This truck is a Kenworth 104. It's got a cummins signature engine. I'm replacing the alternator because as part of the service I pulled off the belt and spun the alternator. It sounded really rumbly and noisy the bearing sounded dry so I decided to replace it. Ideally you should disconnect the batteries because there is live power to the alternator. If you touch your spanner from the power wire to the chassis or even to the body of the alternator itself it will spark and earth out. Once your terminals are removed get a cable tie and tie them together.
One so that one terminal doesn't accidentally drop down and sit in the chassis rail where you forget about it and two so you don't mix it up between terminals. The other option if you don't disconnect the batteries is to be super careful and not let your spanner or wrench touch the chassis or the alternator or anything that it's going to earth out on. This is what I've done here you can see that I have put rag over the terminals so that they can't earth out anywhere. It just saves you a little bit of time not having to disconnect the batteries. Do the same removing the cables off the other terminal and cable tie it. Put some rags so that it doesn't earth out. You'll notice how careful I'm being not to earth out my spanner. Undo the bolts at the top and at the bottom. You will need to get a spanner and hold the nut on the other end. Once the bolts are removed wiggle the alternator out of the bracket. You may have to use a lever bar. The reason is this sliding push it clamps on the mount when you do up the bolt and holds it in firm.
When replacing any part I like to sit them side-by-side and just have a good look at it. See if you can spot any differences. You want to make sure that you have the right alternator for the job. The pulley needs to be swapped over. Get your rattle gun and undo the nut. Notice that the old alternator has an allen key in the shaft to be able to hold the shaft from turning when you're doing up the nut. This allows you to torque it to 70-80 foot pound as it is written on the fan. Unfortunately the new alternator doesn't have this allen key in the shaft so it is impossible to torque it. Leave the nut on a couple of threads, hold the pulley in one hand and hit the shaft with a dead blow soft faced hammer with the other hand. Some manufacturers recommend against this method they say that it puts pressure on the internal components and can damage them. But because we're replacing the alternator it doesn't matter. This pulley is very tight so I'm going to use a puller. You'll notice that I'm using these jaws upside down it's not ideal but it's the only way that you can get something in underneath to hold it. Wind the puller down keeping a close eye on the jaws, making sure that they're not slipping or damaging the pulley.
Fit the pulley on the new alternator making sure to line up the key way. You can lightly tap on the pulley with a soft faced hammer. Once again hitting it too hard would damage internal components. Fit the washer and the nut. Tighten it up with the rattle gun. With no allen key in the shaft this is your only option because it is impossible to stop that pulley from turning while you tension it. Don't try and put a screwdriver or something in the fan to hold the pulley because you're likely to break off one of the fins. Tightening it with a rattle gun is plenty good enough. There's a key way that stops it spinning on the shaft. The nut is just there to hold it in place. Fit your alternator, do up the top and bottom bolts. I always like to check the bolts after using the rattle gun just to make sure that they're nice and tight because you never know the battery could be starting to fail. Before fitting the belt spin the alternator make sure it spins freely and it sounds good and then spin all the idler bearings make sure they sound good. Fit your belt making sure that the ribs are all in the right spot.
Refit your cables, position the terminals in a way so that the wires aren't hitting on each other so that they're sitting nice and flat. Do the nut up tight. Watch that the whole terminal doesn't try and spin so you might have to hold the cable while you're doing it up. Be careful if you over tighten it you can break the terminal off in the alternator. Start the engine. Check that the belt is running correctly and then get your multimeter and check that the alternator is charging and doing its job. You should see around about 14 volts.