AC Blower Fan Not Working in Auto Mode [What Could Be Wrong?]

Having your car’s air conditioner blower fan not come on automatically can be an annoying problem, especially during hot summer weather. The AC system in most vehicles has an “auto” setting that adjusts blower fan speed based on the temperature setting and cabin interior temperature. If the blower fan won’t turn on or change speeds when set to auto mode, there are some things you can check to get to the bottom of the issue.

How Does Auto Mode for the AC Blower Fan Work?

The AC system in your car has a few sensors and electrical components that work together to regulate and maintain the interior temperature when set to auto mode:

• Cabin Air Temperature Sensor – This sensor detects the current air temperature inside the passenger compartment. It communicates this to the AC system control module.
• Desired Temperature Setting – You choose the desired cabin air temperature using the climate control head unit. This setting is sent to the AC control module.
• Blower Motor and Resistor – The blower motor powers the fan blades that push air through the AC ventilation system. The resistor regulates fan speed based on signals from the control module.
• AC Control Module – This is the “brain” of the system that processes inputs from sensors and adjusts climate control components like the blower motor accordingly.

When set to auto mode, the control module reads the cabin air temperature sensor and compares it to your chosen temperature setting. If the cabin air is warmer than the desired temp, it will automatically command the blower motor and resistor to increase fan speed and blow more cool air. As cabin temp reaches the set level, it reduces fan speed. This cycle constantly repeats to maintain the interior temperature.

Common Causes of Blower Fan Not Working in Auto

If the blower fan is not coming on or adjusting speed automatically when the AC is set to auto mode, there are a few key things that could be causing the problem:

AC Controls Set Incorrectly

It may seem obvious, but make sure the AC system is truly set to auto mode. On some vehicles, you have to press the auto button to engage it after turning on the AC. Ensure the temperature is set to a cooler setting than the current cabin temperature.

Electrical Issue with Blower Motor or Resistor

Problems with the wiring to the blower motor or issues with the resistor unit can prevent the fan from operating on auto. Faulty power and ground connections or a damaged resistor can interrupt the control signals.

Failed Sensors

Issues like a malfunctioning cabin temp sensor or AC pressure sensor can provide incorrect data to the AC control module. This may prevent proper auto operation of the blower fan.

Control Module Software Glitch

Sometimes a glitch in the AC control module’s programming can cause it to not register the auto command or fail to adjust fan speed accordingly. Resetting or updating the module may be required.

First Steps to Diagnose the Problem

Here are some initial things you can do to start diagnosing an AC blower fan issue in auto mode:

• Make sure the climate control is truly set to auto mode and the desired temp is cooler than the current cabin air temp. Test if the fan comes on if you manually set the fan speed to high.
• Check for any error codes stored in the AC control module that could indicate a detected problem with sensors, motors, etc. Use an OBD2 scanner to read codes.
• Inspect all electrical connectors and wiring related to the blower motor, resistor, and AC control module. Look for corrosion, damage, or loose connections.
• Monitor AC pressure sensors using a gauge set while the AC is engaged. Unstable pressures or activation of pressure switches can affect system operation.

These basic starting points should give you an idea of where to focus further diagnostic and troubleshooting efforts. In our next section, we’ll go into more ways to zero in on blower motor and resistor issues that could cause problems in AC auto mode.

Diagnosing Blower Motor and Resistor Problems

Issues with the blower motor itself or the resistor that controls its fan speeds are common causes of problems with AC auto mode function. Here are some steps to diagnose these components:

Inspect the Blower Motor and Wiring

The blower motor is the electro-mechanical component that physically powers the fan blades to push air. Start diagnosis by visually inspecting the motor in the HVAC housing and check:

• Buildup of dirt, dust or debris on the motor or fan that could cause seizing
• Damaged or missing fan blades
• Signs of electrical arcing on the motor casing

Also check all wiring and connectors leading to the blower motor. Look for:

• Brittle, cracked or melted wires and insulation
• Loose plugs and terminals
• Corrosion in connectors

Repair any damaged wires or connections and clear any debris buildup on the fan assembly.

