Note: Do not allow sleeving on leads to slide down
leads; phase terminal without sleeves can short
against alternator body.
The diode heat sink assembly is normally checked
using a diode tester. If a diode tester is used, refer to
manufacturer’s instructions for proper connections.
When a diode tester is not available, use an ohmmeter
and refer to the following procedure.
Note: Do not use an AC Device such as a leakage
tester to check the diode heat sink.
TEST 7 - Check Positive Diodes
Refer to Figure 13-6. Set ohmmeter to x100 scale and
make sure ohmmeter is zeroed. Connect one ohmme-
ter lead to the “B+” output stud, connect the other
ohmmeter lead to each of the six heat sink phase
terminals “S”. All six readings should be nearly alike;
either less than 600 ohms or very high. If all six
readings are not alike, the diode rectifier assembly is
defective and front housing assembly must be re-
placed.
TEST 8 - Check Negative Diodes
Refer to Figure 13-6, set ohmmeter to x100 scale, and
make sure ohmmeter is zeroed. Connect one ohmme-
ter lead to “B-“ terminal located on the outside of the
front housing, connect the other ohmmeter lead to
each of the six heat sink phase terminals “S”. All six
readings should be nearly alike; and all should read
very high. If all six readings are not alike the diode
rectifier assembly is defective and the front housing
assembly must be replaced.
Reverse ohmmeter leads, and again observe resis-
tance between “B+” terminal and each of the six heat
sink phase terminals “S”. All six readings should be
very high. If any reading is not alike, the diode rectifier
assembly is defective and the front housing assembly
must be replaced.
STATOR TESTS
NOTE: The front stator related Phase leads are “P1”,
“P2”, & “P3”; The rear ststor phase leads are “P4”,
“P5”, & “P6”
The Alternator has two separate assemblies that will
be checked individually. Make sure all phase leads are
disconnected from the heat sink (Refer to Figure 13-6).
TEST 9 - Check Front & Rear Stator
Set ohmmeter to x1 scale and make sure ohmmeter is
zeroed. Check for open stator winding by connecting
ohmmeter between each successive pair of stator
phase leads (Refer to Figure 13-6: “P-1” – “P2”; “P2”
– “P3”; “P1” – “P3”; “P-4” – “P5”; “P5” – “P6”; & “P4”
– “P6”).
Note: It may be necessary to probe under the sleeves
of the phase leads in order to make electrical contact.
Ohmmeter should read less than 1 ohm between each
pair of stator phase windings. If ohmmeter reads very
high, the stator is open and must be replaced (replace
or repair stator or stator / shell assembly).
Set ohmmeter to x10K scale and make sure ohmmeter
is zeroed. Check for shorted stator windings by con-
necting ohmmeter between each phase lead (“P1”,
“P2”, “P3”, “P4”, “P5”, & “P6”) and the ground terminal
located on the outside of the front housing. Ohmmeter
should read very high. If ohmmeter reads zero for the
related test point, the stator is grounded and must be
replaced (replace or repair stator or stator / shell
assembly).
Note: Grounded stator is difficult to confirm by static
test. Examine stator for burnt insulation or loose coil.
M13003 04/01 Niehoff Alternator Overhaul Manual M13-11
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