* Rheostat (blower motor resistor): This is the most common cause. The rheostat is a variable resistor that controls the fan speed. It's likely one of the resistor windings has failed, leaving only the medium speed circuit intact. This is the path of least resistance, and thus the only one working.
* Wiring issue: A broken or corroded wire in the fan speed control circuit could interrupt the lower speed settings. Check for damaged or loose connections in the wiring harness leading to the blower motor and the rheostat.
* Blower motor switch: While less likely, the switch itself could be faulty. A partially failed switch could only make contact with the circuit for the medium speed setting.
* Blower motor: Though less probable than the others, a failing blower motor could also contribute. It may struggle to start on lower speeds due to worn brushes or a weak motor. However, if the motor was completely failing, you wouldn't even get medium speed.
Troubleshooting Steps:
1. Check the Rheostat: This is usually located near the blower motor or under the dash. It's often a small, heat-sinked component with several wires connected. Visually inspect it for any signs of burning, melting, or loose connections. You can often test it with a multimeter (although, it can be tricky depending on its design). Replacing it is usually the easiest and cheapest solution.
2. Inspect the Wiring: Carefully examine the wiring harness leading from the blower motor switch to the rheostat and to the blower motor itself. Look for any broken, frayed, or corroded wires. Pay close attention to the connectors and ensure they are securely attached.
3. Test the Blower Motor Switch: This is more difficult. You'd need a wiring diagram and multimeter to check continuity and voltage at different switch positions. Often, replacing the switch is simpler than testing it.
4. Check the Blower Motor: If all else fails, test the blower motor itself for proper operation. You might need to disconnect it and apply 12V directly to its terminals (carefully, with appropriate fuses and wire sizing) to see if it runs.
In short: Start by visually inspecting the rheostat and wiring. If those appear fine, you may need a multimeter to further diagnose the problem or consult a wiring diagram specific to your 1972 Chevrolet Blazer to more accurately troubleshoot. Replacing the rheostat is a good starting point as it's the most frequent culprit in this type of issue.
