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Charging Systems

AC203 Alternator Fault Finding

From a BR Training booklet

Proceed with the following in numerical order until fault is located:
  1. Check for missing or loose driving pulley belts.
  2. Start the engines and by means of the tong ammeter check and record output readings -
    1. No Output If no output is obtained from either or both alternators stop the engines and proceed with the defective unit as per item 3.
    2. Batteries Overcharging Batteries overcharging and ammeter indicates high or full output all the time:-
      Check the positive sensing lead and its connection at the regulator unit. If no fault is found, renew the regulator.
      Note: maximum permissible from one alternator - 62 amps.
  3. Examine wiring connections at twin regulator unit, remove and test the appropriate fast fuse. If either fast fuse is found to be defective it should be replaced and the appropriate alternator field should be re-energised. Repeat output readings. (To re-energise the "flash" field, connect a 200 ohm 6 watt resistor between terminals BP and F/P for a few seconds while the engine is running).
    If no output reading is obtained, proceed as follows:-
  4. Start the appropriate engine and put a shorting lead between the regulator terminals FN and BN, run at fast idle only. Output should be noted from the alternator. If no output is obtained, the alternator, stator, slip rings/brushes or main diode network is defective and the following tests must be carried out to find the fault.
    Cables No. 70 and 84 must be disconnected at the BIS before proceeding with Item 5.
  5. Check the rotor windings for short circuits by disconnecting the wires F and A at the appropriate alternator and measure the resistance across the brush box terminals. The resistance should be 10.5 ohms. If correct resistance is not obtained, remove brushes and test direct into slip rings. If again correct reading is not obtained, examine the brushes and slip rings. Replace or clean as necessary to obtain a 10.5 ohm reading.

    Check that the brushes are in good condition and that they protrude from the brush box for a minimum of 1/4".
  6. Replace cables 70 and 84, flash field and re-test for output.
    1. If no output is obtained with field at 10.5 ohms, the appropriate alternator requires renewing.
    2. If output is obtained, proceed to Item 7.
    Tests 7, 8 & 9 to be carried out with the engines stopped.
  7. Where fitted, disconnect the auxiliary supply leads F1 and F2 from the appropriate regulator FP terminals but leave the FP leads in situ.
  8. Connect a proven 24V 2.8 watt lamp across regulator terminals BP and FP. Lamp should light.
  9. Disconnect the FN lead from the regulator. Lamp should go out. If not, the regulator requires changing. The defective regulator should be renewed and the alternator output test repeated.
  10. If tests 8 and 9 appear to indicate the appropriate regulator is satisfactory, re-test the alternator output as per Item 5. If output is then obtained renew the regulator and re-test for output.


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