This post documents replacing the alternator belt on my 2001 BMW R1150GS. Newer R1150GS’s use a slightly different belt and pulley, but the replacement procedure is similar. The major difference is the tensioning of the ELAST belt requires a different tool and procedure.
The tools I used:
- 3/8” torque wrench
- 3/8” socket wrench
- 4mm allen key socket
- 6mm allen key socket
- 13mm wrench
- 13mm socket
- 3/8” short extension
1993 through 2003 50A alternators:
Poly V: 4PK611, BMW part 12 31 1 341 779
ELAST: 4PK592, '04-on Oilheads and most Hexheads,
BMW part 12 31 7 681 841
ELAST: 4PK582, R12RT/Adv, BMW part 12 31 7 690 57
Stock BMW Belt for my bike:
1) There are four nuts holding the alternator cover on. Remove them with the 4mm allen key socket.
2) The cover should pull out slightly and then down.
Alternator belt exposed:
3) Using the 13mm wrench, loosen the three bolts holding the alternator.
4) Pull the alternator down to put more slack on the belt if it didn’t slide down itself.
5) Pull the belt off
Old vs New belt:
Mine was apparently past the 36,000 mile replacement. It was making a bit of a noise. Pieces of the belt had started to come off inside. Checking the belt tension is critical to belt life. Check when you change the oil.
Notice the four ribs on the belt. They match up with the ribs on the pulley. Make sure to line them up when reinstalling the belt.
6) The new belt just barely slipped back on.
7) BMW has very specific belt tensioning specifications. Don’t be tempted to treat this like a car belt and do it by feel.
8) I had to turn the direction adjuster on the torque wrench to “loosen” not the tighten motion to get the alternator to lift.
9) Using the 6mm allen key socket, extension, and torque wrench tighten the adjusting nut to 8 N-m. This will raise the alternator and pull it against the belt.
I found I needed to lift/tilt my gas tank slightly to have room with the torque wrench. See my air filter replacement guide for tips/photos.
BMW makes an adjustment tool that goes on the front rather than the back. Much Easier! I will think about getting it for the next tightening.
Adjuster for Poly V belts: tool 12 3 551, part 83 30 0 401 69
10) While holding the torque wrench, use the 13mm wrench and tighten the other two nuts holding the alternator.
11) Using the torque wrench and 13mm socket, torque all three nuts to 20 N-m.
12) All three tightened up. Use the starter to crank over the engine a couple times, but don’t start it. This will help seat the belt on the pulleys. Loosen the bolts and repeat steps 7-11.
13) Replace the alternator cover and tighten the four bolts with the 4mm allen key socket. The cover is plastic, so be careful not to over tighten.
14) Cover replaced
15) Check the belt again in 6,000 or the next time you change your oil.
16) Go for a ride!
Good pictures/discussion of the Poly-V vs ELAST here.