Testing Field Coils

Internal connections of generator field coils to brushes and terminals are shown in Fig. 5E-2.

Internal connections of generator field coils to brushes and terminals are shown in Fig. 5E-2.

Arrange an ammeter and battery in series with test points connected to leads. NOTE: All 12-volt generators are stamped "12V" following model No. on frame. Use a 6 volt battery for testing 6-volt generators or a 12 volt battery for testing 12-volt generators. During all tests be particularly careful to avoid overloading or shorting ammeter. An overload is indicated by the needle going beyond range of calibrated scale. A direct short is indicated by needle swinging violently to extreme limit of its travel. In either case, contact must be broken instantaneously to avoid damaging the ammeter. In making the following tests, first make only a flicking, momentary contact to determine if a short is present. If ammeter needle does not go beyond calibrated scale, it is safe to make continuous contact. As added precaution, work on a bench with a nonconductive top. Never touch test points together.

Figure 5E-3. Checking Armature for Grounded Winding

1. Remove brushes or insulate brushes from commutator. Touch one test lead to "F" "terminal and the other to any part of the generator frame. There should be no reading. Move first terminal lead to "A" terminal. A reading at either contact indicates a terminal or field coil is grounded to frame. If no reading was obtained, follow further disassembly procedure and eliminate step 2.

Remove generator drive gear using Gear Puller, Part No. 95715-19A.

Press armature out of ball bearing with arbor press and remove. Disassemble terminals, remove field coil leads, inspect terminal components for cracked or worn through insulating materials and, if parts appear serviceable, reassemble terminal components eliminating field coil leads.

2. Recheck terminal to ground contacts as described in step 1. No reading indicates terminals are properly insulated. If reading was obtained in step 1, but not in step 2, field coils are probably grounded.

3. Touch one test lead to either field coil lead and the other to the generator frame. A reading indicates a field coil is grounded and it is necessary to clip the connection between the field coils. Touch test leads to one field coil lead and ground. Repeat process on other coil. A reading indicates a grounded coil which will have to be replaced. If terminals and field coils are in serviceable condition, proceed to step four.

Figure 5E-4. Testing Armature for Short

4. Test field coils, using 6 volt battery for 6 volt generator and 12 volt battery for 12 volt generator, touching test leads to coil lead terminals. Current values should be as follows for double coil: 2 amp. on 6 volt coils, .95 amp. on 1964 model 12 volt coils and 2.3 amp. on 1965 model 12 volt coils. No reading indicates an open coil, a higher reading indicates a shorted coil.

5. Strip back the insulation at point where two field coil leads are joined and file the insulating varnish off a spot on the splice. Connect one test lead at this point, the other at either coil lead. Without moving first test lead, move second test lead to opposite free lead. Current values should be as follows: 4 amperes for 6 volt coil, 1.9 amperes for 1964 model 12 volt coil, arid 4.6 amperes for 1965 model 12 volt coil. No reading indicates an open coil, a higher reading indicates a shorted coil. Faulty parts must be replaced.

6. Touch one test lead to brush holder mounting plate, the other to positive (insulated) brush holder. A reading indicates a shorted holder. Clean thoroughly and recheck. If reading is obtained, replace brush holder mounting plate. Check negative brush holder to be sure it is tight and well grounded.

If field coils, brush holders and generator terminals are serviceable, the trouble is probably in the armature.

Do not remove pole shoes and field coils unless tests previously made proved one or both of the coils to be faulty. When a pole shoe must be removed to replace a field coil, follow the procedure described in "Disassembling Generator."

Figure 5E-5. Testing Armature for Open Circuit

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