7. Alternator: overhaul

  1. Owners should note that since the alternator is a slightly modified version of a unit that can be found on many modern cars, particularly European models, most auto-electrical specialists will be familiar with it.
  2. This means that it may be economically more sensible to take the unit to a specialist lor overhaul than to attempt repairs; certainly an auto-electrical specialist will be able to test very quickly and thoroughly an alternator that is thought to be suspect, even if he cannot obtain the necessary replacement parts from his usual sources. Note also that the manufacturers, Bosch, have their own network of service agents. Owners should investigate the economics of all possibilities before starting work, and should ascertain whether exchange units are available before purchasing expensive replacement parts.
  3. The alternator brushes can be inspected or renewed without removing the alternator from the machine, but disconnect the battery negative lead first.
  4. From the rear of the alternator remove the two screws which secure the voltage regulator/brush carrier assembly. Withdraw the assembly.
  5. Measure the length of each brush projecting from the carrier. If they are worn down to, or below, the minimum projection specified the old brushes will have to be unsoldered and new ones soldered into place. Some skill with a soldering iron will be required; excess heat from the soldering iron could damage the voltage regulator. When fitted, the new brushes must move freely in their holders; ensure that solder does not run down the brush leads. If the original brushes are still serviceable, check that they are both free to move easily in their carrier and that their ends bear fully on the slip rings.
  6. While the brush holder is removed, take the opportunity to clean the slip rings with a cloth soaked in high flash-paint solvent. If badly marked, use a piece of fine glass paper.
  7. To dismantle the unit remove the brush carrier/voltage regulator as described above, and the shock absorber body and fan as described in Section 6. Mark the unit across the mating surfaces of the front housing, the stator windings and the rear cover so that all can be aligned correctly on reassembly. Remove the three retaining screws and withdraw the rear cover far enough to permit removal of the tour small screws which secure the diode plate to the cover inside.
  8. Test the diode plate using a battery and bulb test circuit or an ohmmeter or multimeter set to the resistance scale. When testing rectifier assemblies the important consideration is that each diode must allow current to flow in one direction only, ie current should flow or little resistance should be measured in one direction but when the tester probes are reversed no current should flow or much heavier resistance should be measured. Test between the diode plate surround ( or alternator rear cover) and each stator winding end in turn, then between the B + terminal and each winding end, finally between the D + terminal and each winding end. If current flows in both directions or in neither during any of these tests, that diode is faulty and the plate assembly must be renewed. This means unsoldering the connections between the stator and the diode plate; take great care to ensure that the connections are properly re-made on reassembly.
  9. Test the stator windings by measuring the resistance between each pair of phase outputs. lf the readings obtained differ significantly trom that specified, the stator is faulty and must be renewed. lf its connections are first unsoldered, the stator can also be checked for short circuits to earth, testing between each wire end and the stator body; the coil windings should be completely insulated from the body.
  10. The rotor need only be removed from its front bearing and the front housing if necessary; this task may require the use of a press. Check the rotor exciter winding by measuring the resistance across the slip rings; the reading obtained should be close to that specified. lf the slip rings are worn, scored or distorted, they can be trued up by skimming in a lathe, provided that this does not reduce their outside diameter to less than the minimum specified.
  11. Check the rotor by testing for continuity between both slip rings in turn and each of the rotor steel claw poles; applying up to 80 volts ac. there should be no continuity, ie infinite resistance. lf the rotor or bearings are to be renewed, use a bearing puller to draw off the rear bearing and a hammer and a tubular drift which bears only on its inner race to refit the bearing. Once its retainer plate has been withdrawn, the front bearing can be removed and refitted using a hammer and a tubular drift which bears only on its outer race. Ensure the bearing is square in its housing.
  12. On reassembly, fit the front bearing and fasten securely the retainer plate screws, then press the rotor shaft into the bearing. When soldering the stator connections use the bare minimum of solder possible to achieve a good joint and ensure that the wires are clear of the rotor. Do not forget to align the marks made on removal across the stator, the front housing and the rear cover when reassembling. Tighten securely the retaining screws but do not overtighten them.

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7.4a Remove retaining screws (arrowed) ...

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7.4b ... and withdraw voltage regulator/brush carrier to check brushes

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7.5a Measuring brush projection to determine brush wear

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7.5b When fitting new brushes, great care is required when using soldering iron

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7.7a Before dismantling alternator make reference marks to ensure correct reassembly

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7.7b Remove three long screws from front cover to dismantle alternator

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7.7c Remove four screws trom inside alternator (arrowed) ...

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7.7d ... so that rear cover can be detached from stator and diode plate

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7.8 Diode plate and stator can be tested as described


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