I've seen this happen twice. One of my customers,
an independent shop, had one about two weeks ago.

The 964 alternator setup can be more problematic
than older cars, since there are two belts (two
rotational speeds and twice the possible tension).
Also, since the shaft is longer, there is more
leverage on the shaft. Additionally, the system
uses a fan bearing.

Potential Problems:

1. The fan bearing may have seized causing two
different rotational forces on the shaft.
2. The two belts may have been too tight.

Preventive Action:

1. Replace the fan bearing at 75-100K which is
when the alternator gets replaced. Like the
alt. bearing there's no lubing after the
initial bearing grease.
2. Use extreme car in tightening the belts.

Good Luck
Loren
Systems Consulting (Systemsc.com)

 

Thanks again for all the responses. It appears that the belt was sheared by the fan when it broke loose. It doesn't appear that the alternator seized.

I'm very unhappy that this occurred only 1000 miles after the 60k service but the repair shop claims they did nothing wrong. I've been a customer of theirs for four years and they've always done good work and have an excellent reputation. But I guess past results are no guarantee of future performance.

 

One more question, is there any way to tell if tight belts caused the shaft to break?

 

The latest update: the alternator, fan, belts, and a few other pieces are being replaced. The fan shroud was salvagable. I still have a bad feeling about this happening 1000 miles after a 60k service.

 

Surf, why is your mechanic replacing the alternator if this wasn't the cause of your problem? Just curious.

 

Surf... I am brand new to this board today, and I couldn't help but respond to your post... especially since I was in disbelief WHEN it happened to me. It must be the Calif. air!!!!

Mine just broke last thursday!!! Sheared off right where it exits the alternator. (Which is why the mechanic has to replace it Syd.) One of the two belts looked like it was cut with a knife, so I guess that the fan cut it when it broke loose. The other belt was shredded and the A/C belt just jumped off of the pulley. Alternator still rotated (turned it from the back) without any notice of bearing scrape/failure. The fan hub bearing did not seize (I just replaced it 2 months ago!!), and if anything, my inner belt may have been a tad loose.

Potential differences with mine: Alt. was original (911.xxx.xxx.xx) on a '90 C2 Cab. w/105,000 mi. on it. Age can be just as important as miles!!

I didn't have any warning though. I was on the freeway and, out of nowhere, it sounded like when you run a stick past moving bicycle spokes (Only @ 5K rpm) for about a mile. Then it just popped and the battery and ABS lights came on. The belt light came on later when I re-started to move in-line for the flatbed. (just in case that means something to the trouble shooters out there).

Anyway I got it all back together last night and it fires right up, but I've got other damage that you may want to look out for on yours. My PS pump is making a very loud whirling/whining noise at idle. I figure it's the PS because it gets louder when I turn the steering wheel with the car in place.

Another point for the archives, in case it means anything!! The piece of shaft stayed in the hub and the pulley assemblies were all in tact until I took them apart. After taking the pulleys off, I had a heck of a time pressing out the shaft. I was pressing the hub out of the fan easier than the shaft itself. Finally got out OK.

I hope everything comes out good for you.

Darin
'90 C2 Cab.

 

Darin, that's sounds exactly like what happened to my RSA. The only other collateral damage appears to be one broken pulley . Everything else appears to be OK. I received a call from the repair shop this afternoon informing be that the car is ready. I'll pick it up tomorrow morning.

Here's to that California air! <img border="0" alt="[cherrsagai]" title="" src="graemlins/drink.gif" />

 

Sounds like you guys have done a failure analysis
on the belt. You conclude that the fan cut the
belt, interesting. What in your analysis
provides this conclusion?

Also, how do conclude that neither of the TWO
bearings were not a source of the problem?
Just because they may turn now doesn't preclude
a partial seizure of either at a high RPM.

Since you guys appear to be involved in DIY
maintenance, do you check the tensions of the
belts? Do know whether the TWO bearing have
the proper amount of grease?

Have Fun
Loren
'88 3.2

 

Catastrophic failure of the alternator shaft from fatigue? Fatique being a function of frequency and force. Frequency is you driving down the road and force is the two v-belts flexing the alternator shaft. From the descriptions of the two broken shaft incidents, there were no signs of bearing failures, heat from spinning inner races, grease cooked out of the sealed bearings, etc, I'd say the new belts were overtightned.

<img src="http://boards.rennlist.com/upload/fan.jpg" alt=" - " />

Without any further info, that's my conclusion. <img border="0" alt="[cherrsagai]" title="" src="graemlins/drink.gif" />

Give the belts a check when you get the car back from the shop. Owner manual says "tension is correct when v-belt can be pressed down approx. 5-10 mm by thumb pressure, halfway between the pulleys. Recheck after running engine..." The shop manual says to check it with a tension-0-meter, but isn't that why God gave us opposed, calibrated thumbs?

