Autopsy & resurrection of a high output alternator (2 Viewers)

[Spook50]

Figured I would share this for giggles and for anyone curious about alternator rebuilding. Last summer my high output alternator roasted a bearing so I swapped it out with an OEM that I had kept in storage for many years to use as a backup. Well I suspect the sub zero temps we had a couple weekends ago for several days put enough of a drain on my battery (an AGM from Interstate) to overstress the rectifier in the OEM unit, so I had to take a day and do the rebuild on the high output to get it back in (why hadn't I done it sooner? Juggling way too much and kept procrastinating). So a few days ago I fired up the pellet stove in the shop, threw some DIO on the stereo and went to work.

The victim. This was originally sold as a Beck/Arnley unit that I had gotten back in 2007 or 2008. IIRC that original unit ultimately had either the windings or the rotor go out so I had to replace the whole unit. Took some time but a reputable auto electric shop sourced another 120A unit that when I received it, was identical to the original B/A I had previously.

Close up shot of the info tag. When you research the part number now you find some weird NAPA alternator that's only advertised as an 80A unit, so my impression is that the only high output option anymore is shelling out big bucks for a Mean Green unit, which I've heard mixed reviews of. Honestly I'd love to get a dead one and investigate exactly what failed on it. Also notice how in the upper right corner it says "12V 80A PASS". I remember asking about this when I picked it up and was told that's because the vehicle it's designed for (ie: the US spec FJ62) comes factory with an 80A unit, so the pass/fail is just based on it meeting (in this case exceeding) that spec. Notice the actual current it tested at when at 6000 RPM....

Now before the bearing went out I hadn't noticed any electrical issues beyond the typical voltage drop when at idle with a moderate current load. But when I pulled the rectifier I saw two diodes had at some point blown out:

This is the bearing that had failed. The tail bearing, which holds very little lateral force, unlike the nose bearing which had to take most of the force of the belt tension. Erego the tail bearing is often much smaller than the nose bearing. When I turned it by hand I could feel just how "gritty" it felt and how much it was grinding and eating itself alive.

 

[Spook50]

I never used this small puller for anything else but can't bring myself to get rid of a serviceable tool, so I modified it to remove the tail bearing. When I got the bearing off, both seals fell right off of it and I could see just how badly overheated the balls inside had gotten by their discoloration.

Once everything was apart I used my Fluke 87V to test both the rotor and the case windings. I couldn't get any pictures because these tasks honestly required three hands and until I spend more time at Hanford I've only got two. Basically you want to make sure there's about 2Ω of resistance between the slip rings (differs among alternators but this rotor was spec'd for 2Ω. I had notes from long ago that I referenced so to be completely honest I can't remember where I had gotten that spec). Then make sure neither slip ring is shorted to the core. Now if you want to have a little fun, you can set a socket a couple inches away from the rotor and then apply 12V to the slip rings. Bam the rotor assembly is now an electromagnet. A strong one at that. That magnetic field is how the core of the alternator generates the 3-phase AC that's then rectified and controlled by the voltage regulator. The field connections use a small amount of current to create that magnetic field through brushes that contact the slip rings.

For the stator windings, all I needed to do in the case of my unit is make sure the resistance is consistent between all combinations of pairs of leads coming off the windings and nothing shorted to the housing.

New bearings received. I made sure to get Koyo bearings this time. The 6202-2RS is the small tail bearing, and the Denso bearing (which in actuality is a Koyo 437) is the larger nose bearing.

New nose bearing pressed into place and secured with a mounting plate. In this case the mounting plate is critical because there's a shoulder on the rotor assembly and on the pulley. When the pulley is installed and the nut torqued down, the inner race of the bearing is sandwiched between the two. With the plate holding the bearing in place in the housing, this prevents the rotor assembly from migrating to the rear and causing catastrophic damage to the internal fan and the rear housing.

Since I had to replace the rectifier, I opted to replace the voltage regulator and the brush assembly as well. Those two parts were probably fine, but since new parts are inexpensive, replacing them with new while I have the alternator apart is cheap insurance.

The new rectifier installed. In my case this is an INR743P rectifier (which crosses with the Denso 021580-6010) with 8 diodes, each rated at 400V and 60A. The assembly overall is designed for Denso 136A alternators but is specified to accommodate 180A of current (which I imagine would generate quite a bit of heat and cause premature failure at that point). You can see one screw is pretty well blocked by the mounting ear (honestly the only aspect of this alternator that I hate with the ferocity of the winter cold). This always gets mangled when I remove it, so I ordered new screws from McMaster that are Torx drive. These are T20 sized, but with a skinny T15 driver I can get up to it to snug it down. I now have plenty of extras so that I can just replace that screw in future teardowns.

When I removed the long screws securing the voltage regulator to the housing, both had seized. I managed to get them out but the threads were pretty well galled, so again new screws from McMaster and this time a touch of blue Loctite to prevent future problems. Voltage regulator is an IN436 which crosses to several Denso part numbers so I won't list them all here.

 

[Spook50]

All new parts installed and ready for the rear cover. Honestly I really didn't need the brush assembly because there was hardly any wear on the old one, but my OCD wouldn't let that one go, so a new Denso 021620-2720 was installed too.

Reassembled and ready to be installed back in my 62.

While I had the truck parked I pulled the battery and put it on my Noco charger inside to get a nice easy top off. Freshly charged battery and all new alternator components and she fired right up, giving me a full 14.2V just off idle with high beams, both fans and wipers running full bore. At idle it sat at 13.6v under load. Much less voltage drop under load with a 100% capable rectifier, which I expected once I saw the blown diodes in the old part. Nice to have gotten that all fixed and squared away now.

I did not have to do a break-in procedure for the magnets in the rotor since obviously that already had plenty of use on it. A new rotor requires a break-in to maximize the life of the magnets, which obviously isn't the case with me this time.

Next up (when I get to it), autopsy and resurrection of an OEM alternator, which may or may not get a write-up