Digital Corpus
Top Post Dawg
This is a thread for the culmination of info to rebuild the Bosch 123-505 alternator in the B4 Passat that is rated for 120 amps. If any other TDI alternators are wanted to be placed. There is a lot of scattered information around online and not every place you hunt has all of the details. It's usually encompassing the voltage regulator, the carbon brushes and slip ring, the bearings, or a mix thereof, but not all of this. Any new information towards rebuilding these alternators will be updated in this post when possible. This first post will be specs and details, the second post for parts, and the third for and index tutorials/how-tos.
When first searching this, I came across one thread on the forums:
http://forums.tdiclub.com/showthread.php?t=408651
For general part number references, woodauto.com has been cited:
http://www.woodauto.com/Unit.aspx?Man=BOSCH&Ref=0123505011
This thread is my continuation of the sort with more detailed specs for the bearings and carbon brushes that I've dug up over the past few months.
Bearings
As stated in the aforementioned thread, the bearings in our particular alternator are 6203 and 6303, which are 17x40x12 mm and 17x47x14 mm respectively. Reputable brands are always suggested over the forum-known C.R.A.P. and the bearings should preferably be sealed on both sides, which limits their maximum RPM. From what I've read, such brands in no particular order are Boca, OSR/AST, FAG/INA/Schaeffler, KML, Koyo, Nachi, NTN/SNR, SKF, and Timken. Most sealed bearings are rated for 11,000-15,000 RPM from what I've been able to gather.
But what RPM rating do we need???
The answer to this is really very simple if you have caliper or measuring tape. Since we're working with a pulley system, we just need the ratio of the harmonic balancer to the alternator pulley. Now, you can find the specs of similar HBs online, but to be precise the circumference/diameter we're after is where the lays, not the OD of the edge portion to keep the belt in check.
I measured ~20.25-20.325" for the circumference of the HB, which is the updated one from idparts.com FYI. This gives a diameter, averaged between the two, of 164 mm. My caliper reports ~49.5mm for the diameter of the Gates clutched pulley. This means that the alternator is being driven at ~3.313:1 compared to the engine. Set a redline of 5000 RPM, the alternator is spinning at ~16.6K, and for a 6000 RPM engine this means almost 20K for the alternator.
Please keep in mind that if you see just one max speed rating for a bearing listed, I'd imagine its safe to assume it's the "oiled", not "greased" speed rating. The latter will always be lower and I've seen disparities of 8K RPM, i.e. 20K for oiled and 12K for greased. Now, I've spent hours trying to go through various catalogs, but I've not found one as comprehensive as Boca's searchable catalog.
Now, if you've looked around at various manufacturers' sites, you'll note various C0/C3/C5 or even C2 tags onto the part number/name. This is the rating for radial clearance. Since both bearings on this alternator are interference fit, choose a C3 or C5 bearing. One with a C0 rating has tighter than normal clearances and C0 is standard. If a standard C0 bearing is used, its possible that the positive radial clearance will be completely taken up or maybe not be enough, thus hastening a short life for the bearing.
Voltage Regulator
When the voltage to our alternator drops to below 14-14.4V when, it typically means that our VR is "bad". Usually meaning that the carbon brushes have been worn too far and are not making appropriate contact with the slip ring. It is common practice to replace the VR with a new one from any one of our reputable vendors. However, if you have a drill and a soldering iron, there is a less expensive option.
VR Brushes
As just mentioned, the brushes on the VR are primary part that needs to be replaced on our alternators in order to keep them running and charging our batteries. Instead of spending $30-$60 on replacing your VR when its brushes have worn off, most have either picked up new ones or scavenged one from compatible alternators in the junk yard. However, it is possible to replace the brushed on the VR with a little elbow grease if you have a drill and a high capacity soldering iron.
