Rattling Noise from Alternator area

[autobonrun]

I'm getting a low pitch thumping/rattling noise from my engine compartment that I've isolated to the fan/alternator. It may be the alternator bearings. How can I tell if the bearings are bad? There appears to be play in the thrust direction but not side to side. I may just replace them while I have it off.

Also, there is a cover on the back of the alternator with an air deflector riveted to it. It's a grey plastic housing with six nuts that fasten it to the alternator. I notice that the rivets/deflector is loose. This could also be the source of the noise.

First, can the rivets be tightened? If so how?

If not, what is this cover called and does Pelican sell it.? I can't find it on the Pelican site?

[Jim Sims]

Some axial play is normal and okay but any side play means the bearings are failing or the plastic bearing retainer is failing. Address this now as it will leave you "dead in the water". Can take out the fan and fan shroud if the bearings fail at speed. Once the fan goes of course, one cannot drive the car without overheating and possibly (likely) seizing the engine.

The air deflector has been called an air guide, air duct or hub extension depending upon the vintage of the catalog. The latest part number of the version with the cut-out to accomodate the alternators 1982 and newer (also fits the older cars) with the internal voltage regulator is 91110603305. Type in the number in the search box and it will come up on Pelican; it is $81.24. One could drill out the loose rivets and perhaps redo them with a pop rivet tool if the shaking hasn't widened the holes in the fiberglass body of the hub extension. One might tighten them by pushing in epoxy and letting it cure or one could replace the rivets with small screws and nuts held tight by red Loctite.

Cheers, Jim

[autobonrun]

Excellent Jim, I'm on it. Thanks much.

[tsniu]

Thought I would throw in my experience since this board has always been entertaining and informative for me.

Jim has some good ideas to start. When I heard rattling from my alternator/fan assembly, I also thought bearings, time for a new alternator, etc. But when I took the alternator out, I could see that the ribs in the fan shroud/housing whatever had begun to fracture on the ribs that hold the alternator and the fan was now striking, ever so slightly, the fan housing, and only intermittently. The whole thing had to be replaced. Incidentally, I was told that the air deflector may be, or was temporarily, NLA.

Good luck. Tom

[Rodney Nelson]

This happened to me about 2 weeks ago. All of a sudden there was a large vibration coming from the engine compartment. The engine was running fine. I put my hand everywhere trying to find out where the vibration was coming from, it was from the fan housing.

I removed the alternator the bearings seemed to be fine with no play, however, I was conviced the bearings were the culprit. When the bearings are under a load they will act different.

I took it to the local alternator shop and to my suprise the guy at the desk knew it was from a 911. I told him check everything out and change the bearings!!!!!!!

It cost me $43.00, new bearings and brushes. I put it in and what do you know? No more Vibration.

This was the cheapest Porsche fix ever!!!!

Good luck!

[autobonrun]

Unfortunately my problem was not just the bearings but the shaft as well. Evidently either the shaft size had decreased causing the bearings to go, the bearings were not reinstalled correctly during the previous rebuild, or the failing bearings damaged the shaft. In either case, the repair was a new alternator. This is a first. This alternator lasted about 15 years so I should not complain. Previous rebuilds have only been brushes and sanding the commutator bars.

Thanks for the help. I'll also make sure I get 6 shims on this time. I don't know if having only 5 was the problem but I'll make sure I have the correct number. I only had two between the pulley sections and three inside the pulley half.