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Alternator steel slip rings form insulating layer


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mmo
New User

Aug 23, 2018, 1:31 PM

Post #1 of 6 (125 views)
  post locked   Alternator steel slip rings form insulating layer  

My vehicle is a nearly 30 years old Nissan truck with a Z24S engine. It has a Mitsubishi 55A alternator with external voltage regulator. This alternator worked fine until two months ago, when the alternator warning light started to come on intermittently, and the voltage sagged. I made it home on battery power, removed the alternator and opened it. Not surprisingly the brushes were worn down, with one of them making poor contact. It had sparked and slightly eroded its slip ring.

I sanded the eroded slipring resonably flat, and replaced the brushes by the only fitting ones I had on hand. The original Mitsubishi brushes are deep black, while the new ones look more grayish and shining. Since the rear bearing was dry and slightly rough, I noted the bearing sizes and put the thing back together for temporary use, until I buy new bearings and brushes. I live in the hills far away from any town, and depend on the truck to get to town.

I easily found the exact bearings, but not any brushes that would be certain to be the right type. This alternator has steel sliprings, which seems rather uncommon to me, and might need some specific brush formulation. Anybody knows? The only brushes I could find in the proper size, that are different from those I put in, are copper dust loaded ones. The originals definitely do not look like containing any metal dust.

Well, after only a few hundred kilometers driving, before finding the time to open the alternator again, it suddenly stopped working. No intermittence at all, just OFF, like if a plug had fallen off or something. Alternator warning light ON and battery voltage down. I was on the highway when that happened. I checked for anything bad in the wiring, and not finding anything visibly abnormal, I took the gamble and drove home on battery power, a two-hour trip with headlights on...

After one and a half hours, and the battery still holding up at 11.5V, suddenly the alternator sprang back to life, and worked normally for the rest of the trip.

I removed the alternator again and opened it. I could see nothing wrong. There was only a very slight film on the sliprings, and the brushes had just seated themselves a little bit.

Since I had it open anyway, and now had the new bearings at hand, I fully disassembled it, replaced the bearings, turned the slip rings on my lathe to remove the slight unevenness that had been caused by the sparking and sanding, then used sandpaper from 180 grit down to 400 grit to remove the machining marks. I don't have any tool to really measure the surface roughness, but under an USB microscope I can see a fine roughness remaining which might be reasonably close to the 1 to 2 micrometers recommended for slip rings.

While doing the electrical tests before reassembly, I stumbled upong something unexpected: The freshly machined and sanded slip rings are not conducting easily! I have to exert significant pressure with the test lead to get them to conduct! And they have a resistance proportional to contact pressure, from kiloohms at moderate pressure down to several ohms when I press firmly. The steel alloy used in those Mitsubishi sliprings must have formed an oxide layer, while I sanded them!

Is this normal??? Will the carbon brushes break through the oxide? Could this phenomenon be the reason for my alternator quitting for one and a half hours and then returning to operation by itself? And should I keep the grayish brushes I have in now, or should I install those with copper dust in them?

It's interesting to note that the steel shaft of the rotor, and the bearing races, have excellent suface conductivity, far better than the slip rings...

Of course I will thoroughly check the regulator and the wiring. But I would be very grateful if somebody could shed some light on the strange behaviour of these steel slip rings, suggest what to do, and also suggest whether to keep the shiny grayish carbon brushes or replace them by the copper-loaded ones.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Aug 23, 2018, 1:46 PM

Post #2 of 6 (119 views)
  post locked   Re: Alternator steel slip rings form insulating layer  

Why are you even messing with this thing? Nobody outside of manufacturers repair alternators any more.
They aren't built of high enough quality to invest the time and parts in, especially since replacements are so much cheaper these days.

Cut your losses and buy a reman.

Replace the regulator at the same time.



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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



(This post was edited by Hammer Time on Aug 23, 2018, 1:46 PM)


mmo
New User

Aug 23, 2018, 2:02 PM

Post #3 of 6 (111 views)
  post locked   Re: Alternator steel slip rings form insulating layer  

I'm messing with it in part because I'm a tinkerer with lots of time available, and also because this old Japanese alternator seems to be better made than any replacement I can get. Professionally remanufactured ones are not available where I live. The choice would be either a very crudely repaired one, by some guy who works pretty much with a sledgehammer and chisel, or a new Chinese-made universal replacement unit. The ones I have seen don't even have a rear bearing, just a sleeve...

So I would still be grateful for a technical answer to my questions.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Aug 23, 2018, 2:08 PM

Post #4 of 6 (107 views)
  post locked   Re: Alternator steel slip rings form insulating layer  

I don't think you are going to get any other answer here because like i said, nobody does that any more.
You obviously have Internet because you are posting here. If you have Internet and an address, you can have anything shipped to you. You don't need local suppliers.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.



mmo
New User

Aug 23, 2018, 2:23 PM

Post #5 of 6 (101 views)
  post locked   Re: Alternator steel slip rings form insulating layer  

That's precisely a problem I have: No address. I live in the woods. No shipping company delivers here. I do have a PO Box in the nearest city, but postal shipping is limited to modest weights. An alternator most likely will exceed that. I do purchase lots of things over the web, from anywhere in the world - but only small enough, lightweight items.

I have tried once to arrange in-office pickup with UPS, but it was a nightmare. Also shipping an alternator internationally with such a company is very pricey.

So, I will try to get my old alternator into a reliable shape again, and if that fails, I will locally buy a Chinese replacement.

More than anything, I love to UNDERSTAND things. Right now I'm deeply intrigued by how exactly a slipring is supposed to work, if it forms an insulating layer so easily.

I do realize now that I might not get a technical answer, simply because nobody here knows. Before posting I thought that some people might be around who have a good understanding of these things, rather than just replacing complete units. Some old-time electromechanic, who learned his trade before the age of the throw-away-and-replace approach.

Please don't take me wrong. I fully realize that in this time of high income, high labor charges, and cheap robotized manufacture, it's often more economical to replace than to repair. But it's very boring too! Repairing, and learning in the process, is more attractive to me.


Hammer Time
Ultimate Carjunky / Moderator
Hammer Time profile image

Aug 23, 2018, 2:32 PM

Post #6 of 6 (95 views)
  post locked   Re: Alternator steel slip rings form insulating layer  

I'm sure there must be shipping locations somewhere near you. You can't use the cost excuse either as there are many, many companies that ship for free.

I wish you luck with your boredom and intrigue but there are reasons it's not practical to attempt to repair these things.

Sorry we couldn't help you here.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.







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