The brushes in my Huco VR are 4x6x19mm. If you measure from the press fit braided wire, it's 16mm. However, the depth of the socket in the Huco VR for a brush is 1" and it will accept 1/4" wide brushes as long as they are less than 0.17" thick. In metric this equates to 6.35mm x 4.32mm maximum dimensions. With the slip ring worn down, you need ~0.25" of the brush protruding from the VR in order to make contact, thus you need to find a replacement that measures no less than 1.25", ~32mm, combined length of the brush and wire/shunt and spring length. The uncompressed spring length is ~36.5mm or ~1.437".
Slip Ring
This is what the brushes make contact with. It is a wear part, but from the 3-4 slip rings I've seen, they have a fair amount of meat on them and will last 200K miles or more. In order to replace the slip ring, you need to remove the smaller bearing on the alternator. It's relatively easy to press on/off either of these, though the soldering of the terminals on the stock SR is a bit tedious. This is because the wires are pressed on and lightly soldered. I'm not sure of the method specifically, but having a 30w soldering iron, patience, and a small flat head screwdriver got them free. I resoldered the new ring on with SAC-305 solder.
Alternator Pulley
If you've come across this thread, you probably know that we have a solid pulley from the factory and that the clutched pulley from the MK IV Jetta will fit our alternator. This is a worthwhile upgrade to reduce wear on the accessory belt's hardware and the alternator itself. If you're looking for OE PN's regarding the solid pulley, you'll have to look elsewhere at this time. Anyhow, a clutched pulley alloys the alternator to be spun in one direction. Until it fails, which it then becomes a solid pulley effectively, the backlash from the accessory/serpentine belt isn't transferred into the alternator. This consequently also reduced the amount of load on the serpentine tensioner spring and reduced the dynamic load on the bearings in the system.
There are 2 pulleys that fit onto our alternator, though only one of them is proper and that is the Gates 37010P. However, it is $70-$80 and I already had a failure on the one I've used. The alternative is the ALH clutched pulley but it has 2 technical downsides that should be made known. The largest and most significant is a simple one where instead of having a diameter of 49.4 mm for the belt, it has a diameter of 55.6 mm. This lowers the alternator's RPMs by ~12%. If you do a lot of short trips, this can impact your battery life in the long run. The second difference is the offset. Though it's a 5 mm difference, depending on how your shaft was cut, it can be low like mine and only stick out ~2.5 mm. The closest pulley to the alternator is the air conditioner and a little trig says that this offset creates a hair over 1° angle on the belt due to the offset. So why would you use the ALH pulley? Well, the INA version of it is half the cost of the Gates pulley at most. The other upside of spinning the alternator slower is longer life of all rotational components. Call me biased, but I do long enough drives that the RPM difference matters not and the cheaper pulley makes everything cheaper to maintain.
Tensioner Pulley
Though not a part directly from the alternator, it's within close proximity and as of late, several individuals have had failures. Don't let that worry you as this is a part that is supposed to be swapped with every timing belt and won't typically fail with this change out frequency. However, due to it's construction, it is a throw away part and this doesn't sit well with me for long term maintenance. There seem to be some alternates available but these are tentative as they require documentation as to what needs to be changed or added to let them work.
Amazon
You'll notice that a good portion of the links below are from Amazon.com. This was done so as the part is readily available there and if you happen to be one of the many Amazon Prime members, it's likely that you'll be able to get the part without having to pay shipping and quicker than most other sources. Effort has been taken to try and ensure all links are valid, but you're encouraged to lookup the references yourself in case I become delinquent in keeping this thread up-to-date.
Cost
You've probably noticed that "lifetime" alternators from your local parts store have a tendency to not last long in our cars. This is why identifying the individual parts is so significant. All in all, a full R&R of the alternator will cost:
To-do List
Voltage regulators and carbon brushes need to be compiled and evaluated along with the clutched pulleys and metal tensioner rollers. These will be added when time permits.
When first searching this, I came across one thread on the forums:
http://forums.tdiclub.com/showthread.php?t=408651
For general part number references, woodauto.com has been cited:
http://www.woodauto.com/Unit.aspx?Man=BOSCH&Ref=0123505011
This thread is my continuation of the sort with more detailed specs for the bearings and carbon brushes that I've dug up over the past few months.
