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[Datsun 1200 encyclopedia]

Alternator diagnoses

From Datsun 1200 Club

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Categories: Charging System | Diagnoses and Corrections

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.


Requirements for Testing

The battery must be fully charged. A run-down battery can cause the alternator and voltage regulator to not work correctly.

Charge the existing battery (or swap in a fully-charged battery) before continuing.

First Test

Turn key to On/Run. Engine not running. Look at dash gauges. Is the IGN/CHG lamp lit up?

  • Yes: proceed to further tests
  • No: test the bulb before continuing. Remove the wiring connector from the Voltage Regulator and ground (earth) the WR wire with a small jumper wire. Repeat the test. If the lamp still does not light up, replace the burned-out dash bulb and perform this test again.

A Note About Wiring

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 - Neutral point. This is connected to the dash Lamp. This is NOT optional, the alternator in a 1200 car will not function with this disconnected. (Some later Datsuns have a parallel resistor in the dash, so it will charge even if the light is broken).

Basic Testing

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)
    • If it reads less voltage than that, continue on to the next steps
  • Measure Ohms between F & E terminals. It should be approxmiately 5 ohms (with ohmmeter on lowest range).
  • 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. This bypasses the regulator wiring
    3. Jump the F terminal to Battery +, momentarily
    4. Test with headlight main beams on
    18733.jpgAlbum 18733
    Output at 1100 RPM should be 12.5 volts or higher with headlights on
Do not exceed 1100 RPM when in test configuration

Voltage Output

For a lead-acid battery, the proper charging voltage depends on the temperature. For the charging system in normal operation, the following chart shows the expected readings at the Battery for the stock voltage regulator.

24525.jpgAlbum 24525

20C/68F: 14.1 to 15.2 volts

In no case should be it be lower than 13.85 or higher than 15.6 (with headlights off, engine at 1200 RPM or higher). Check for clean wiring connections if that occurs.

If voltage is outside this range, check and clean all the wiring connections involved. Then repeat the test. If it is still outside the specified voltages, replace the voltage regulator.


Charge Light Dimly Glows

The charge lamp is wired so it has 12 volts from the battery applied to one side of the light, and the alternator F terminal grounds the other side. So with key ON, engine OFF, the lamp will light up.

When the alternator starts to charge, the F terminal applies voltage to the the light. Since both sides of the lamp are at positive voltage, the light goes out.

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 glowing dimly while idling with headlights ON ... IF the two voltage tests checks out OK)

NOTE: Worn brushes can cause flickering dash lamp, or intermittent charging. Very easy to check -- see Alternator Maintenance#Brushes

No Output

Is the fan belt missing?

Can be caused by wiring, alternator, or voltage regulator. Always check the wiring first!

A broken wire (open connection) or corroded connection/loose connection in the wiring harness can cause this. Especially inspect the wire connectors and terminal connections -- clean, repair or replace as necessary.

On the alternator, it could be minor (sticking or dirty brushes, dirty slip rings, loose connection or broken wire) or major (defective stator, rotor, diodes).

Regulator stuck open can cause no output. See Voltage Regulator Maintenance.

Low Output

Can be caused by wiring, alternator, or voltage regulator.

First, check the Fan Belt tension. If very loose the alternator will slip instead of turn.

Second, clean the battery cable connections. Then retest.

Connection between Alternator BAT terminal and battery could be damage. Inspect the large wire carefully. With engine running, measure voltage AT THE ALTERNATOR. If it is above 13.5 volts, but the battery is below 13.5, the wire is bad.

Regulator may be defective. Sometimes adjusting it will fix the problem.

Alternator causes could be minor (brush problems, dirty slip rings) or major (defective stator, coil, diode).

Whining Sound

If the alternator emits an electical whine, one of the diodes may have gone bad.

Grinding/Roaring Sound

The bearings may be on their way out. See Alternator Maintenance.

Battery Overflowing

Overflowing, constant venting or bubbling of a battery is usually caused by overcharging (voltage in excess of normal).

But first, let the battery settle down and check the fluid level. If it has too much electrolyte (e.g. someone added to much water), that is the problem.

Excessively High Voltage

Wiring is unlikely to cause excessive voltage unless two wires are shorted together. Do a quick look to see if your wiring harness is smashed -- or damaged by rubbing engine parts. Also look for modified or jury-rigged wiring -- a previous owner may have connected the wires incorrectly.

Test at the battery terminal -- not at the battery cables ends on the terminals:

  • If the battery voltage is less than 15V but the terminals or main wires are +15V, this is a "bad connection" problem. See Loose Ground
  • If the battery voltage is higher than 15V, most likely, voltage in excess of 15.5 volts is caused by a bad voltage regulator. See Voltage Regulator Testing.

Alternator could be the cause if the (internal) N wire is shorted, broken, or incorrectly fitted around the brush holder. Remove the brush holder and inspect that wire.

NOTE: On a IR type Sunny (late 1200 Sunny Truck, or USA B210 or B310) the number one cause of voltage over 15.5 volts is: Someone fitted the wrong alternator. These cars are supposed to have an internally-regulated alternator, and if an (otherwise identical looking) externally-regulated unit is fitted, it will overcharge.

Engine Won't Stop Running

External regulator may be shorted. Unplug regulator to see if the engine stops.

Or, on an alternator swap, it is wired incorrectly. Wires are connected in the wrong places.

Charge Lamp Always On

Most likely causes for a charge lamp "always on":

  • External regulator Lamp relay is stuck. Replace regulator.
  • Wiring is incorrect. Correct it. 

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