1. Charging System Testing and Service
Unit 5
Charging System Testing and Service

2. Introduction Almost every system relies on: Automobiles have:
12-volt storage battery
Electrical and electronic components
Engine starting system
Electrical charging system
Electronic control modules
Networking systems

3. Charging Systems—Charging Systems Output Test (1 of 4)
Charging systems are voltage regulated.
Alternator maintains a set voltage.
Voltage drops as electrical load current increases.
Voltage regulator increases current output.
Increases system voltage

4. Charging Systems—Charging Systems Output Test (2 of 4)
Start alternator output tests with voltmeter testing of regulated voltage.
Voltage at which regulator allows alternator to create only a small charge
Should be between specified minimum and maximum regulated voltage

5. Charging Systems—Charging Systems Output Test (3 of 4)
Then check charging system output.
External electrical load to reduce battery voltage
Tricks regulator into making alternator produce maximum amperage output
Read by inductive ammeter
If within 10% of rate output, still okay

6. Charging Systems—Charging Systems Output Test (4 of 4)
If less than 10%, indicates:
Faulty regulator
Faulty alternator
Excessive voltage drops on alternator output or ground circuits

7. Charging Systems—Testing Charging System Circuit Voltage Drop (1 of 6)
Excessive voltage drop may cause:
Battery not fully charged—reduced voltage to battery
Battery charged but alternator at higher voltage—may overheat

8. Charging Systems—Testing Charging System Circuit Voltage Drop (2 of 6)
Problem shown by where voltage is sensed.
Alternator—battery undercharged
Battery—Alternator at higher voltage
External alternator output circuit has:
Wires and fuse or fusible links
Ground wire back to battery

9. Charging Systems—Testing Charging System Circuit Voltage Drop (3 of 6)
Voltage drop may occur in:
Output current circuit
Ground circuit
Terminals and connectors (most common)
Small amount of resistance can cause significant voltage drop.

10. Charging Systems—Testing Charging System Circuit Voltage Drop (4 of 6)
Voltmeter or DVOM measures voltage drops on all circuit parts.
Voltmeter with min/max range setting:
Records and holds maximum voltage drop occurring in particular operation cycle
Only works when circuit under load, current flow
Always perform test with circuit on.

11. Charging Systems—Testing Charging System Circuit Voltage Drop (5 of 6)
To measure with DVOM:
Most efficient to first measure voltage drop on both entire positive and negative sides
Connected in parallel across the:
Component
Then narrow down to individual components
Cable
Connections

12. Charging Systems—Testing Charging System Circuit Voltage Drop (6 of 6)
Example:
Connect black probe to alternator’s output side.
Connect red probe to positive post of battery.
Operate charging system under heavy load.
Record voltage drop across output side.
Perform same test on ground side.

13. Charging Systems—Replacing an Alternator (1 of 3)
Electrical faults:
Worn and noisy bearings
No-charge
Other internal or external mechanical damage
Undercharge
Overcharge
Mechanical faults:

14. Charging Systems—Replacing an Alternator (2 of 3)
Take precautions when changing alternator.
Battery voltage present at output terminal
Disconnect at least one battery terminal from battery.
Do not operate alternator with battery or terminals at back of alternator disconnected.

15. Charging Systems—Replacing an Alternator (3 of 3)
Usually between 31.8 and 14.5 volts. If not:
Alternator
Internal or external voltage regulator
Drive belt loose
Built-in voltage regulating circuit in ECM/BCM
Drops in charging or voltage sensing circuit
If not the problem, check for faulty:

16. Charging Systems—Diagnosing Undercharge and Overcharge (1 of 5)
Undercharging
Overcharging
Electrical system not fully functional
Short life for electrical component
Battery not fully charging
Overcharges battery
Increases gassing and water loss
Sulfation and early battery death

17. Charging Systems—Diagnosing Undercharge and Overcharge (2 of 5)
Voltage regulator maintains constant voltage output from alternator to max output current.
Always check specs for regulated voltages.
In non-PCM–controlled 12-volt charging system:
Regulated alternator output voltage
If outside specs, further testing needed

18. Charging Systems—Diagnosing Undercharge and Overcharge (3 of 5)
If overcharging:
Faulty regulator
Worn brushes
Voltage regulator
High resistance in rotor
Regulator ground
Open/shorted diode
Open/shorted stator windings
Shorted cell on battery
If undercharging:
Loose drive belt

19. Charging Systems—Diagnosing Undercharge and Overcharge (4 of 5)
Check easiest and most common faults first.
Drive belts slipping
Shorted battery cell
Inadequate battery capacity

20. Charging Systems—Diagnosing Undercharge and Overcharge (5 of 5)
Test voltage drops on output and ground circuit if voltage output is low.
If none found, fault is inside alternator.
Faulty regulator
Worn brushes
Faulty rotor

21. Summary
The first item that should be checked if an alternator is not charging according to the manufacturer’s specifications is the belt tension and condition.
Charging system voltage matching specification under load is the critical factor.
Before determining alternator failure, verify all inputs and connections, including alternator ground.

22. Credits
Unless otherwise indicated, all photographs and illustrations are under copyright of Jones & Bartlett Learning.