Alternator noises

[240ZMan]

I've got an '80 alternator in my '73 240 and for the last year it has worked great. But last night I noticed the headlights dimming now and then and the ammeter showing a slight discharge when they dimmed, even when the engine was at reasonable revs.

When I got home I checked the wiring all around, wiggled everything and the problem dissapeared. That is, until today when again it intermittently showed up. I noticed that when it dims, there is a slight growl or hum from the alternator that you can hear in the cabin as well as with the hood up. Also, there is a smell while it's making the noise if I put my nose down near that part of the engine compartment. I've smelled that smell before, but can't quite place it. But it's not insulation burning.

In any event, I suspect I'm going to need to replace the alternator, but my question is if anyone has experienced the noise? Mostly out of curiosity, I don't understand why it's making a speed dependent noise when it's not putting out enough current?

Anyone experienced something like this before?

[mlc240z]

sounds like a bad bearing. if you haven't done any major internal damage yet, you can just replace the bearings. generally cheaper than new alternator. remove the belt and spin the pulley to feel any binding or excessive resistance with condition of bearings.

[sblake01]

IIRC, diodes on their way out can also cause that scenario. I would remove it and have it bench tested.

[240ZMan]

sounds like a bad bearing.

I agree, that was my first thought on the sound. But I don't understand how that would affect its ability to charge? When the sound is present the alternator can barely power the ignition. As soon as I turn on the headlights the ammeter shows I'm discharging the battery.

[Bambikiller240]

Since power output is fluctuating, I agree with Stephen, I'd suspect the diodes first. Bad bearings shouldn't affect power output until they are almost frozen solid.

[mlc240z]

power fluctuation from bad diode i can understand. but the growl/hum (bearing moan) noise would point me to a bad bearing possibly slowing down the rotation or heat buildup leading to power drop. spin test might narrow it down or eliminate bearing as a suspect.

[Bambikiller240]

I hear what your saying, but if the rotation is being slowed down, your gonna smell burning bearings, and fan belt, and likely here a screech from the belt slipping as well. Complete failure will follow shortly. I wouldn't expect bearing noise to be intermittant either.

Either way, once I confirmed one problem or the other in the alt, I wouldn't be mucking about trying to "fix" either. I'd get a new/rebuilt alt, install it and take the car for a nice drive, then have a beer when I get back.

[240ZMan]

Either way, once I confirmed one problem or the other in the alt, I wouldn't be mucking about trying to "fix" either. I'd get a new/rebuilt alt, install it and take the car for a nice drive, then have a beer when I get back.

I'm thinking along the lines of Carl. Either way I'm going to replace the alternator.

I did pull it out this morning. It spins quite freely. The only play in the bearing/shaft is front to back, and very little. I can't feel any sideways at all.

The only thing stopping me from just replacing it with a rebuilt (besides needing to finish mowing the lawn:) ) is that a few weeks ago I had problems getting the engine started (my mistake in setting the timing) and eventually the battery needed a boost. It's not a new battery, and I can't help but remember reading people who replace one of the two and then later found out the problem was the other. And given the problem is intermittent, if the alternator tests good on the bench, then I don't know anything more.

I did measure the voltage across the battery while the moaning sound was present, and it measured about 12.8V. That makes me think the problem ISN'T with the battery as a dead cell would likely drop it further, don't you think?

[First Gen Z]

You are right about voltage drop with a dead cell. However, the battery should read 13.6 - 14.2 when the engine is running. I think your alternator has just commited suicide.

[240ZMan]

I'd get a new/rebuilt alt, install it and take the car for a nice drive, then have a beer when I get back.

Rebuilt alternator is installed and so far all is good. Opening beer now...