Blower Motor Inoperative on One Speed - Electrical Diagnosis
A 2018 heavy truck with a 4-speed manual HVAC blower and a separate A/C request switch has no blower operation on any speed, but the A/C compressor engages when requested. The blower fuse is intact. The blower resistor block is mounted in the evaporator case and shows corrosion. Battery voltage is 12.6V at rest and 13.9V running. The customer says the blower worked intermittently for a week, then quit. A previous technician replaced the blower motor, and it still does not run. You have a DVOM, a clamp-on ammeter, and a wiring diagram showing a relay controlled by the body controller, a 30A fuse, the resistor block, and a ground at the right kick panel.
1. Lay out a logical voltage-drop-based diagnostic path to find why the blower does not run, given the motor was already replaced.
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Work from the source to the load, using voltage drops rather than resistance checks on a live circuit.
- Confirm power at the fuse and relay. Check for battery voltage at the 30A fuse (both sides) and at the relay terminals 30 and 86. If the relay is not being commanded, check the body controller output at terminal 85/86.
- Check relay operation. Command the blower on and confirm the relay clicks and terminal 87 goes to battery voltage. If the relay does not pull in, check the control ground/command from the body controller and the relay coil.
- Check voltage at the blower motor high-speed terminal. With the relay closed and speed on high, you should see near battery voltage at the motor. A significant drop (more than ~0.5V) indicates high resistance in the relay contacts, fuse, or wiring.
- Check the ground side. Measure voltage from the motor ground terminal to battery negative while commanded on; more than ~0.2–0.3V indicates a bad ground at the right kick panel.
- Check the resistor block and speed switch path. For lower speeds, voltage passes through the resistor block; corrosion there can open the circuit. Verify continuity/voltage through each resistor leg.
- Check the motor itself. Even a new motor can be faulty - bench-test or substitute a known-good motor and measure current draw (a seized or shorted motor draws high current; an open motor draws none).
The intermittent-then-quit history plus corrosion at the resistor block suggests a connection or resistor failure, but confirm with voltage drops before replacing parts.
2. You measure 12.4V at the blower motor high-speed terminal with the relay commanded on, but the motor does not run, and the ground voltage drop is 0.05V. The motor draws 0A. What is your conclusion and next step?
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Interpretation: Voltage is present at the motor (12.4V vs. 13.9V running is about a 1.5V drop, which is more than ideal but not enough alone to prevent operation), the ground is good (0.05V), and the motor draws 0A - meaning no current is flowing through the motor. That points to an open circuit inside the motor or between the motor terminal and its brushes/windings, not a supply or ground problem.
Next steps:
- Verify the 12.4V is actually at the motor terminal and not just upstream - back-probe at the motor connector.
- Remove the motor and bench-test it directly across a battery with a fused jumper; confirm it does not run and draws no current.
- Inspect the connector and pigtail for corrosion or a broken wire at the motor; the resistor block corrosion suggests moisture intrusion in the case, which can also corrode the motor connector.
- If the motor is confirmed open, replace it and correct the moisture source (drain, seals, resistor block gasket).
- After repair, verify all four speeds and measure current draw on high to confirm it is within spec.
Note the 1.5V drop on the feed; investigate the relay/fuse/wiring after restoring operation, since it will cause slow blower speed and heat at connections.