Alternator diagnoses

(Difference between revisions)

Revision as of 21:24, 10 January 2012

The alternator should output 13.5v to 15v at 1200 RPM. A voltmeter is the only tool needed for testing. Further, he charge lamp on the instrument panel should come on before the engine starts, and go off while the engine is running. If it is not working correctly, then follow the testing procedures to find the cause.

Basic Testing

The alternator has four terminals:

• A - alternator connects to Battery cable + 12V via large white/red wire.
• E - earth/ground. Connected to case of alternator, hence though bracket to the engine. Engine is grounded by black cable (part of the two-cable negative battery cable)
• F - the Field. This needs 12V to get the alternator working. The Voltage regulator turns this on/off (12V) rapidly to achieve an average of 13.5-15.0 volts
• N - Nuetral point. This is connected to the dash Lamp. This is optional, the alternator will function with this disconnected.

Test Procedures

• With engine off, check voltage at battery terminals. If it is less than 12.0 volts, stop. Charge the battery before continuing. A low battery can prevent a good alternator from putting out correct voltage.
• With engine running at 1100 rpm, measure Voltage across battery cable. If it reads between 13.5-15.0 volts, everything is fine (or over 12.5 volts headlights on main beams)
• Measure Ohms between F & E terminals. It should be approxmiately 5 ohms (with ohmmeter on lowest range). If higher or lower, remove alternator and inspect.
• Perform Regulator Bypass Test
  1. Be sure to test at A & E terminals, to bypass external wiring problems
  2. Disconnect T-shaped connector (F+N) at alternator, to bypass the regulator wiring
  3. Jump the F terminal to Battery +
  4. Test with headlight main beams on

  

  Output at 1100 should be 12.5 volts or higher

Charge Light Dimly Glows

The charge light is wired so it has 12 volts from the battery applied to one side of the light, and when the alternator starts to charge, it applies voltage to the other side of the light. When you turn the key on, the alternator is not turning or making electricity, and the charge light is on, brightly. After the engine starts, the alternator makes voltage and the light turns off.

With the headlights on, under some situations it may be OK that the light glows dimly. Test as follows:

Engine speed 1,500 rpm, Headlights ON: Dimly glowing charge light -> Engine idling (Measure the voltage across B and L terminals:

• more than 0.5v -> fault alternator (not regulator)
• Less than 0.5v -> OK

Engine speed 1,500 rpm, Headlights OFF:

• More than 15.5v -> faulty voltage regulator
• 13v to 15v -> Let engine idle, Lighting switch ON:
  • Charge light on -> faulty alternator (not regulator)
  • Charge light off: OK

(it's OK for charge light to be vglowing dimly while idling with headlights ON ... IF the two voltage tests checks out OK)

Brushes

Brush comparo:

• Left: worn down to the limit line
• right: New

These just unbolt from the rear of the alternator, very easy to check and replace if necessary.

Rotor

After removing the brushes, you can test the rotor. Or disassemble the alternator.

Using an ohmmeter, check between the two copper slip rings:

• Should be around 4.4 ohms (with ohmmeter on lowest range)
  • No continuity means the winding is broken or detached
  • Lower ohms may indicate a short, perhaps from bad insulation

    Neither condition is easily fixable. Replace it.

Also check for isolation:

• There should be no continuity (even using 1M-ohm scale) between either ring and the rotor metal or shaft.

  If there is, it is shorted. Replace it.