John Deere stx38 - Manual (Page 120)

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3/21/97
CIRCUIT OPERATION AND DIAGNOSIS ELECTRICAL
IGNITION CIRCUIT DIAGNOSIS STX30 & STX38 (SN —210000)
When diagnosing an ignition problem, isolate the magneto circuit from the ground circuit by separating the engine
connector. If the engine will not start check the magneto circuit first and them check the ground circuit. If the engine
will not shut off, check the ground circuit. Remember the engine is stopped by grounding the ignition coil through
either the key switch or the start relay. The start relay must be energized to prevent the ignition coil from being
grounded.
Test Conditions:
• Park brake engaged • Spark plug lead connected to Spark Tester
• Transaxle in neutral • Key switch in start position
• PTO switch disengaged • Meter negative (-) lead on engine ground
• Engine connector disconnected • Meter positive (+) lead on numbered test point
Test/Check Point Normal If Not Normal
1. Spark tester Spark test indicates hot blue spark No spark—test ignition module,
check armature air gap and
flywheel magnets.
Spark—check spark plug gap and
condition or replace spark plug. If
plug is good, continue testing
ground circuit.
Test Conditions:
• Engine connector connected • Key switch in run position
Test/Check Point Normal If Not Normal
2. Battery positive terminal. 11.8—13.2 volts Test battery
3. Key switch terminal B. Battery voltage Check battery cable connection,
fusible link or circuit breaker, and
200/210 red wires.
4. Key switch terminal A. Battery voltage Replace key switch.
5. PTO switch—yellow wire. Battery voltage Replace 510 yellow wire.
6. PTO switch—720 purple wire. Battery voltage Replace PTO switch.
7. Neutral start switch—720
purple wire.
Battery voltage Test 720 purple wire.
8. Neutral start switch—710
purple wire.
Battery voltage Replace neutral start switch.
9. Key switch terminal S1. Battery voltage Test 710 purple wire.
10. Fuse—714 pur/blk wire. Battery voltage Test 714 pur/blk wire.
11. Fuse—715 pur/blk wire. Battery voltage Replace fuse
12. Start relay terminal 85—
715 pur/blk wire.
Battery voltage Test 715 pur/blk wire and diode.