DIY - Alternator Testing and Rebuild

[McGyver]

Alternator Rebuild and Testing

Alternators for our cars tend to be robust and the go-to solution for a bad alternator is to just replace the voltage regulator. However, there are many more components that should be checked/replaced to ensure your alternator is working correctly. Depending on the extent of the rebuild and your access to equipment, it’s not hard to fully rebuild your alternator.

I found a VW manual for Bosch alternators. It's attached to this post as a PDF.

Goal:

Replace all wear components and test for functionality.

Warning:

I’m not an electrical engineer, I cobbled together this information from multiple sources, and I’m writing this a little over 4 years from when I did the work. Follow at your own risk, I may be wrong or have forgotten to include a critical step.

Approach:

1. Disassembly
2. Rotor
3. Stator/Rectifying Diode Assembly
4. Voltage Regulator
5. Assembly

Tools:

Socket wrenches, screw drivers, flush cutter, penetrating oil, multimeter, adjustable power supply, soldering iron, solder, solder wick, emery cloth, anti-seize, spray paint, shop press, lathe

Parts:

I purchased everything from the website below. I’m not affiliated in any way, they just had everything I needed in one place. Prices are from January 2025. I’m not sure rebuilding at home is the cheapest option, but it is a fun project.
https://www.maniacelectricmotors.com/90ampboalfor4.html
Front Bearing - NTN 6303-2RS 17x47x14mm $18.98
Rear Bearing - NTN Bearing 6201-2RS 12mm X 32mm $14.98
Brushes - 5x8x18mm 51mm Lead $11.49
Capacitor - Replacing Bosch #s 0290800005, 0290800036 $7.29
Insulator – 92202011 $12.29
Bearing Ring - 32x35x18mm $14.98
Subtotal: $80.01

Disassembly:

See the picture below for an exploded view of a typical Bosch alternator. It is slightly different from the one on our engines.



Remove the 24-mm bolt by either holding the fan or using a hex socket to hold the rotor shaft. Remove the parts in this order: Nut, Washer, Plate, Outer Pulley Half, Inner Pulley Half, Spacer, Plate, Fan. Don’t loose the woodruff key for the rotor shaft!





Remove voltage regulator, terminal nuts, terminal insulation, and capacitor.

• Remove the outer four Phillips head bolts to separate the housing. Make sure to use penetrating oil on the back side as these will be corroded into place. These bolts are really hard to find if you strip the head or have to cut them. Length = 95mm, Thread Size = M5, Thread Length = 30mm
• Remove the rear housing. Remove the 6 bolts to release the Stator/Diode Assembly. These are M4x0.7 bolts if you need to replace them. Don’t loose the insulator on the “B+” terminal!
• Remove the four inner Phillips head bolts to release the stator from the front housing. These are M4x0.7 x 20mm. Separate the rotor from the front housing.
• Remove the front and rear bearings from the rotor using a press

De-solder the stator from the rectifying diode.



You now have a handful of parts to check and rebuild.



Rotor:

The rotor in an alternator is the rotating part that generates a magnetic field. As it spins within the stator, it induces an alternating current (AC) in the stator windings. This magnetic field is crucial for the process of converting mechanical energy (from the engine) into electrical energy.

Other than replacing the bearings, this section should only be attempted if you have access to a lathe. Use a low speed to avoid damaging the rotor.

Test the continuity across the two slip rings. Per the manual I found, set ohmmeter on lowest scale and connect across slip rings. Resistance should be 2.8-3.2 ohms for 65 through 90-amp alternators. If resistance is too low, the rotor has a short circuit and must be replaced. If the resistance is infinity (no continuity), the rotor has an open circuit and must be replaced. Mine measured 2.6Ω, good enough for me.



Hold the rotor as shown to polish the front shaft using emery cloth. The goal is to remove surface contamination, not any shaft material.



Hold the rotor by the front shaft and measure the minimum slip ring diameter (copper rings on the back shaft). Cut the slip rings until they are uniformly smooth. Don’t go to the bottom of any deep grooves.
The unworn section on mine measured 27.59mm and the worn section measured 26.28mm. The generic guide I found states that the minimum slip ring diameter is 25.4mm, but I don’t know if that applies here. I did not try to remove and replace the slip rings as they need to be pressed and soldered on.



Press the new rear bearing on the rear shaft. Use a socket to press on the inner race. If the bearing is only sealed on one side, the open side must face away from the rotor.



Flip rotor over. INSTALL THE BEARING RETAINER, then press on the front bearing. Use the spacer to press on the inner race. If the bearing is only sealed on one side, the open side must face the rotor.



