Identifying your ignition system:
* Distributor with a vacuum advance canister and rotating weights inside: This is a mechanical advance system.
* No distributor or a distributor without obvious mechanical advance components (weights, vacuum canister): This is likely an electronic ignition system controlled by the computer (ECM).
Adjusting Timing (Mechanical Advance System - Proceed with extreme caution):
1. Safety First: Disconnect the negative battery cable.
2. Locate Timing Mark: Find the timing marks on the harmonic balancer (a pulley on the crankshaft) and the timing pointer on the engine block or cover.
3. Find the Timing Light: You'll need a timing light to accurately adjust the timing.
4. Connect Timing Light: Clip the timing light's inductive clamp around the number one spark plug wire. Connect the light to the battery.
5. Set Engine to Idle: Start the engine and let it warm up to operating temperature.
6. Shine the Timing Light: Shine the timing light onto the harmonic balancer. The timing mark should align with the pointer. The specified timing is usually found on a sticker under the hood, often around 10-15 degrees BTDC (Before Top Dead Center) at idle. Consult your owner's manual for the specific specification.
7. Adjust Timing: If the timing is off, locate the distributor. It should have a clamp or other mechanism to rotate it. Loosen the distributor clamp slightly.
8. Slowly Rotate the Distributor: Carefully rotate the distributor slightly clockwise (this advances the timing) or counter-clockwise (this retards the timing) to align the mark with the pointer. Tighten the clamp once the timing is correct.
9. Recheck Timing: Recheck the timing with the timing light.
10. Test Drive: Carefully test drive the vehicle and listen for any pinging or knocking which indicates incorrect timing.
Adjusting Timing (Electronic Ignition System - Generally NOT Adjustable by the User):
Electronic ignition systems in '89 Chevy trucks are controlled by the Engine Control Module (ECM). The ECM automatically adjusts the timing for optimal performance and efficiency. Attempting to adjust the timing on this system manually is generally not recommended and can damage your vehicle's ECM.
Instead of adjusting the timing directly, consider these options if you suspect a timing issue:
* Check for Diagnostic Trouble Codes (DTCs): Use an OBDI scanner (for 1989 models) to check for any codes stored in the ECM's memory. These codes can indicate problems with sensors or other components that affect timing.
* Professional Diagnosis: Take your truck to a qualified mechanic for proper diagnosis. They have the tools and expertise to diagnose and fix timing-related issues in modern electronic ignition systems.
In summary: For an '89 Chevy truck, you're highly likely to have an electronic system. Trying to adjust timing yourself on an electronic ignition system is risky and likely won't improve anything; you'll probably damage your vehicle's computer and possibly even the engine. Take it to a mechanic for diagnosis and repair. Only attempt the mechanical adjustment method if you are absolutely certain your truck has a mechanical advance distributor, and even then, proceed with caution. Always consult your owner's manual for specifics.
