I was able to prowl Home Depot as never before, as I needed a couple of hours to allow the engine to cool.  Then I was able to nurse the car home before it could overheat.  Even at that, engine overheat was a real risk .  And the whole thing was an extreme inconvenience.  Cost, if you replace the belt and both pulleys: about $100.  That's $50 a year.  Less than the cost of a tow, let alone a head gasket replacement. And by the way, if an aftermarket parts place offers cheap and less cheap pulleys, I'd get the more expensive ones.  No, the pulleys are not interchangeable - the tensioner pulley's hub is thicker than the fixed pulley's hub.

Here's a picture and description of the belt routing.  

To install the serpentine belt,  you must remove the pulley from the tensioner assembly bracket (hereafter called the tensioner pulley, as opposed to the fixed idler pulley). 

To remove and replace either the serpentine belt or the tensioner pulley, you must keep the belt tensioner compressed; to do this, you will need a tool to hold the parts in place.  Tool specs.  The tool is absolutely necessary, as the tensioner will separate and be ruined if you try to remove the tensioner pulley nut without having the tool in place.  This is because you must pull forward on the wrench to remove the left-hand-threaded tensioner pulley nut - without the tool in place, this will put a pulling force on the tensioner and it will pull apart and be ruined.  Here's how the tool is used.

The tensioner pulley bolt has LEFT-HANDED threads - DO NOT ATTEMPT TO LOOSEN OR REMOVE THE BOLT BY TRYING TO TURN IT IN THE "NORMAL" LOOSENING DIRECTION.  Picture of the bolt.  The left-hand threads allow you to put a socket on the pulley bolt (19mm), driven by a long handled breaker bar, and push the pulley toward the back of the car without the nut coming loose; this compresses the tensioner.  

Did I mention that the tensioner pulley bolt has LEFT-HANDED threads and that you should NOT ATTEMPT TO LOOSEN OR REMOVE THE BOLT BY TURNING IT IN THE "NORMAL" LOOSENING DIRECTION?  Did I mention that?  

And that the retainer tool is absolutely necessary if you're going to remove the tensioner pulley, as the tensioner will separate and be ruined if you try to remove the tensioner pulley nut without having the tool in place?

Did I mention those two things?  OK.  Don't write me and tell me I should be more explicit.

Using a long 1/2" breaker bar from above and a 19 mm 12-point socket on the tensioner pulley bolt head, push slowly and firmly - pushing fast will just increase the resistance and make it more difficult.  Use a 12 point socket so you can get better wrench positioning - a 6 point socket isn't fine enough.  Start with the breaker bar as far forward as you can position it and still get the socket in place.  You can have an assistant push the wrench while you work from within the wheel well (car on a stand, wheel removed, inner fender liner removed  (fender liner removal instructions).  When the tensioner is fully compressed by pushing the breaker bar as far to the rear as it will go, your assistant can gently release pressure on the wrench as you position the retaining tool.  Picture.  If you cannot get an assistant, you can use a heavy duty bungee strap (the solid rubber kind), about 18" long, to secure the wrench in the aft-most position - secure the strap on the torque arm, pull it taut, wrap it around the end of the breaker bar handle, then secure the other end to the torque arm.  Picture.  This isn't easy, and it's dangerous - if something slips, the breaker bar will launch into you or your surroundings, and it will do so with one hell of a lot of stored energy.  I'm serious - you're much better off with an assistant.  If you try to do this yourself, you're going to have to be strong, resourceful and damned careful.  Once the assembly is tied down, you can go into the wheel well and install the retaining tool (it should just fit into position in the notches provided).  Now you can release the wrench and bungee strap.

Because the belt loops through the 'U' formed by the tensioner pulley bracket, you must remove the tensioner pulley to remove the belt.  Remove the bolt from the tensioner pulley (turn the bolt to the RIGHT, or toward the front of the car - because the bolt has left-hand threads, it loosens in the opposite direction of a "normal" bolt).  Remove the bolt, then the pulley and then the belt.  Inspect both pulleys by spinning them; if either pulley sounds or feels the slightest bit rough, it should be replaced (see the dire warning at the top of the page).

Belt replacement: Drape the belt over the top two pulleys (alternator and AC compressor).  You might have to twist it to get it past the retainer tool. Route it past the front of the water pump pulley, then let the slack hang down to the ground.  Poke the left side forward into the space between the alternator and steering pump pulleys, then pull a loop forward and loop it around the fixed idler pulley (the smooth back side of the belt contacts this pulley).  Picture.  Gather the bottom of the belt below the steering pulley in a loop and pull it up between the steering and crank pulleys, beneath the fixed idler and into a loop between the sides of the tensioner pulley bracket.  Picture.  Slip the tensioner pulley into place and install the bolt (turn it counterclockwise/left to tighten the left-hand threads).  Use the breaker bar and socket to snug up the bolt by pushing toward the rear.  When it's just snug, release pressure and reposition the breaker bar just a little less than full forward.  You're going to push the wrench back to tighten the nut, then you'll release the holding tool and let the breaker bar come forward to take up the slack in the belt - if you start out with the breaker bar too for forward, you might run out of room and get the breaker bar jammed up as you release the slack.  Pushing against the force of the tensioner should be enough to tighten the bolt, but you can give it an extra, quick jerk to cinch it a bit tighter if you want.  Release the temporary tool, slacken the wrench and remove it.

IMPORTANT: Carefully inspect the belt to make sure it's centered on all the pulleys.  It's very easy to get the belt off-center and up on one of the sharp shoulders of a pulley.  If you do that the belt will appear to work fine, but it last only a few miles and then fail.  Trust the voice of experience from the big dope who didn't check his work carefully once.  Once again I was very lucky - it failed as I pulled into a parking space at work the next morning.  Inconvenient, but manageable.  (Did I mention that I always carry an old belt as an emergency backup?) (And the restraining tool?)  (And a small but full set of sockets and wrenches, and pliers, and a screwdriver, and duct tape and Vice Grips and baling wire and ...)