QSB 6.7 Alternator Pulley Issues

[bdunigan]

I have been fighting stretchy belt noise on my port QSB 6.7. I replaced what I thought was a defective fan hub and the noise diminished. I replaced an alternator with suspect bearings and the noise didn’t change. Finally, I bought a Dayco laser alignment tool (referenced in the Cummins manual as 93874). The vibration damper and fan hub pulleys lined up perfectly. However, the Seaboard replacement alternator pulley was off by one pulley groove. Not Seaboard’s issue though as the pulley is the correct Cummins 3918275. I was able to reverse the pulley, shim it, and get it to line up with the fan hub pulley. Noise gone! In fact, the port engine is so quiet I can now hear a chirp in the stbd engine. But this is only a temporary fix. The factory pulley has a flange (circled in attached drawing) that rides against the cooling fan. I would say the flange is about 3-4mm thick which is the about the same as the width of a single pulley groove. This flange causes the pulley to extend out of alignment by one groove and causes the chirp. Factory flaw? In my mind, yes….either the pulley design or a flawed alternator mounting. The alternator is mounted only one way and cannot be adjusted forward or backward. It can only rotate to tension the belt and that rotation is also linearly fixed.  Unless there is a factory fix, I will have to machine the pulley flange to properly correct this issue. This issue has come up here in the past and, in one case, Cummins suggested the 6.7 alternator mounting was indeed slightly out of alignment. Maybe this flaw is causing not only the noise but the failure of the fan hub bearing due to the belt not running perfectly straight. I can’t fix the mounting issue but I can modify the pulley to correct it. All suggestions and comments are appreciated.

Topic

[Tony Athens]

Certainly an odd ball glitch.. “Good Job” hanging in there.

[bdunigan]

Pulley and Laser Pics

Today I wrapped up the pulley mod to correct the misalignment that was causing belt chirp & snap. The first pic is the Dayco laser alignment check of the factory pulley. The 2nd pic shows the misalignment of the factory pulley…one full groove or 3.5mm.  The 3rd pic is the modded pulley with shows near perfect alignment. The 4th pic shows a detail f the modded pulley. In short, 2mm was milled from the outer race and 3.5mm was milled from the inner flange surface. No more noise. I checked the stbd pulley at it was off by 1/2 groove or 1.75mm. This causes very minor noise at cold idle and the noise goes away after warm up. The stbd alternator is factory original. Like the port side, the crank pulley and fan hub lined up perfectly. So, if there is a lesson here, it is that the QSB6.7HO alternator mounting and tensioning system is subject to lateral variation that can cause these alignment issues. Mine was simply worse than most.

[bdunigan]

Thanks, Tony. I am having the pulley machined to correct the misalignment. When it is ready to be installed, I will take pics…before and after….and post. The pulley mod will solve my problem. Just soooooo weird that this misalignment exists without clear reason. I removed the mounting bracket on Sunday and saw no issues or possibility of lateral movement. It came out and reinstalled without issue. So if the bracket is the issue, it has to be a factory machining fault (if I understand your comment correctly).

[Tony Athens]

When you look really close and the alternator bracket, it is no t “really”  pinned.

Yes, it has pins BUT  they are merely push up against the bracket and can be tightened OUT of  ALIGNMNENT–Seen this not less that 2 times now on new engines that would  EAT a belt at low hours–A complete pain in the butt to deal with on the boat unless you have a spacious engine room

 

MICKEY MOUSE in so many words & obviously designed by a couch engineer.

[bdunigan]

QSB6.7 425HP, SN: 73430746, 725 hours

Tony, the alternator I replaced with your 22si unit was the original 4003445. Your replacement alternator is SKU: ALT-22SI-130-3 and it has the Cummins 3918275 pulley. As far as I am concerned, yours is a direct replacement and is not the issue.

I didn’t take any pics yesterday while I was diagnosing the issue with the Dayco laser but the alt pulley was out of alignment (too far forward) by one groove or rib. I wish I had taken photos. 

I don’t believe the alternator was installed incorrectly since, even though the pulley was off by one groove, the laser line was parallel to the pulley groove. There was no ‘axial’ or ‘angular’ misalignment that would indicate a mounting issue since the laser line was not at an angle to the pulley groove.  That said, I could be wrong. The mounting bracket is fixed with locating pins so there is no variability there. The alternator ears that straddle the mount leave no room for forward or backward play. IOW, the there is no way to move the alternator further back to get the pulley lined up correctly.

Over the next week or so, I will have to remove the pulley and have it modified. At that time, I will take pictures and post. I will report back then.

Thanks, Tony, for your response.

[Tony Athens]

6.7 Pulley alignment issues?

I’d like to get a little more info on this as this one is totally  new to me.. I have seen some issues with the alternator’s  “stretchy belt drive”, but not this one.

They were related to the actual mounting alignment of the alternator itself as,  IMO,  not full totally proof or KOSHER as it can be installed in a tweaked out of alignment way. 

6.7 or 6.7SL? 

Serial number?

hours?

What can you sent me off line as to pictures details,. etc..

Original Pulley ?

Original alternator?

[Author]

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