Solving The Alternator/Regulator Mystery

01/09/15 (Note that the diagrams and figs. have not been published yet... soon)

Faced with a faulty G.M. alternator/regulator most mechanics "go weak in the knees", reach for the phone and order a rebuilt. Why? Why lose money by "sending out" or lose money from related sales and labor charges? And most important--why put your reputation on an item that has been rebuilt by someone else?
Section I-Alternator Testing and
Section II--Disassembly, will cover the testing, repair and replacement of the most common alternator components including the regulator, rectifier bridge, diode trio, brushes and bearings.


Alternator Testing

Three basic problems can exist with the charging system:
A. Failure to charge
B. Low charging rate
C. Overcharge

Let's deal with them one at a time...

A) Failure To Charge

1. Make sure fan belt is adjusted to proper tension.
2. Connect voltmeter to BAT terminal on alternator with negative meter lead to ground.
3. Run engine 1500 to 2000 rpm with all lights and accessories ON. If the voltmeter reads from 12.8 volts to 15 volts the alternator is good, and the problem is in the external wiring shown in Fig. 1. If voltage is less than 12.8 volts, ground the field winding by inserting a screwdriver into the test hole in the end frame. (Fig. 2).

CAUTION: Do not force the screwdriver more than 3/4 inch into end frame.
If voltage increases to 13 volts or more the internal regulator unit is defective.

If there is no appreciable increase in voltage,the alternator is defective. Check bridge rectifier, diode trio, brushes and field windings. (See Section II).

B) Low Charging Rate

1. Make sure the fan belt is adjusted to the proper tension.

2. Connect a 10 ohm resistor rated at 6 watts or greater, a 10 volt voltmeter, an ammeter of greater range than the alternator output and a carbon pile resistor as shown in Fig. 3.

3. Note output current rating stamped on alternator frame.

4. Run engine at about 2500 RPM and adjust carbon pile to obtain rated alternator output as indicated on ammeter. If you get within 10% of rated output alternator is good.

5. If alternator is not within specifications, ground the alternator field by inserting a screwdriver into the test hole in the end frame. (Fig. 1): if output rises to within 10%: of rating then the voltage regulator is faulty and must be replaced. If output is still not within specifications then check diode trio, rectifier bridge and stator windings. (Section II).

C) Overcharge

With voltmeter connected to BAT terminal vary the engine speed. If voltage is uncontrolled with engine RPM, and rises to 16 volts or more with increasing RPM, then either a brush clip is grounded or the regulator is faulty and must be replaced.


Disassembly Component Testing and Repair

After completing the testing above, you probably have a good idea what's wrong. This section will describe a step by step strip down of the alternator and will indicate at which step each major component can be tested and replaced.
The index below provides a quick reference for the disassembly steps required to replace the various components.
  For example, if your only replacing the stator winding, you only need to do steps 1 thru 3, and step 5.

To Replace, Disassembly, or Service Steps:

Stator Winding 1 thru 3, and 5
Diode Trio 1 thru 3, 5. and 7
Rectifier Bridge 1 thru 3, 5_ 7, and 10
Brush Assembly 1 thru 3, 11 thru 14 except in step 11 remove all three screws on brush assembly, Bend lead of diode trio up and carefully to allow brush assembly to slip out Voltage Regulator Same as for Brush Assembly
Bearing (Slip ring End Frame) 1 thru 3. 5,7, 10 thru 12, 15 thru 17
Slip Rings 1 thru 13; 13 (c). 18
Rotor (Field) 1 thru 3. 13 (c). 19
Bearing (Drive End Frame ) 1 thru 3, 13 (c); 19, and 20

(For reassembly, reverse the disassembly steps.)

1. Secure the alternator in a vise by tile large mounting flange only.

2. Remove the 4 casing bolts. Carefully pry apart the Slip Ring End Frame and Drive End Frame. (See Fig. 4). Remove loose brush springs and reposition brushes in holder temporarily.

3. Protect the bearing and end of shaft by masking with tape.

Grounded Brush Lead Clip

4. Set ohmmeter to lowest scale and measure between brush clip and frame. (Fig 5). Do not measure from screw head since it is normally ground. Note reading and then reverse leads.

Meter should indicate high resistance. If meter reads zero then clip is grounded, probably due to missing or defective insulating sleeve or washer Stator Winding.

5. Remove the three nuts connecting the stator windings to the bridge rectifier and lift stator out of the end frame. (See Figs. 6 & 7).