I just replaced my alternator with a rebuilt due to failed brush rigging. I'd hate to have my newly painted fan (bright orange, by god!) take a spin around my engine compartment..... <img border="0" alt="[crying]" title="" src="graemlins/crying.gif" />

 

Agreed! There is a real frequency, though:

It is the impulse/varying load developed by the
voltage regulator which is a switching regulator;
i.e. when the sense voltage reaches the set value
(14.5 volts) the regulator switches the field
current off to stop the charging. When the voltage
drops below the lower sense value, the regulator
switches the field current "on". This causes
a pulsating load on the alt. shaft which as
mentioned can fatigue the metal when the belts
are too tight.

There really isn't enough data without simulating
this under lab conditions to determine the actual
failure. As mentioned before, always check the
belts' tensions. The leverage on the shaft is
much greater on the long 964 alt. shaft than the
early (pre '90) non-Bosch alternators.

Note: The 964 alt. is the first Bosch alt. used
in a 911 type car since about 1970.

Good Luck
Loren
'88 3.2

 

Surf... I'll drink to that. I like the icon with the never empty cup. I could use a few rounds lined up in a row right now. I hope you get MANY miles out of your RSA, this time, before you have to visit another shop.

Loren... The conclusion on the belt was due to the fact that the cut was clean on one of the belts, like a knife did it (other belt was shredded and I could tell which was which). The end of a fan blade would be the likely culprit in that part of the engine compartment. Belt fatigue (split on a seam) is not impossible obviously, but the belt was 2 months old!! I did consider the possibility that a belt broke first and possibly jammed in between the blades and caused a seizure (ref: my prev. comment about stick in the spokes of a bike wheel). However, the lights didn't come on until the noise went silent (shaft broke), not right after a break as I would have expected a mile earlier.

The fan bearing was 2 months old also. The one I replaced stuck just a little, this one is smooth. It also appears that no damage occurred to the new one in this latest incident. I did the VERY scientific process of spinning on my finger and feeling for any "Roughness" in the rotation. The alternator bearings did not appear to have, even, a minor sign of failure comming on, but when compared with the new one the new shaft did spin a bit more freely (just checked by spinning the shaft and noting the free travel). How do you check for proper bearing grease? These aren't serviceble post production... as YOU note in a reply futher up.

Loren and Mojorizing... (Belts) I did follow the manual on belt tightening specs. As you know new belts stretch in the first few hundred miles of use. So I felt that my belts may have become a bit too loose... For the sake of argument let me just ask from your experience, How many shims should be inside the pulley to achieve the proper tension?

Regarless, my conclusion is that my Alt. was just toast waiting for it's jam...PUN intended. In Surfs case, it may be a bit premature unless a prev. owner (If there was one??!!) drove it hard. After all his is an RSA!!

Anyway, thanks for the questions that continue to challenge my process of reasoning. It can only help me to be a better DIYer.

Darin
'90 C2 Cab.

 

I picked up my car this morning. Interesting that the high-pitched whining noise that occurred immediately after the 60k service is no longer there. The mechanics continue to insist that they did nothing wrong. The shop's owner returns from vacation Monday and I'm going to have a chat with him. All of this has really dampened my enthusiasm...

 

Ahh!! Mine didn't have that high-pitched whine prior!!! Sounds like a bearing WAS bad!!??!! However, it remains to be seen whether or not it was the result of the mechanics work. Good luck on making your case.

What part of Ca. are you in? Would you mind sharing the name of the shop? To date, I have done all of my own work, but I know eventually I will have to count on a shop for something that I am not willing to get involved in. Also, for comparison, How much did this fix-it job cost you? Alt. and 3 belts ran me $215.00 plus an hour and a half of my time. If you don't want to share I COMPLETELY understand. I guess if anything, I most want to justify my time spent on the car.

Thanks,

Darin

 

Thanks for the e-mail.... I really hope that you can get a resolution with the shop owner. I can understand how you might be suspicious of them developing future business when you took it in for the 60k service.

Good luck!!!

 

Hi Daring,
FYI: I have a 10 X 763 v-belt on the fan pulley with 4 shims and a 9.5 X 776 belt with 2 shims in the alt. pulley. Your shims will vary with belt width and length. I think the manual calls for a 9.5 wide belt on both pulleys, but this is what I use.

Later, mojo <img border="0" alt="[bigbye]" title="" src="graemlins/xyxwave.gif" />