1. Check the Blower Motor Resistor:
* Location: The resistor is usually located behind the glovebox or under the dash, near the blower motor. Consult your owner's manual or a repair manual for the precise location on your specific Suburban model.
* Function: The resistor regulates the speed of the blower motor. If it's faulty, only the highest speed (which bypasses the resistor) will work.
* Inspection: Visually inspect the resistor for any obvious signs of damage, such as burning, melting, or loose connections. A multimeter can be used to test its resistance values; the values should match those specified in a repair manual. If it's burned out or has incorrect resistance, it needs replacement.
2. Check the Heater Switch:
* Accessibility: This often requires removing the dash panel around the switch. Be cautious and take pictures before disconnecting anything.
* Testing: This is more involved and might require a wiring diagram. You'll need to test the continuity between the switch's terminals and the wiring leading to the blower motor resistor using a multimeter. If the switch isn't supplying power to the lower speeds, it's likely faulty and needs to be replaced.
3. Check Wiring and Connections:
* Visual Inspection: Look for any broken, frayed, or corroded wires in the harness leading from the switch to the resistor and the blower motor. Pay close attention to connections – ensure they're secure and free of corrosion.
* Continuity Test: Use a multimeter to check the continuity of the wires between the switch, resistor, and blower motor. A break in the wiring will prevent lower speeds from functioning.
4. Check the Blower Motor:
* While less likely: A completely failed blower motor could *appear* as a high-speed-only problem, as the only thing you can hear is the full power running. If the other steps show no issues, carefully test the motor for functionality (possibly by applying 12V directly to its terminals – exercise caution, as direct wiring can be dangerous).
Important Notes:
* Safety First: Disconnect the negative battery terminal before starting any electrical work.
* Repair Manual: A repair manual specific to your 1992 Chevy Suburban is highly recommended. It will provide detailed diagrams and specifications for your vehicle's heating system.
* Professional Help: If you're not comfortable working with automotive electrical systems, it's best to take your Suburban to a qualified mechanic.
The most likely culprit is either the blower motor resistor or the heater switch itself. Start with the resistor, as it's generally easier to access and replace. If that doesn't solve the problem, you'll need to move on to checking the switch and wiring.
