Nicole

So, Bill took the car home last night (and hit the 100k Mile mark on the way).

We started it cold on the lift, and put it in gear. Bill listened with the hose, and found the noise clearly to be coming from the alternator - most likely from the inside.

We also found that putting load on the alternator by turning on the lights increases the rattle, or can trigger it again after it has already disappeared.

So, we are curious what inside an alternator might rattle under load. Any ideas?

This was a BOSCH rebuilt unit; I have to look-up when exactly it was installed. Maybe there is warranty on it - that woudl be nice!

Otherwise I'll live with it for a while - too many other projects to do right now.

Mrmerlin

Thats good that you have narrowed things down, could you have a stone or a loose nut rattling around inside the alternator or the airduct housing.
It shouldnt take more than 2 hours to RnR the alternator at Bills (sorry for volunteering your time Bill)

Bill Ball

Nothing rattled when I hit the alternator with a hammer. Funny that the noise disappears as it warms up but it comes back when you turn on the lights. Turn off the lights and it goes away immediately, etc. Not likely a rock inside. Not sure what it is, but I guess we should take it off an open it up.

Charley B

An alternator builder with a warped sense of humor, no doubt. Reminds me of the old story of the ball bearing in the cross member from the factory.

Ron_H

The problem is ......BILL. How do I know? When Bill gets around a 928 and starts wrenching on it, strange noises never before heard tend to develop out of nowhere. Ask Bill how I know this. And then when the 928 is taken away from Bill, the noises disappear. Weird.

Tails

Some thoughts:

I would not like to have a stone or a nut rattleing around inside the alternator.

There are only a few moving parts in an alternator:
The roller bearings,
The armature, and
The pulley.

The roller bearing could be worn or loose and rattle at start up or when the load is increased.

The armature, I would not like to have anything loose on or around the armature.

The pulley, could it be loose on the shaft and at start up or when load is applied it
rattles until the alternator warms up?

The armature (rotor) rotates in a magnetic field, the strength of which is influenced by the load.

We also know the alternator gets quite hot and this is dependent on the load and its location in the engine bay.

As previously stated, look forward to hear what the fault is when located.

Tails 1990 928 S4 Auto

69gaugeman

Maybe the laminations on the stator are loose (riveted I think) As it heats up the plates expand to put pressure on the rivets again. Not an expert but just thinking that might be an explaination

Bill Ball

The pulley and fan are tight. The shaft has zero play. With the belt off, it spins nicely.

What inside would make a rattle noise under increased electrical load (lights on)? It's not just heat, as once warmed up, we still got the nosie when the lights were switched on. It's an immediate effect and stops as soon as the lights are switched off. As Tails said, there's not much inside. Rod may be on to something. At this point, the noise is just an annoyance.

Nicole, if we crack into it, I'd like to have a replacement at hand in case something inside falls apart. I have my old one with a bad bearing. I just need to fix that and we'll have a spare.

Nicole

You mean... We're actually going to take it apart? That'll be interesting, but I agree it would be good to have a spare on hand, just in case. One never knows...

I so envy my friend in Germany with his 26k km 95 GTS - that engine runs sooo quiet and smooth...

Bill Ball

Nicole: We might as well. Hopefully we'll find something loose that can be refastened.

Nicole

OK, sounds like another interesting adventure... But let's finish the brakes first, OK?

AO

Nicole- Are you getting your hands dirty on this too?

Bill Ball

Andrew: No charge, no warranty, just dirty hands.

Nicole

I usually get the gloves dirty... Hands only sometimes.

Garth S

As Rod noted, the stator plates came to mind .... but they are normally tightly wrapped by the 3 sets of field windings and varnished solid ( re insulation). Perhaps the first thing easily checked is to remove the pulley before dropping the alt - and check that the 4 longish M5 bolts that bind the bearing end housings to the stator body are tight. Inside the alternator, it is not too likely that the regulator is not screwed dodn tightly ... but it is a possibility. Also, on certain Bosch alternators, the kidney shaped diode plates are screwed down .... or should be. One or more of the through bolts are covered by an insulating spacer, which remains quiet if tight.