Testing Diodes with Bosch Tester

E1 Testing diodes using Bosch diode tester





It is not necessary to disconnect stator phase terminals from diodes. Bosch diode tester operates on alternating current, and measures ability of diodes to rectify current.

E2 Mark alternator





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

E3 Remove pulley

Counterhold Tool 998 6683:

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

E4 Disassemble alternator





Clamp alternator in a vise and separate two housings using two screwdrivers.

Use: Do not insert screwdrivers more than 2 mm (1/4 in) otherwise stator coil may be damaged.

NOTE SPECIAL CASE:

Twin Parallel-Connected Power Diodes:

Bosch N1 14 V 70 A 20, N1 14 V 90A alternators with twin, parallel-connected power diodes. When testing stator and power diodes, always disconnect and remove stator phase connections and diode parallel connections, and test components separately.

Booster Diodes:

Bosch N1 14 V 31/80 A with booster diodes. Test diodes according to E6-E8. Booster diodes increase power at high revolutions.

E6 Positive power diodes





Connect meter probes between B+ and diode terminals (three measurements).
Read upper section (large diode symbol).
Green section = diode OK
Red section = defective diode (open circuit)
Zero = defective diode (short circuit)

E7 Negative power diodes





Connect probes between D- and diode terminals (three measurements).
Read upper section (large diode symbol):
Green section = diode OK
Red section = defective diode (open circuit)
Zero = defective diode (short circuit)

E8 Exciter diodes





Connect probes between D+/61 and diode terminals (three measurements).
Read lower section (small diode symbol):
Green section = diode OK
Red section = defective diode (open circuit)
Zero = defective diode (short circuit)


E9 Testing diodes disconnected from alternator, using 40 V test voltage





Red probe = (+), and black = (-).
Read left side of meter. To check condition of diode
Connect probes to diode casing and terminal leads. Diode is OK if reading = 1 mA (transit direction), and 0 mA (blocking direction).
- 0 mA in both directions = open circuit
- 1 mA in both directions = short circuit
- Max. current leakage in blocking direction = 0.8 mA at +25'C (77°F), at diode casing.

E10 Determining diode polarity





Positive: Connect black probe to diode casing and red to diode terminal.
- pointer should move to 1 mA.

Negative: Connect red probe to diode casing and black to diode terminal.
- pointer should move to 1 mA.

E11 Stator





Resistance measurement
Connect meter probes between stator phase terminals (three measurements).
Read right side of meter (0-2 ohms). Resistance should be same in all three readings.

Refer to specifications for exact readings.

E12 Insulation test using 40 V test voltage*

carried out if negative diodes are OK.:

*This test can only be carried out if negativce diodes are OK. Connect black probe to stator ground. Hold red probe against diode terminals (three measurements). Read left side of meter (0 - 1 mA). If insulation is OK, pointer will deflect slightly to left (caused by leak current from negative diodes). If pointer strikes left stop position, this indicates poor stator insulation.

E13 Rotor





Resistance measurement
Connect meter probes to slip rings.
Read right side of meter (0 - 20 ohm). Check specifications for appropriate alternator.
Low resistance (approx. 1 ohm) = short circuit
High resistance = open circuit

E14 Insulation test - 40 V test voltage





Connect probes between stator ground and one of slip rings.
Read left side of meter (0 - 1 mA).
Insulation is OK if pointer stays at zero.

E15 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.

E16 Carbon brushes





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

E17 Insulation test - 40 V test voltage





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 zero = good insulation
- 1 mA = good connection

E18 Bearings





Removal: Replace bearings if worn or damaged. Use a puller to remove bearing.

E19 Installation:





Use a sleeve to press on bearing inner race.