MAINTENANCE OF M939 SERIES VEHICLES - OD1661 - LESSON 1/TASK 2
Since the starter current is above 100 amps, SGT Metcalfe is
directed to No-Go test 81, which tests the batteries.
No-Go Test 81.
Each battery is best tested individually. During
keep the engine from starting.
Step 1. In step 1, SGT Metcalfe installs the test probe cable W2 and
performs the offset test.
To accomplish this he connects the test
probe cable W2 to J4 on the VTM.
Then he shorts the red and black
Now he dials 89 into the TEST SELECT switches,
pressing and holding the TEST button until the CAL message appears on
When the CAL message appears on the display, SGT
Metcalfe releases the TEST button and waits for the offset value to
appear on the display. The offset value should be between -6.8 and
+6.8. Since the offset value was +2.5, SGT Metcalfe proceeds to step
2, conditioning the current probe and performing the offset.
Step 2. To condition the current probe, SGT Metcalfe first connects
the red clip lead of cable W2 to the positive post of the battery
He then connects the black clip lead of cable W2 to
the negative post of the battery being tested.
After these connections have been made, SGT Metcalfe connects plug P1
of transducer cable W4 to jack J3 on the VTM. He connects plug P2 of
transducer cable W4 to the current probe. He then clamps the current
probe around the battery cable which connects the series pair of
batteries which includes the battery to be tested. SGT Metcalfe then
points the arrow on the current toward the negative post connected to
the battery cable as shown in figure 20 (on the following page).
After this has been done, he attempts to crank the engine for several