C.E. Niehoff & Co. C653/C653A/C625 Troubleshooting Guides - Manual (Page 9)

Page 9 TG13G
Section D: A2-214 Advanced Troubleshooting
(CONT’D)
Repair vehicle circuit to IGN terminal.
Vehicle charging circuit test is complete.
Set DMM to diode test. Connect black lead of DMM to B+ terminal on alternator. Connect red
lead to pin B on harness plug. DMM should read voltage drop. Reverse leads. DMM should
read OL.
Check continuity of thermal switch inside control
unit: Remove drive end cover on alternator. With
DMM, check continuity between pin B on harness
plug and diode shown in Figure 9 below. Does
continuity exist?
Yes No
Yes No
Alternator is
defective.



Thermal switch
in control unit
is defective.

Figure 9 – Diode Arrangement inside C653 Drive End Housing
USE THIS DIODE

Chart 4 – Continuation of Chart 3 as Noted