Bearings
As stated in the aforementioned thread, the bearings in our particular alternator are 6203 and 6303, which are 17x40x12 mm and 17x47x14 mm respectively. Reputable brands are always suggested over the forum-known C.R.A.P. and the bearings should preferably be sealed on both sides, which limits their maximum RPM. From what I've read, such brands in no particular order are Boca, OSR/AST, FAG/INA/Schaeffler, KML, Koyo, Nachi, NTN/SNR, SKF, and Timken. Most sealed bearings are rated for 11,000-15,000 RPM from what I've been able to gather.
But what RPM rating do we need???
The answer to this is really very simple if you have caliper or measuring tape. Since we're working with a pulley system, we just need the ratio of the harmonic balancer to the alternator pulley. Now, you can find the specs of similar HBs online, but to be precise the circumference/diameter we're after is where the lays, not the OD of the edge portion to keep the belt in check.
I measured ~20.25-20.325" for the circumference of the HB, which is the updated one from idparts.com FYI. This gives a diameter, averaged between the two, of 164 mm. My caliper reports ~49.5mm for the diameter of the Gates clutched pulley. This means that the alternator is being driven at ~3.313:1 compared to the engine. Set a redline of 5000 RPM, the alternator is spinning at ~16.6K, and for a 6000 RPM engine this means almost 20K for the alternator.
- If you're the easy going TDI driver who never goes over 3500 RPM, then you can pick nearly any sealed bearing that is rated for 12,000 RPM.
- But say your turbo is clean(er) with occasional spirited driving topping off with 4500 RPM, then the average bearing rated for 15,000 RPM bearing will be fine.
- Though if you're more *spirited* than that, but don't ever plan on taking your TDI beyond 5000 RPM, then a bearing rated for 17,000 RPM is perfect.
- Lastly, if you're the type who's not afraid of 5500-6000 RPM your choices become limited to needing ratings for 18,200 RPM to 19,900 RPM.
Please keep in mind that if you see just one max speed rating for a bearing listed, I'd imagine its safe to assume it's the "oiled", not "greased" speed rating. The latter will always be lower and I've seen disparities of 8K RPM, i.e. 20K for oiled and 12K for greased. Now, I've spent hours trying to go through various catalogs, but I've not found one as comprehensive as Boca's searchable catalog.
Now, if you've looked around at various manufacturers' sites, you'll note various C0/C3/C5 or even C2 tags onto the part number/name. This is the rating for radial clearance. Since both bearings on this alternator are interference fit, choose a C3 or C5 bearing. One with a C0 rating has tighter than normal clearances and C0 is standard. If a standard C0 bearing is used, its possible that the positive radial clearance will be completely taken up or maybe not be enough, thus hastening a short life for the bearing.
Voltage Regulator
When the voltage to our alternator drops to below 14-14.4V when, it typically means that our VR is "bad". Usually meaning that the carbon brushes have been worn too far and are not making appropriate contact with the slip ring. It is common practice to replace the VR with a new one from any one of our reputable vendors. However, if you have a drill and a soldering iron, there is a less expensive option.
VR Brushes
As just mentioned, the brushes on the VR are primary part that needs to be replaced on our alternators in order to keep them running and charging our batteries. Instead of spending $30-$60 on replacing your VR when its brushes have worn off, most have either picked up new ones or scavenged one from compatible alternators in the junk yard. However, it is possible to replace the brushed on the VR with a little elbow grease if you have a drill and a high capacity soldering iron.
The brushes in my Huco VR are 4x6x19mm. If you measure from the press fit braided wire, it's 16mm. However, the depth of the socket in the Huco VR for a brush is 1" and it will accept 1/4" wide brushes as long as they are less than 0.17" thick. In metric this equates to 6.35mm x 4.32mm maximum dimensions. With the slip ring worn down, you need ~0.25" of the brush protruding from the VR in order to make contact, thus you need to find a replacement that measures no less than 1.25", ~32mm, combined length of the brush and wire/shunt and spring length. The uncompressed spring length is ~36.5mm or ~1.437".
