Troubleshooting a heater fan blower on a 1984 Chevy S10 requires testing the entire circuit leading to the fan. The fan motor is located under the hood, on the passenger-side firewall next to the wheel well. The circuit consists of the fuse, relay, control panel, heater-blower motor resistor and the blower itself. The most common reason for a blower-motor failure is the blower-motor resistor. This item is used to vary the speeds of the blower.
Open the hood. Locate the blower motor on the passenger side of the firewall. Notice the blower-motor resistor just to the right of the blower. Check the blower-motor resistor first, since it is the most likely candidate for failure. Check the voltage at the resistor with the voltmeter by connecting the black lead to a good ground. Turn the voltmeter on and adjust it to read on the 20-volt scale. Turn the key to the "On" position. Turn the heater blower on.
Pull the electrical connector off the blower-motor resistor. Probe the connector for battery voltage, which is 12 volts on one terminal. If there is no voltage, the fuse and relay must be checked next. You need voltage to the resistor for the fan to work. If there is voltage at one terminal, reinstall the connector on the resistor.
Replace the fuse if blown. If it is good, check the relay by pulling the relay out of the fuse block under the hood on the driver’s fender well. Leave the black lead from the voltmeter on a good ground and probe the terminals from which the relay was pulled. With the key off there should be one terminal with battery power. Now turn the key on and the blower on. There should be one more terminal with power. This terminal is from the blower-fan switch. If no power is present, the fan switch is bad. If there is power, the relay is bad.
Check for power at the blower-motor electrical connector (assuming that the fuse and relay were good and that there is power at the electrical connector on the blower motor resistor). Use the red lead on the voltmeter to probe the electrical connector on the blower motor. If there is no power present with the key on and the blower-motor fan switch in the on position, the blower-motor resistor is bad and needs to be replaced. If there is power at the blower motor, the blower motor is bad.
