NOTE: The ECM cannot read base timing. If base timing is incorrect, spark advance curve will be incorrect. Base timing may be set using either a timing light or magnetic timing probe. See Fig 2 .

Ensure engine is at normal operating temperature. With A/C off, operate engine at idle speed. Ensure CHECK ENGINE light is off. Ground ALDL diagnostic connector. See Fig 1 . This causes timing advance control to switch from ECM to HEI ignition module in distributor.
CHECK ENGINE light will flash while ALDL is grounded. Connect inductive pick-up timing light to No. 1 spark plug wire. Notch in crankshaft pulley should align with stationary pointer on front of engine. See Fig 2 .
The mark may fluctuate to each side of pointer, but should be centered on the pointer. This position is 8 degrees BTDC. Loosen distributor hold-down bolt and rotate distributor base to adjust (if necessary).
To set base timing, a magnetic type pick-up may also be used. Install timing probe in engine block, just below oil filter. See Fig 2 . With engine at normal idle, set timing to 10 degrees BTDC. Rotate distributor base to adjust (if necessary).
Remove jumper wire from ALDL connector. Ensure CHECK ENGINE light stops flashing.

Fig 1: Identifying ALDL Diagnostic Connector Terminals

Courtesy of GENERAL MOTORS CORP.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Ignition Timing

Use the Manual With the Right Hardware

When to See a Mechanic

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