Slip Ring
This is what the brushes make contact with. It is a wear part, but from the 3-4 slip rings I've seen, they have a fair amount of meat on them and will last 200K miles or more. In order to replace the slip ring, you need to remove the smaller bearing on the alternator. It's relatively easy to press on/off either of these, though the soldering of the terminals on the stock SR is a bit tedious. This is because the wires are pressed on and lightly soldered. I'm not sure of the method specifically, but having a 30w soldering iron, patience, and a small flat head screwdriver got them free. I resoldered the new ring on with SAC-305 solder.
Alternator Pulley
If you've come across this thread, you probably know that we have a solid pulley from the factory and that the clutched pulley from the MK IV Jetta will fit our alternator. This is a worthwhile upgrade to reduce wear on the accessory belt's hardware and the alternator itself. If you're looking for OE PN's regarding the solid pulley, you'll have to look elsewhere at this time. Anyhow, a clutched pulley alloys the alternator to be spun in one direction. Until it fails, which it then becomes a solid pulley effectively, the backlash from the accessory/serpentine belt isn't transferred into the alternator. This consequently also reduced the amount of load on the serpentine tensioner spring and reduced the dynamic load on the bearings in the system.
There are 2 pulleys that fit onto our alternator, though only one of them is proper and that is the Gates 37010P. However, it is $70-$80 and I already had a failure on the one I've used. The alternative is the ALH clutched pulley but it has 2 technical downsides that should be made known. The largest and most significant is a simple one where instead of having a diameter of 49.4 mm for the belt, it has a diameter of 55.6 mm. This lowers the alternator's RPMs by ~12%. If you do a lot of short trips, this can impact your battery life in the long run. The second difference is the offset. Though it's a 5 mm difference, depending on how your shaft was cut, it can be low like mine and only stick out ~2.5 mm. The closest pulley to the alternator is the air conditioner and a little trig says that this offset creates a hair over 1° angle on the belt due to the offset. So why would you use the ALH pulley? Well, the INA version of it is half the cost of the Gates pulley at most. The other upside of spinning the alternator slower is longer life of all rotational components. Call me biased, but I do long enough drives that the RPM difference matters not and the cheaper pulley makes everything cheaper to maintain.
Tensioner Pulley
Though not a part directly from the alternator, it's within close proximity and as of late, several individuals have had failures. Don't let that worry you as this is a part that is supposed to be swapped with every timing belt and won't typically fail with this change out frequency. However, due to it's construction, it is a throw away part and this doesn't sit well with me for long term maintenance. There seem to be some alternates available but these are tentative as they require documentation as to what needs to be changed or added to let them work.
Amazon
You'll notice that a good portion of the links below are from Amazon.com. This was done so as the part is readily available there and if you happen to be one of the many Amazon Prime members, it's likely that you'll be able to get the part without having to pay shipping and quicker than most other sources. Effort has been taken to try and ensure all links are valid, but you're encouraged to lookup the references yourself in case I become delinquent in keeping this thread up-to-date.
Cost
You've probably noticed that "lifetime" alternators from your local parts store have a tendency to not last long in our cars. This is why identifying the individual parts is so significant. All in all, a full R&R of the alternator will cost:
- ~$50 in parts if VR is good: This is both bearings, brushes & slip ring
- +$40-$60 if new VR is required or you don't want to change the carbon brushes
- +$25-$65 for a new clutched pulley
- +$60 for a bearing puller and alternator drive bits
To-do List
Voltage regulators and carbon brushes need to be compiled and evaluated along with the clutched pulleys and metal tensioner rollers. These will be added when time permits.
Last edited:
...google-fu is like a martial art...it needs to be practiced 
