Engine and Engine AggregatesAlternator
Alternator
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Measurement: |
Harmonic content of the charge voltage when engine is idling. |
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Check Objective: |
Check the charging operation and the excitation and rectification diodes of the alternator. |
Preparation
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Switch off the engine and ignition. Check connections to alternator, starter and battery are secure. Check tension of alternator belts. |
Connections
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INPUT A |
COM/TRIGGER |
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Connect the red measurement leads to the B+ or D+ (terminal 61) on the alternator. (The battery can also be temporarily connected). Ensure correct polarity! |
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Procedure Start the engine and allow it to idle. If the battery has been connected, electrical loads (e.g., headlamps, heated rear screen) must be switched on and the engine speed must be increased. Reference Curve | 
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The appearance of the measured curve can vary according to the condition of the engine (load, battery charge, engine speed). As a result, the following fault diagrams can only serve as a guide for evaluation. However the deviations arising from a fault are generally so major that definite allocation of the fault should nevertheless be possible. Fault Diagram 1 Symptoms: One harmonic is missing | 
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Possible Causes |
Countermeasures |
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Faulty excitation diode (open-circuit) |
Repair or replace alternator. |
Notice: Voltage transients occurring in the plotted curve can be generated by the alternator or result from interference from the ignition system. This does not indicate a fault.
Fault Diagram 2 Symptoms: Broad interruptions in the plotted curve and heavily distorted harmonic waves. | 
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Possible Causes |
Countermeasures |
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Faulty excitation diode (short-circuit) |
Repair or replace alternator. |
Fault Diagram 3 Symptoms: Plotted curve contains positive voltage spikes | 
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Possible Causes |
Countermeasures |
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Faulty positive rectification diode (open-circuit) |
Repair or replace alternator. |
Fault Diagram 4 Symptoms: Only two half-waves visible | 
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Possible Causes |
Countermeasures |
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Faulty positive rectification diode (short-circuit) |
Repair or replace alternator. |
Fault Diagram 5 Symptoms: Very narrow interruption in the plotted curve | 
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Possible Causes |
Countermeasures |
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Faulty negative rectification diode (open-circuit) |
Repair or replace alternator. |
Fault Diagram 6 Symptoms: Broad interruption in the plotted curve (only two phases visible) | 
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Possible Causes |
Countermeasures |
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Faulty negative rectification diode (short-circuit) |
Repair or replace alternator. |
Fault Diagram 7 Symptoms: Negative spikes after every harmonic | 
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Possible Causes |
Countermeasures |
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One phase open-circuited or two phases short-circuited together. |
Repair or replace alternator. |
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09 Engine
