1. Power Supply Check:
* Battery Voltage: Verify the battery voltage is within the acceptable range (typically 12.2-12.8 volts when the engine is off). A low voltage can cause intermittent or complete failure of electrical components.
* Main Power Wire: Locate the main power wire supplying the power to both electric seats. This will likely be a fairly heavy-gauge wire. Trace this wire from the battery (or a main power distribution point) to where it connects to the seat control system. Look for any breaks, corrosion, loose connections, or damage. A multimeter can be used to check for voltage at this point.
* Grounding: Poor grounding can also cause electrical problems. Check the grounding points for both seats. These are usually metal straps or bolts attached to the vehicle chassis. Clean them thoroughly with a wire brush and ensure a solid connection.
2. Seat Control Module/Switch:
* Switch Check: The problem might be in the switch itself for either the driver or passenger seat. Try wiggling the switches while checking for power going to the seat motors with a multimeter (make sure to disconnect power before doing so). Often a failing switch will initially manifest as an intermittent problem before completely failing.
* Control Module (if applicable): Some vehicles have a separate control module that manages the power seats. This is less common on older vehicles, but possible. If your Envoy does have one, you'll need to locate it and check its connections. Consult a wiring diagram (see point 4).
3. Wiring Harnesses:
* Inspect Wiring: Carefully examine the wiring harnesses running from the control switch to each seat motor. Look for any pinched, frayed, or damaged wires. Pay close attention to areas where the wiring might rub against metal parts of the vehicle.
* Continuity Test: Use a multimeter to check for continuity in the wiring harnesses. This will confirm if there's a break in the circuit. You'll need a wiring diagram (see below) to identify which wires go to which motor.
4. Wiring Diagram:
* Obtain a Wiring Diagram: This is the most crucial step. A wiring diagram specific to your 2002 GMC Envoy will show the exact location and routing of the power seats wiring, helping you trace the problem more efficiently. You can find wiring diagrams online (sometimes for a fee) or through a repair manual.
5. Seat Motors:
* Test Motors (Advanced): If you've ruled out everything else, you can test the motors themselves. This requires disconnecting the motors from the wiring harness and applying power directly (use caution, as incorrect voltage or polarity can damage the motors).
Important Safety Notes:
* Disconnect the battery's negative terminal before working with the vehicle's electrical system to prevent accidental shorts or shocks.
* Use caution when working with electricity. If you're not comfortable working with electrical systems, it's best to take your vehicle to a qualified mechanic.
By systematically checking these areas, you should be able to pinpoint the cause of the problem with your power seats. Remember that a wiring diagram is your best friend in this process.
