The following table provides an overview of the alternator check procedure.
Battery Check
1.) Measure Off-Load Voltage
- Voltage measurement at battery terminal 30 and battery terminal 31.
| Off-load voltage, reference value: | 12,8 - 13,2 V |
|---|
| Reference value not attained: | Charge or replace battery |
2.) Battery Test:
- Test battery with tester KM-J-42000
Alternator Check
3.) Visual Testing:
- Connect battery to supply system
- Ensure V-belt tension complies with auto manufacturer data
4.) Measure No-Load Control Voltage
- Start cold engine (no consumers)
- Engine speed 2000 rpm
- Measure control voltage at alternator connection B+ and alternator housing (not at battery terminals)
| Control voltage, reference value: | 13,7 - 14,8 V |
|---|
| Reference value not attained: | Vehicle with petrol engine: continue with test step 5 |
| | Vehicle with diesel engine: continue with test step 7 |
| Reference value attained: | continue with test step 9 |
5.) Test Preexciting Circuit:
- Remove terminal L wiring harness plug
- Voltage measurement at terminal L and battery terminal 31
- Ignition ON
- Engine OFF
| Voltage, reference value: | less than 5 V |
|---|
| Reference value not attained: | - test wiring harness between alternator terminal L and engine control unit for short-circuit against voltage, earth and break |
| | or |
| | - replace engine control unit |
Important: Components must be replaced in the specified sequence.
| Reference value attained: | continue with test step 6 |
6.) Preexciting Circuit:
- Connect terminal L wiring harness plug
- Voltage measurement (with adapter cable) at terminal L and battery terminal 31
- Engine RUNNING
- switch on consumer (lights, heated rear window, heater fan, etc.)
| Voltage, reference value: | greater than 5 V (depending on engine speed and output of the connected consumer) |
|---|
| Reference value not attained: | - test wiring harness between alternator terminal L and engine control unit for short-circuit break |
| | or |
| | - replace engine control unit |
Important: Components must be replaced in the specified sequence.
| Reference value attained: | - replace alternator control unit (if control unit is listed separately according to EPC) |
|---|
| | or |
| | - Replace alternator |
Important: Components must be replaced in the specified sequence.
7.) Test Preexciting Circuit:
- Remove terminal L, F wiring harness plug
- Voltage measurement at terminal L, F and battery terminal 31
- Ignition ON
- Engine OFF
| Voltage reference value, terminal L: | greater than 8 V |
|---|
| Voltage reference value, terminal F: | less than 5 V |
| Reference value not attained: | test wiring harness between alternator terminal L, F and engine control unit for short-circuit against voltage, earth and break |
| | or |
| | - engine control unit |
Important: Components must be replaced in the specified sequence.
8.) Test Preexciting Circuit:
- Connect terminal L, F wiring harness plug
- Voltage measurement (with adapter cable) at terminal L, F and battery terminal 31
- Engine RUNNING
- switch on consumer (lights, heated rear window, heater fan, etc.)
| Voltage reference value, terminal L: | greater than 5 V (depending on engine speed and output of the connected consumer) |
|---|
| Voltage reference value, terminal F: | greater than 5 V |
| Reference value not attained: | - test wiring harness between alternator terminal L, F and engine control unit for break |
| | or |
| | - replace engine control unit |
Important: Components must be replaced in the specified sequence.
| Reference value attained: | - replace alternator control unit (if control unit is listed separately according to EPC) |
|---|
| | or |
| | - Replace alternator |
Important: Components must be replaced in the specified sequence.
9.) Alternator Charge Testing:
- Position clamp-on ammeter as close to alternator B+ connection as possible
- Measure control voltage at battery terminal 30 and battery terminal 31
- Start engine
- Engine speed approx. 2000 rpm
- Charge the supply system (battery) with 50 % of the current strength specified on the rating plate.
Example: for an 80 amp (A) alternator, the following consumers would be switched on to achieve a charge of 40 A:
| Lights: | approx. 12 A |
|---|
| Heated rear window: | approx. 10 A |
| Fog lights: | approx. 8 A |
| Interior fan: | approx. 8 A |
| Air-conditioning unit: | approx. 8 A |
| Total: | approx. 46 A |
- Here, the control voltage must not fall more than 0.5 V below the reference value.
| Control voltage, reference value: | 13,7 - 14,8 V |
|---|
| Reference value not attained: | continue with test step 10 |
| Reference value attained: | see instruction for static current measurement |
10.) Charge Test 1, Charging Current Circuit:
- Connect bridge cable between battery terminal 31 and alternator housing:
- Control voltage at battery terminal 30 and battery terminal 31
- Here, the control voltage must not fall more than 0.5 V below the reference value.
| Control voltage, reference value: | 13,7 - 14,8 V |
|---|
| Reference value not attained: | continue with test step 11 |
| Reference value attained: | High contact resistance between battery terminal 31 and alternator housing |
11.) Charge Test 2, Charging Current Circuit
- Connect bridge cable between battery terminal 30 and B+:
- Measure control voltage at battery terminal 30 and battery terminal 31
- Here, the control voltage must not fall more than 0.5 V below the reference value.
| Control voltage, reference value: | 13,7 - 14,8 V |
|---|
| Reference value not attained: | Replace alternator |
| Reference value attained: | High contact resistance between battery terminal 31 and alternator housing |
Oscilloscope - Reference Images:
- Test alternator with oscilloscope (connect as for voltage measurement to B+ and alternator housing)
Alternator with phase error
Alternator with diode error (plus or minus diode break)
Alternator with diode error, plus or minus diode (short-circuit - alternator excited)
Alternator with diode error, plus or minus diode (short-circuit - alternator not excited)
Instruction for Static Current Measurement:
Unpermissibly High Battery Discharge May Also Result from Drawing of an Increased Static Current.
- Drawing of the static current is dependent on:
| - vehicle equipment |
|---|
| - control unit tracking time |
