Here's a general guide. This is not a substitute for a repair manual specific to your 1990 GMC Sierra's engine. The exact procedure and tensioner location vary slightly depending on the engine (e.g., 4.3L V6, 5.0L V8, 5.7L V8, 6.2L V8, etc.). You must consult a repair manual for your specific engine to be certain.
Tools You'll Likely Need:
* New serpentine belt: Make sure to get the correct size and type for your specific engine. Your local auto parts store can help you identify the correct belt using your vehicle's VIN.
* Wrench(es): Various sizes depending on your tensioner and other pulley bolts. A socket set with extensions will likely be very useful.
* Socket wrench: A ratchet is extremely helpful.
* Pliers or a belt tensioner tool: Some tensioners require a special tool, others can be manipulated with pliers (carefully!). Again, your repair manual will show you the correct method.
* Gloves: To protect your hands.
* Jack and jack stands (highly recommended): This provides better access and a safer working environment.
* Repair manual specific to your 1990 GMC Sierra: This is crucial.
General Steps (Consult your repair manual for specifics!):
1. Locate the serpentine belt: It's the long belt running around various pulleys (alternator, power steering pump, water pump, etc.).
2. Identify the belt tensioner: This is a device that allows you to relieve tension on the belt so you can remove it. It might be spring-loaded or hydraulic. Your repair manual will show you its location and how to operate it.
3. Loosen the belt tensioner: Use the appropriate wrench or tool to release tension on the belt. This will create slack in the belt, allowing you to remove it from the pulleys. Be very careful not to let the tensioner spring back and hit you when you release it. Often, you need to use a wrench to hold it in place while you slide the belt off.
4. Remove the old serpentine belt: Carefully guide the belt off each pulley, starting with the one that has the most slack.
5. Install the new serpentine belt: Route the new belt around the pulleys in the same order as the old one. Make sure it sits correctly in the grooves of each pulley. Your repair manual will have a diagram showing the correct routing.
6. Tighten the belt tensioner: Once the belt is routed correctly, slowly release the tensioner, allowing it to clamp the belt. Ensure the belt is properly seated on all pulleys.
7. Check belt tension: The belt should have a slight amount of deflection (give) when pressed with your thumb. Your repair manual will likely specify the correct amount of deflection. If the tension is too tight or too loose, you'll need to adjust it.
8. Reconnect the battery terminal.
9. Start the engine and check for any issues: Listen for any squealing or unusual noises.
Important Considerations:
* Broken Tensioner: If the belt tensioner is broken or seized, you'll need to replace it as well. Don't force anything; you could cause more damage.
* Pulley Alignment: Make sure all pulleys are properly aligned. Misalignment can quickly wear out the new belt.
* Belt Routing: Getting the belt routed incorrectly is a common mistake. Double-check against your repair manual's diagram.
This is a general overview. You absolutely must consult a repair manual specific to your 1990 GMC Sierra and its engine for detailed instructions and diagrams. If you're not comfortable performing this repair yourself, take your truck to a qualified mechanic. Improper installation can lead to serious damage to your engine.
