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, run engine at idle speed. Ensure Service Engine Soon (SES) light is off. Ground ALDL diagnostic connector. This causes timing advance control to switch from ECM to HEI ignition module in distributor. See Fig 1 .
SES light will flash while the ALDL is grounded. Connect an inductive pick-up timing light to coil wire or spark plugs No. 1 or 4. Notch in crankshaft pulley align with stationary pointer on front of engine.
The mark may fluctuate to each side of pointer, but should be centered on the pointer. This position is 8 degrees BTDC. See Fig 2 . Loosen distributor hold-down bolt and rotate distributor base to adjust (if necessary).
On 1.6L engines, a magnetic type pick-up may also be used. Install timing probe in engine block, located 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 the jumper wire from the ALDL connector. Ensure SES light stops flashing.

Fig 1: Identifying ALDL Connector

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