Stator/Rectifying Diode Assembly:

Test all 6 combinations of stator leads. Per the manual I found, set ohmmeter on lowest scale. Resistance between leads should be approximately 0.09-0.10 ohms for 65 through 90-amp alternators. If the resistance is incorrect, stator has open or shorted windings and must be replaced.



Test resistance between each lead and the stator core. Per the manual I found, set ohmmeter on 1000 scale. No continuity should exist between each lead and core. If continuity exists, stator is grounded and must be replaced.



The rectifying diode in an alternator's charging system serves to convert the alternating current (AC) produced by the alternator into direct current (DC). Alternators generate AC, but most vehicle electrical systems and batteries require DC to function properly. The diodes are arranged in a bridge configuration to ensure that the current flows in one direction only, thus providing a steady DC output to charge the battery and power the electrical system.

The image below shows the wiring diagram for the stator/rectifying diode assembly.



There are three tests for the rectifying diode assembly. Per the manual I found, set ohmmeter on 100 scale. For each test below, test continuity across the specified point and each of the stator terminals. Then reverse the leads and repeat the test. Ohmmeter should indicate continuity in one direction only. Replace diode assembly if any test fails.

1. Positive Diode Test – Test from “B+” terminal to each stator terminal.
2. Negative Diode Test – Test from Negative Plate to each stator terminal.
3. Exciter Diode Test – Test from “D+” terminal to each stator terminal.

Assuming all tests have passed, this is a good time to paint the outside of the stator. Then solder the stator leads to the appropriate terminal.



Voltage Regulator:

The voltage regulator in an alternator controls the output voltage to ensure it remains within a safe range, typically around 13.5 to 14.5 volts. It adjusts the alternator's field current to prevent overcharging or undercharging the battery, ensuring stable operation of the vehicle's electrical system and preventing damage to the battery and other components.

You will need a power supply capable to producing more voltage than the specification on the regulator, which should be around 14.0 volts. Connect the power supply to the regulator as shown (small clips on the body). Set the multimeter to the 10 setting and connect a lead to each brush as shown. Be sure to avoid short circuits.



With the power supply below the voltage regulator cutoff, you should see the same voltage on your multimeter (minus a small drop). Once the power supply exceeds the cutoff, you should see less than 1 volts on your multimeter.





Assuming your voltage regulator is working correctly, it is time to replace the brushes. Use solder wick to remove the solder holding the brush leads in place and make sure the hole is clear for the new lead.





Slip the spring over the lead on the new brush and slide the lead through the hole in the voltage regulator. Solder the lead into place and trim excess lead. I tinned the end of the lead to keep it together when I pushed it through the hole. I used tape to hold the brush in place while I soldered the lead. It is probably a good idea to test the regulator again to make sure nothing got fried from the heat.



Assembly:

This is a good time to paint the housing. Make sure to mask off the threads in the rear housing and the bearing locations in each housing.

• Install the bearing ring in the rear housing
• Install insulator in hole for “B+” terminal
• Install the stator/rotor assembly into the rear housing. Use anti-seize and fasten the 6 bolts.
• FULLY seat the rotor bearing into its retainer in the rear housing. You risk the brushes bridging the slip rings, by passing the voltage regulator, if the rotor is not fully in place.
• Install the bearing spacer on the front shaft
• Install the front housing and use the 4 short Philips head bolts to secure the bearing retainer. Use blue Loctite.
• Install the 4 long Philips head bolts to hold the housings together. Use anti-seize.
• Rebuild the fan assembly. Don’t forget the woodruff key! Use blue Loctite on the nut.
• Install the insulator, washers, and nuts
• Install the voltage regulator. Use anti-seize.
• Install capacitor

* missing insulator in “B+” hole*

[82eye]

great work. i now know why i am stuck with buying alternators lol

[reelizmpro]

Great write up. Our alternators and starters are pretty well built. I've actually never had to replace an alternator on our cars, only change rubber bushings (E30 M3) and voltage regulators. I looked into replacing the brushes but decided it wasn't worth the trouble since I found some Hella replacements for like $20 or so.

I once overtorqued the nut for the small terminal post behind the alternator years ago. It was spinning and the alternator would not charge. I had another laying around so I swapped it out. When I tore into the alternator I found that the bolt was only held captive by plastic so it had spun and broken contact. All I had to do was resolder it, secure the bolt and I was back in business. That diode assembly plate was pricey when I looked for a replacement.