Regulator-Test
This test can be performed to determine an alternator or regulator malfunction. This test does not cover all the failure possibilities, but verifies a few common problems. This test requires additional test equipment besides the 6V-7070 Digital Multimeter. See the test equipment chart for all of the needed equipment.
Alternator output voltage is regulated by controlling field coil current. The regulator senses output voltage. If the voltage is low, the regulator allows field current flow and the voltage builds. When output voltage exceeds the upper limit, field current is turned off. A properly functioning alternator and regulator will have out put voltage of 28.0±1.0V for 24V alternators or 14.0±0.5V for 12V alternators. The voltage should not vary more than approximately 0.3V during this process. This test measures voltage when the regulator turns field current on and off and determines if there is an open or short in the diode trio, field coil or regulator.
| Test Equipment Chart | ||
|---|---|---|
| Quantity | Symbol | Description |
| 1 | VPS | 0 to 40V, 6A Variable Power Supply |
| 1 | S1 | Single Pull, Single Throw Switch |
| 1 | S2 | Single Pull, Double Throw Switch |
| 1 | T1 | 6.3V, 0.3A Filament Transformer |
| 2 | R1, R3 | 500 Ohm Potentiometers (Any type) |
| 1 | R5 | 18 Ohm Resistor (1/2 Watt) |
| 1 | R2 | 180 Ohm Resistor (1/2 Watt) |
| 1 | R4 | 270 Ohm Resistor (1/2 Watt) |
| 1 | F1 | 1A Fuse |
| 1 | V1,V2,A | 6V-7070 Digital Multimeter or Equivalent (V1 Measured On AC Scale) |
 | |
| Illustration 1 | g01065398 |
Turn-On Circuit (B) Wire (C) Wire | |
The Prestolite SCJ series alternators require a test circuit generating a regulator turn-on as shown in Illustration 1. This circuit must be connected to the regulator before doing the regulator test.
- Make the turn-on circuit as shown in Illustration 1 from the equipment listed in Table 1.
- Remove the four regulator screws and pull regulator away from the alternator far enough to see the internal wire connections.
- Disconnect the two yellow phase leads from the stator terminals on rear of alternator.
- Connect the phase leads to wires (B) and (C) of the turn-on Circuit.
- Put voltage selector (S2) on 12V or 24V. Adjust potentiometer (R1) or (R3) until voltage (V1) is 0.4V.
- Connect variable power source (VPS) positive (+) lead to the alternator "B+" terminal of the alternator. Connect VPS negative (-) lead to the "B-" terminal of the alternator. Adjust VPS to 28.0±1.0V for 24V alternators or 14.0±0.5V for 12V alternators until ammeter (A) first indicates current draw. Measure voltage (V2) and write this measurement down as turn-on voltage.
Ammeter (A) should read field current. It should be as follows:
- If ammeter (A) indication is zero amps, the diode trio, field coil or regulator are open. If ammeter (A) indication is too high (see Step 7), the field coil may be shorted. The regulator is also bad if the turn-on voltage is not 28.0±1.0V for 24V alternators or 14.0±0.5V for 12V alternators.
- If the measurements in Step 7 and Step 7 are correct, go to Step 9. If they are not correct the alternator and/or the regulator are bad.
- Adjust variable power supply (VPS) to the turn-on voltage as measured in Step 6. Slowly increase the voltage until ammeter (A) indicates zero amps. Write this voltage down as turn-off voltage.
- The difference between turn-off voltage and turn-on voltage must be no more than 0.3V. A larger value indicates a regulator malfunction. Also, ammeter (A) should drop sharply to zero amps. If not, the regulator is faulty.
- Adjust VPS until ammeter (A) indicates current draw. Next, disconnect one of the yellow phase leads connected to the turn-on circuit. This should cause ammeter (A) to read zero amps. If not, the regulator is faulty.