Testing with an Ohmmeter

F1 Diode testing





Use a universal ohmmeter, if a diode tester is not available.
A 12 V test lamp or a battery powered buzzer can also be used.

F2 Resistance measurement





Use an ohmmeter when measuring resistance of stator winding.
Range 0 - 50 ohm.

F3 Remove pulley

Counterhold Tool 998 6683:

Unscrew center nut. If necessary counterhold pulley.
Remove long screws holding alternator housing together.

F4 Mark alternator





Mark front and rear housings so that they can be reassembled correctly.

F5 Disassemble alternator





NOTE: Clamp alternator in a vise and separate two housings using two screwdrivers. Do not insert screwdrivers more than 2 mm (1/4 in.) otherwise stator coil may be damaged.

F6 Before testing





Remove stator phase terminals from diodes. A short circuited diode can affect test.

Twin Parallel-Connected Power Diodes:

F7 NOTES: Bosch N1 14 V 70 A 20, N1 14 V 90A alternators. Disconnect twin, parallel-connected power diodes and proceed to F8.

*SPECIAL CASE: Bosch N1 14 V 31/80 with booster diode; *Carry out test according to E6-E8 in Testing Diodes using BOSCH TESTER.


F8 Diode test





Select ohm x 10.
Diode resistance should be low in transit direction and high (infinite ohm) in blocking direction.
- high resistance in both directions = open circuit
- low resistance in both directions = short circuit

F9 Positive power diodes





Connect meter probes between B+ and diode terminals, one at a time (3 measurements). Swap probes and take 3 more measurements.

F10 Negative power diodes





Connect meter probes between D- and diode terminals, one at a time (3 measurements). Swap probes and take 3 more measurements.

F11 Exciter diodes





Connect meter probes between D+/61 and diode terminals, one at a time (3 measurements). Swap probes and take 3 more measurements.

F12 Soldering diodes terminals





Use a 100 W soldering iron. Protect diodes from heat damage by holding base with pliers.

F13 Stator





Resistance measurement.
Connect meter probes between stator phase terminals (3 measurements in all). Resistance should be same for all readings, see appropriate specification.

F14 Insulation test - stator





Select ohm x 1000.
Connect one meter probe to stator body and other to a phase terminal.
A high (infinite ohm) resistance should be recorded.
A 40 V test lamp can also be used for this test.

F15 Rotor





Resistance measurement
Select ohm x 1.
Connect meter probes to slip rings.
- low resistance (approx. 1 ohm) = short circuit
- high resistance (infinite ohm)) = open circuit

F16 Insulation test - stator





Select ohm x 1000.
Connect meter probes between stator body and slip ring.
A high (infinite ohm) resistance should be recorded.
A 40 V test lamp can also be used for this test.

F17 Machining





Slip rings which are pitted or damaged should be machined. Do not machine to less than the minimum specified diameter. After machining, check for out-of-round with a dial indicator. Clean slip rings with a fine-grade wet sandpaper.

F18 Carbon brushes





Measure length of brushes between contact surface and brush holder. Replace brushes if worn or damaged.

F19 Insulation test





Make sure that brushes are insulated from each other, and that:
- The positive brush is insulated from ground
- Negative brush is connected to ground
- Pointer is 0 = good insulation
- 1 mA = good connection
A 40 V test lamp can also be used for this text.

F20 & F21 Bearing





Use a puller to remove bearing if worn or damaged. Install using a sleeve to press on bearing inner race.