PSA: If you can remove your alternator, you can probably rebuild it. (1 Viewer)


... pronounced "Jason"
Aug 10, 2011
Clarksville, TN
or "how I learned to trust but verify when the parts store tests your alternator and tells you it is bad."

Also, this is assuming that your rotor and stator windings are good. Replacing the brushes, regulator, and rectifier is pretty easy.

After watching lots of vidjeos and reading a lot of pre-newtonian physics explanations of how alternators and regulators work, I decided to do a partial rebuild for about $50 instead of buying a new or remanufactured alternator.

for posterity
1990 HDJ81 1HD-T
27060-17040 - 100 or 110A depending on where you look

I bought all of these parts off of Alternator Rebuild Kit | Alternator Repair Kit | Starter Kits (not affiliated)

Alternator Regulator Denso 126000-0610, -1440, -1441 - 80904388
Denso Alternators,Rectifier, 8 Diodes, - 77904015
Alternator Voltage Regulator for 104210-2090, -4750, -4790, -6131 - 80904455: Alternator Brushes Nippondenso (2 Pack) 4.8-5mm Tall x 7mm Wide x 15/14mm Long x 48mm Long Lead - 7390901

I swamped my alternator going through a mud hole and got the "christmas tree lights" on the dash. I was worried about the high A/T oil temp and T.belt lights and did not notice that the Alt light was on and the batt volts were going low until later. I was able to air up and get home from about an hour away.


To get the thing out, I struggled a bit due to the AC compressor, then later found these instructions on Radd Cruisers, which roughly matched what I ended up doing.

One thing I noticed was that the pulley tensioner may have been a bit loose, but I definitely saw the wheel on the alt spinning, so I think it was a legitimate failure of the electronics.

I took it to Advance Auto and had them run it on their bench and they said it was dead (sweet confirmation bias). They seemed to struggle to get it set up, and maybe their rig had an issue (foreshadowing).

On the bench I did all of the 1HD-T FSM charging system checks (spins good, rotor and stator field winding continuity and ground resistance checks, slip ring runout, brush length) and everything came out ok except one diode didn't seem to work right and I got some funny readings with the multimeter. The FSM doesn't tell you how to check the regulator and I couldn't make a bench tester to give me any readings that meant anything, but since the Voutput was low on the charging circuit, I figured it could just as well be the regulator as the diodes. Since the cost was negligible, I replaced the regulator and diode rectifier plate. The brushes and slip rings (collector) were nominal, so I ended up not needing the brushes.

The new rectifier plate came without the gray epoxy coating, but the diodes are all potted and I don't know exactly what was on the old one so I rolled the dice and installed it anyway.


As for the regulator, the new one seemed to be missing a component, but all pictures online of the replacement regulator seemed to be the same. I figured that the electronics package has been redesigned and made smaller over the last 30 years and eliminated that UJT or whatever that thing is on the bottom.


I took it back to Advance Auto, and to my chagrin, the guy told me "It's f*cked, pardon my language." I was pretty crestfallen, but was determined to figure out what the issue was.

Back home again, I tried to make a bench tester and apply 12v on the IG and Sense and to get it to light a 12v light bulb and measure the output on the B+ post while spinning with a drill. It was all pretty inconclusive but I felt like with new electronics and good winding the thing should work, so I reinstalled it anyway.

Boom, it worked great and made 14-15 volts at the batteries. I dunno what witchcraft is going on in there that I can't measure, but the end result was that the parts store tested it bad and it works in the truck, so I am happy.
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