C.E. Niehoff & Co. N1225-1/N1237-1/N1505-1 Troubleshooting Guides - Manual (Page 6)

Page 6
TG0017B
(CONT’D)
Section 3: Advanced Troubleshooting
Yes No
Alternator is defective.
Unplug alternator-to-regulator 4-socket harness from regulator. Connect red lead from DMM to pin A in
plug. Connect black lead to pin D in plug. Does resistance read 1.8 ± 2.2 ohms?
Chart 3b – No 28 V Alternator Output – Test OVCO Circuit
Yes No
Replace existing regulator with known good regulator.
Run engine. Does OVCO trip?
Alternator is defective. Original regulator is
defective.
 
Chart 3a – No 14 V Alternator Output – Test OVCO Circuit
Yes No
With red lead from DMM connected to pin A in plug, connect black lead to B– terminal. Does
resistance read OL (out of limits)?
Alternator is defective.

Chart 3 – OVCO Trip – Determine 28 V or 14 V
Yes No
28 V side tripped OVCO circuit.
Go to Chart 3b.
With meter red lead on 28 V B+ at battery and black lead on chassis ground, start engine.
Watch meter dial: Does meter read charge voltage above 29 V?

14 V side tripped OVCO circuit.
Go to Chart 3a.



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Figure 6 – Alternator-to-Regulator 4-Socket Harness Plug
SOCKET CONNECTIONS
A F–
B Phase Signal AC
C B–
D 28 V B+
Yes No
Original regulator
is defective.
Replace regulator with known good regulator.
Run engine. Does OVCO trip?
Alternator
is defective.
 
Yes No
Alternator is defective.
Unplug alternator-to-regulator 4-socket harness from regulator. At receptacle on regulator, connect red lead
from DMM to pin C. Connect black lead to B– terminal. Does resistance read OL (out of limits)?
 