G643(E) Service Manual Chapter 4. Engine Electrical System 123
Ignition Timing Adjustment for G643
Engine
The ignition timing advance of G643 is controlled by
the SECM. The initial ignition timing setting of the
distributor is described in the following steps. It will
be necessary to mechanically adjust the distributor
to an initial setting for startup, and then adjust the
final timing using the Service Tool.
1. Be sure engine is turned OFF.
2. Remove distributor cap (Figure 34).
3. Disconnect the fuel lock-off connector at the lock
off so that fuel cannot flow during cranking.
4. Disconnect the 4-pin connector (Figure 35) to the
ignition module. This eliminates the +12 Vdc
power on Pin D and puts the ignition to a default
value not controlled by the SECM.
5. Connect standard inductive timing light (Figure
36) to the #1 cylinder.
6. Manually bar the engine so that the timing
indicator on the crankshaft pulley lines up to 0°
Top Dead Center (TDC) of the #1 cylinder (Figure
37).
7. Verify that the distributor rotor is lined up with #1
cylinder on the distributor cap (Figure 38).
(1) lgnition Coil. (2) lgnition Module.
Figure 34. GM Ignition Module
Figure 35. Four-Pin Connector to Ignition Module
Figure 36. Standard Inductive Timing Light
8. Using standard inductive timing light with no offset
adjustment set the initial timing to 4° BTDC
(advanced) by rotating the distributor while
cranking the engine with the ignition key.
9. Tighten the distributor hold-down bolt to the
specified torque value and re-connect the 4-pin
ignition module connector and the fuel lock-off
connector.
Figure 37. Engine Set at 0° Top Dead Center
(TDC)
DISTRIBUTOR
IGNITION
MODULE
1
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