6. Use ohmmeter on lowest scale to check for continuity of stator windings. (See Fig. 8). Measure between pairs of terminals. Meter should read the same (almost zero ohms) between all combinations of terminals. An open indicates a defective stator.

Diode Trio

7. Remove screw connecting diode trio lead clip to brush lead clip and lift out (Fig. 9). Note position of insulating washer. It must be reassembled in same position.

8. Set ohmmeter on lowest scale and connect between single brush lead and one of the three stator leads of diode trio. (See Fig, 10). Note reading then reverse ohmmeter leads. There should be one low reading and one infinity reading. Repeat with ohmmeter connected from single lead to remaining two stator leads of diode trio. If you don't get these readings on any of the tests then the diode trio is defective and should be replaced with a STANDARD diode trio #D-5.

Rectifier Bridge

9. To test the bridge connect an ohmmeter between the grounded heat sink and one of the three bridge terminals (See Fig. 11). Then reverse the meter leads and note the reading. If both readings are the same the bridge is defective and must be replaced. A good unit is almost shorted in one direction and almost infinity in the other direction. Repeat this test on the remaining two bridge terminals. Now repeat the three measurements but this time between the insulated heat sink and the bridge terminals. ( Fig. 12). Again. if any two readings are the same when the meter leads are reversed the bridge is defective and must be replaced. Use a STANDARD silicon rectifier bridge #D-4.

10. To replace the bridge remove one attaching screw and the BAT terminal screw (Fig. 13). Do not lose the toothed washer under nut of battery post. Disconnect condenser lead and remove bridge.

NOTE: The insulator between the insulated heat sink and the end frame must be returned to the same position when reassembling.

Brushes and Voltage Regulator

11. Remove remaining two screws holding brush assembly. (Fig. 14). Note which screws have insulating washers which must be replaced in the same position when reassembling.

12. Remove brush holder and brushes. (Fig. 15). The voltage regulator may now also be removed. (Fig. 16). If preliminary tests had indicated a faulty voltage regulator it should be replaced at this time with a STANDARD VR-111 regulator.

13. If the brush assembly is to be re-used inspect assembly for following:
a Brushes should be at least 1/4" long
b. Brushes and springs must be free of corrosion, undamaged, and free of oil and grease. Clean the brushes with trichlorethylene in a well ventilated area.
c. Insert spring and brushes into holder and check that they are Free to move without binding. Insert wooden or plastic toothpick into bottom hole in holder to retain brushes during reassembly (Fig. 17).

NOTE: Be sure to remove the toothpick after the two alternator frames are reassembled. (Fig. 18).

14. If inspection of the brush assembly indicates any fault, replace brushes with STANDARD RX-103 Brush Set or RX-103A Brush Holder Assembly.

Bearing (Slip Ring End Frame)

15. Support the end frame on the inside with a hollow cylinder which has an inside diameter large enough to allow the seal and bearing to pass. A large deep socket can be used. Remove bearing by pressing it from outside to inside. Bearings are permanently lubricated. Do Not Lubricate. If bearing is dry or worn, replace it.
16. To replace bearing, use a flat plate placed against bearing and press from outside towards inside. A large vise is handy as a press but again support end frame with cylinder on inside to prevent distortion or cracking of the frame.

17. From inside the end frame, insert seal and press flush with housing.

Slip Rings (Drive End Frame)

18 Inspect slip rings for dirt or scoring. They may be cleaned with #400 grit sand paper, while rotating the rotor to make sure rings are concentric. (Do Not use emery) If slip rings are deeply scored or out of true concentric by more than .001 the entire rotor must be replaced as a unit.


19. Check for open field winding using an ohmmeter on lowest scale. Measure across both slip rings. (Fig. 19). If meter indicates open field winding, replace rotor. To remove rotor assembly from drive end frame (Refer to exploded view Fig. 20):
Remove pulley nut while preventing shaft rotation with a 5/16 inch Alien wrench.
Remove pulley, fan and spacer.
Remove rotor and internal spacer drive end frame.

Bearing (Drive End Frame)

20. Remove three screws from bearing retainer plate on inside of drive end frame (Fig. 21); remove plate and bearing. Do not attempt to lubricate bearing.

Congratulations! If you followed the steps outlined, you have completed a STANDARD PLUS Alternator/Regulator repair job. You also know that you have used the best parts available.