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Old 04-30-2014, 09:52 PM
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HOWTO - Change the Rectifier in a 3G Alternator

This thread will discuss how to swap out the rectifier in a Ford large case 3G alternator. This is not a thread about the 3G upgrade generally. Iíll assume you know what it is and why a person might want one. There is unending discussion of these swaps over on the Full Size Bronco forums, and plenty here on FordTrucks too.

Here is a particularly good thread to get you started, especially the first post: 3G Interchange Facts.

Or if you prefer to do all your reading on our own dear forum, hereís another good thread right here in the 80-86 section: 3G Alternator Install With Pictures.

What I want to talk about is what you almost canít find on the internet, or at least I wasnít able to (not much). And that is how to upgrade a 130 amp 3G alternator to 160 amps. The largest 3G alternators you can buy today, or find in a junkyard, are going to be rated 130A (large case Ė small cases are 95A). I wouldnít even have known about the 160A upgrade but they were sold standard by Ryan at RJM Injection Tech and in fact I think ALL his alternators came with the 160A rectifier upgrade standard.

Then RJM went out of business and Ryan has so thoroughly dropped off the face of the earth not even the Bronco guys have been able to track him down.

I figured it should be easy to upgrade one myself, but when I searched internet on how to do it all I found was people saying, "Don't bother, just get one from RJM." That was good advice back in the day because not only did you save yourself the trouble, he was also selling them already converted for cheaper than you can buy the parts to do it yourself today.

But that was then and this is now.

So first, what is the 160 amp upgrade? It is a swap of the bridge rectifier (series of diodes) in the alternator to a heavier-duty version that can safely handle the higher current. I donít know which rectifier Ryan was using but this is the only upgraded rectifier I was able to even find for sale in all of cyberspace: Part # 502073 Ford 3G type Heavy Duty Bridge Rectifier, sold by AlternatorParts.com. As of today, they want 60 bucks for it. (This same place will sell you the alternator already upgraded but they want $270 for it which I thought was excessive).

If you read their description you will see they claim this rectifier is actually rated to 200 amps. Now Ryan used to have a discussion about this on his website, but I donít think it is available anymore because much of his storefront site has been taken down. As I recall, he said that while some of the components may actually be rated at 200A, you would never get that much out of the alternator in practice. In his testing he found that you could only get around 160 amps output before belt slippage but even that was using either a 6-groove serpentine or dual V-belt setup (and donít even bother to try it on a single V was his advice). So that is why Ryan rated his alternators at 160 amps even though maybe the bridge rectifier could handle 200.

Anyway, even though AlternatorParts.com sells the rectifier as a 200A model Iím assuming I wonít get more than 160A out of it. (By the way, if you dig around on their site, you will see the version they sell already assembled is also only rated at 160 amps even though it has the 200 amp rectifier in it).

But of course 160 amps should be plenty more than enough, I'm not greedy.


Ok, on with the swap. Here is a photo of all the parts so you can see what Iím going to talk about next:

Click the image to open in full size.
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Old 04-30-2014, 09:53 PM
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Swapping the Bridge Rectifier

To swap the old rectifier for the new, I had to:

1) Remove the voltage regulator, sorry, I didn't take a picture (it's just four T20 Torx screws). This is probably not strictly necessary but it's easy enough to do and saved me from potentially damaging the brushes as I disassembled the rest of the alternator.


2) Unbolt the alternator case halves and separate them (three bolts, easy).


3) The front half of the case fell off without effort. However the Rotor shaft remained wedged into the rear case half, as the rotor shaft is pressed into a bearing and the bearing is tightly pressed into the rear case. Between the Rotor and the rear case are the Stator and the Bridge Rectifier, so I had to get this all separated. I didn't know any good way other than prying with a long screwdriver as gingerly as I could, a little bit at a time as I worked my way around. I almost thought Iíd never get it but after about 30 minutes of wiggling it came apart. Here you can see the rear case and the Rotor with the bearing still on the shaft. I put some antiseize on my bearing before I re-assembled everything in case I have to get this thing apart some day in the future.



4) Once I had separated the rotor from the rear case I was left with just the Stator which is soldered at six points to the Bridge Rectifier. Here you can see the comparison: the new rectifier has a thicker heat sink which appears to be made of copper, rather than the thinner aluminum one of the stock. The diodes are clearly larger. And the battery output is a larger 8mm copper threaded stud compared to what I believe was a 6mm on the stock (my new rectifier did not come with a nut for this stud, you may want something nice like this 8mm Stainless Tooth Washer nut, McMaster-Carr part number 90923A241).



5) After a brief experiment trying to unsolder the Stator from the Bridge Rectifier, I decided I could save myself a lot of time and just cut it off with diagonal cutters, which I did. But donít cut the wires short, you need the full length of them! I cut the old Bridge Rectifier tabs instead.



6) I then cleaned the tips of the Stator wire which was easy enough to do with a regular soldering iron. I then got them bright and shiny with some rubbing alcohol. Here they are before with the remnants of the old rectifier tabs:



7) And here they are cleaned up:



8) I almost forgot the plastic piece off the old rectifier, this is an insulator to make sure none of the stator wires touch the case. Donít throw it away!



9) Next the part I feared the most, soldering the old Stator to the new Bridge Rectifier. For this you will need to fire up a pretty hefty heat source. I tried four different ones, a small butane torch first, followed by my Radio Snack 100 watt gun, followed by my Radio Snack 230 watt gun, followed finally by my very beefy
Weller 100 watt iron Weller 100 watt iron
that finally did the job the best. The iron is less wattage maybe but has a very wide and flat tip so could transfer more heat.


10) The Stator wires were hardly long enough to reach my new rectifier so I had to use a good deal of solder to fill in the gaps. For some reason I chose to use Harris Stay-Brite lead free silver solder along with their flux, but this was almost certainly unnecessary. Iím sure any 60/40 rosin core would be fine, and maybe preferable.


11) Most important was to get a very good bond. That means plenty of heat on the joint so as to get sufficient solder flow in all direction and a clean, shiny result. The only way to do this is with a sufficiently powerful iron, trying to use something that canít put out enough heat is just asking for a cold solder joint. (This photo only shows 4 connections, there are two others not visible for a total of 6).



12) Once the new rectifier was on, I re-assembled the alternator in the reverse order I took it apart. You can "clock" (rotate) the rear case relative to the front at this time if you want your voltage regulator connection to face a different direction.
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Old 04-30-2014, 09:53 PM
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160 Amp Upgrade Conclusion

In conclusion - most people (including myself) don't really need an alternator to put out 160 amps. If you are thinking of an upgrade, swapping in a bone-stock 3G large case alternator (rated at 130 amps) is easy and inexpensive and a very good idea, and will be worlds better than what Ford put in the trucks of our era. Many people get one from a bone-yard from later models and so the total upgrade cost can be minimal.

But if you really wanted a 160 amp alternator after having seen them on RJM's website all these years, it is possible to do it in one evening for about $60 more than the cost of your stock 3G. The process is not difficult; getting the rotor off the rear case without breaking the thin aluminum was the hardest part for me. But you certainly need a very good soldering iron/gun and a bit of experience to tell a good solder joint from a bad one. Other than that I think this is something anyone could do.
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