Onan MJA Series Service Manual page 69

67 STARTING SYSTEM
Check the charge condition of the battery with a hydrometer. SpeCific
gravity should be between 1.290 and 1.225.
If
not, recharge the bat ­
tery.
If
the battery won't recharge, replace
it.
With the starting motor operating, check the voltage drops (1) from the
battery ground terminal post (not the cable clamp) to the cylinder block
(2) from the cylinder block to the starting motor frame and (3) from
the battery positive post to the battery terminal stud On the solenoid.
Normally, each of these should be less than 0.2 volts.
If
extra long
battery cables are used, slightly higher voltage drops may result.
Thoroughly clean all connections in any part of the circuit showing ex­
cessively high voltage drops.
If
starting motor tests are required, remove the motor from the plant
and test it on a bench. Test the free running voltage and current.
Limits are given in the Table of Clearances and Limits (sect 9).
Using a spring scale and torque arm, test the stall torque, Fig 3.6.B.
The torque will be the product of the spring scale reading and length of
the arm in feet.
If
free running speed is low, and a high current draw with low stall
torque exists, check for tight, dirty, or worn bushings, bent armature
shaft, or loose field pole screws allowing armature to drag, shorted
armature, or grounded armature or field.
A low free speed with low torque and low current draw indicates an open
field winding, high internal resistance due to poor connections, defec ­
tive leads, dirty commutator, broken or worn springs, worn brushes,
or scored, worn, or dirty commutator .
High free speed with low developed torque and high current draw indi ­
cates shorted fields. Since there is no easy way to detect shorted field
coils, replace and check for improved performance.
The voltage drop across the solenoid on the starting motor should be
less than 1. 50 volts.
If
more, remove it for repair.
VARIABLE RESISTANCE
~
J
0
0
~
!
j
0
0
o
o
KNIFESWfTCH\
~
~A~RJ
ZQJ
l..
,8ATTERY
STORAGE
- - - - - 1 2 " - - - - - '
FIG 3.6.B - STARTING MOTOR STALL TORQUE TEST