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Old 12-20-2015, 09:23 PM
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nograin nograin is offline
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Location: Philadelphia, Pa., USA
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12SI Alternator Guts and related stuff

Why do some 12SI alternators have a resistor from the diode trio to ground and others don't?

All of the alternators I've opened the past week or so have this resistor (see photo). It goes across the brush holder and voltage regulator. The MR 253 shop manual, the parts book, and the photos in the Haynes manual don't show this resistor. One I checked was 40 Ohms.


Also, to make it easier to troubleshoot the circuits, I sketched out the charging circuit wiring diagram (below). The 84-86 factory foldout is near impossible to follow. I used the template from links near the bottom of post 3, Jeepforum Wiring 101

Let me know if you see any mistakes. It's specific to my '85 GW. Feel free to take and modify it for other years, turn it into function diagrams or whatever you need. Hopefully flkr is set so it will allow you to download the picture full size. (Its a gif)

Adding some explanation. C100 is the Bulkhead Connector. The dashed red line is the wire with red insulation and white trace. The Alternator is actually grounded to the engine even though I didn't put a ground symbol on it. The switch to ground with an N next to it is the Neutral Safety Switch.
The junction between the Ammeter and B2 also feeds the Headlights and other circuits that don't require a key ON or Accessory to work (like the Dome light).
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'85 Grand Wagoneer
360 727auto, NP229
body by beer (PO)
carries wood inside
no "wood" outside
My other car is a fish

Last edited by nograin : 12-20-2015 at 09:46 PM.
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Old 12-20-2015, 10:17 PM
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serehill serehill is offline
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Great diagram

Good detail

I would think it's to reduce alternator hum.
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Edlebrock performer intake
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If you can't make it better why waste your time. No use repeating the orignal mistakes. I'm to old to push it that's why.

Last edited by serehill : 12-21-2015 at 08:51 AM.
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Old 12-21-2015, 05:52 AM
JeepJeepster JeepJeepster is offline
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My 7278-9 12SI doesn't have that resistor. I just had it rebuilt at a local shop and it still doesn't have that resistor. No capacitor either.

Nice diagram. Amazes me someone thought it was be a good idea to send the Bat wire off the alternator inside the Jeep. If they wanted an amp meter there are much better ways...

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1998 Jeep ZJ 5.9
1994 Jeep ZJ I6
1989 Jeep Grand Wagoneer 360

Last edited by JeepJeepster : 12-21-2015 at 06:02 AM.
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Old 12-21-2015, 04:49 PM
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nograin nograin is offline
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Well, here's what I learned from the gentleman at Weitzel's D.C. Electric in Norwalk, Ohio.

Capacitors
The big round capaciter was used on the first years of the 12SI and also in the previous 10SI models. The later 12 SIs got a small black rectangular capacitor that tucks under the rectifier's negative side on the right. (I presume the change was for better heat dissapation or manufacturing costs.) They do the same thing.

Resistor from Trio to Ground

These are to help trip the idiot light. Of course the SJ's that got the 12SI alternators don't have said lights. Instead they have the ammeter and a resistor. So those 40 ohm resistors aren't needed in our Waggys, and that's why they're not in original alternators or in parts book.
-------------------------------
I'm going to spend some time looking for voltage drops, loose connections and grounds. Then go back to getting some more juice out of the alternator. I also confirmed that the only things needed to upgrade to a 94 amp alternator is the stator and a higher amp rectifier (identified by the curved fins). I think this assumes that the rotor is really a 12SI rotor not a 10SI rotor swapped in during remanufacuring. That's harder to spot.

Props to Mr Weitzel or whomever answered the phone. I called there because someone suggested his e-bay store as a source of parts and a little poking around with google showed it was a real repair shop. So he also has a lot more inventory than ebay shows
__________________
'85 Grand Wagoneer
360 727auto, NP229
body by beer (PO)
carries wood inside
no "wood" outside
My other car is a fish

Last edited by nograin : 12-21-2015 at 04:57 PM.
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Old 12-22-2015, 05:56 AM
JeepJeepster JeepJeepster is offline
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Location: Kentucky
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If I knew that I would've ordered a rebuild kit off him instead of paying someone else to rebuild it. I wanted new bearings and didn't want to screw around with that. Rebuild kit was ~$30, paying someone else to rebuild it was ~$45. I enjoy doing things myself though. I had put a new regulator in it but got a cheap one, which I regretted. Figured he would replace it since it was a full rebuild, no dice. He replaced everything BUT the regulator since he knew I had just replaced it. Trying to save a penny he was.

I searched for awhile trying to figure out what I had. Not much info on FSJ's alternators. Most people don't seem to know they have the 10 or 12si alternator. Ended up finding a parts list on a chevy forum, thankfully my tag was still legible.
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