Check Blower Motor Power and Ground Circuits

Use a multimeter to test for proper voltage power and ground at the blower motor plug. You should get ~12V on the power wire when the fan is commanded on. Lack of voltage indicates a blown fuse, bad blower relay, or issue with the resistor circuit.

Verify Blower Motor Resistance

Test the winding resistance between the motor terminals. Consult a service manual for your specific vehicle’s specs. Resistance should be fairly consistent – a drastic difference indicates internal winding damage.

Watch Blower Motor Operation

With the AC on, visually inspect the blower motor while switching between off, low and high speeds. The motor should smoothly increase and decrease speed as commanded. Delayed start/stop, hesitation, or grinding indicates a bad motor needing replacement.

Inspect Blower Resistor Unit

The blower resistor modulates voltage and current to vary fan speed settings. When removed, inspect for:

• Overheated, discolored or cracked resistor element
• Melted or broken internal solder joints
• Broken wire connector tabs or loose plugs

Any signs of heat damage or severed internal connections means the resistor is faulty and must be replaced.

By methodically testing wiring, power circuits, resistance, and visually observing the blower motor and resistor, you can zero in on root causes of problems that will affect operation in AC auto fan mode.

Troubleshooting AC System Sensors and Switches

In addition to the blower components, improper input signals from sensors and switches in the AC system can also disrupt auto operation of the fan. Here are a few key sensors to check:

Cabin Air Temperature Sensor

Use a scan tool to read the temperature value from this sensor in real-time and compare it to the actual cabin air temp. If the values don’t align, the sensor could be faulty and needs replacement.

AC Pressure Switches

Pressure switches cycle the AC compressor clutch on and off based on system high and low pressure cut-off points. Monitor pressures with gauges and watch switch activation. Any irregularity in cut-in/cut-out indicates a bad switch.

AC Pressure Transducer

This sensor provides a variable voltage signal to the AC control module based on system pressure readings. Out-of-range voltage signals can confuse the computer and cause erratic operation.

Ambient Air Temperature Sensor

This detects outside air temperature, which the AC module factors in when regulating interior fan speed and airflow in auto mode. Malfunction here can impact that calculation.

By testing the key sensors and switches, you can uncover any that are providing inaccurate data to the AC module computer and causing the irregular auto operation.

Proper signaling from devices like the cabin temp sensor is critical for the AC system to accurately adjust blower motor speed when set to auto mode. Faulty readings will lead to abnormal fan operation. Thorough troubleshooting focused on the sensors, switches, wiring, blower components, and modules will help uncover the root problem.

Frequently Asked Questions

Why does my blower fan stay on high speed when set to auto mode?

A stuck open pressure switch, faulty cabin temp sensor reading very hot, failed resistor, or control module software issue could cause this.

The blower fan is delayed turning on when I turn on the AC. What’s wrong?

This could indicate a bad blower relay, motor resistor issue, or problem with the control module power circuits.

My fan only works on high speed in auto mode. Other speeds don’t work. What now?

Most likely the blower motor resistor is damaged and needs to be replaced. This controls voltage to vary fan speeds.

I hear a loud grinding noise from the blower motor on auto mode. Cause?

Sounds like the blower motor bearings are worn out. The motor will need to be replaced to resolve this issue.

AC cuts in and out in auto mode due to low pressure. Fan also surges on/off. Why?

Most likely a blocked evaporator or issues with the AC compressor are causing intermittent low pressure cut-outs and erratic fan operation.

Conclusion

Troubleshooting an AC blower fan that is not working correctly in auto mode requires methodically checking the various components and circuits involved in the climate control system. Issues with electrical connections, the blower motor, resistor, sensors, switches, and even software glitches in the AC control module can contribute to problems.

By first verifying proper system settings and error codes, then testing wiring, power circuits, component operation, sensor readings, and pressure switch activation, you can isolate the source of the problem. Don’t forget to also inspect for physical damage, debris, corrosion, or loose connectors.

Replacing a faulty blower motor, resistor, sensor, or pressure switch should ultimately resolve AC blower fan operation issues in auto mode. A wiring repair or fuse replacement may also be required. For recurring control module or software issues, a system recalibration or firmware update from the dealer may be necessary.