Owner's Problem Description
The owner reports that the fan speed in their 1995 Lincoln Town Car only operates at high speed, regardless of the automatic temperature control or manual settings. They wonder if this could be a default setting and inquire about the location of the fan motor speed relay switch. The owner mentions that earlier models had a resistor under the hood, but they were informed that the 1995 model differs in design.
Professional Technical Analysis
The 1995 Lincoln Town Car is equipped with an automatic temperature control and a blower motor that exhibits a singular operational issue where the fan speed only functions at the high setting. This vehicle, part of a model range known for its luxury features, may have a blower speed controller (BSC) located on the evaporator case, which is accessible from the engine compartment. Given that the owner is unsure whether the issue is due to a relay or a resistor, it is essential to conduct a thorough diagnostic to assess the potential causes. The BSC controls the voltage supplied to the blower motor, converting signals from the climate control module into a variable ground feed. When the motor operates solely on high speed, it may indicate a failure in the BSC or the blower motor itself, which could be caused by internal shorts or failures in the control circuitry. Additionally, the vehicle's wiring may have suffered from wear or corrosion, particularly given its age and potential exposure to environmental factors. Safety concerns arise as a malfunctioning blower could impact cabin comfort and defrosting capability, while improper function could lead to overheating of the blower motor. The current condition of the vehicle shows that while the automatic system operates, it does not provide variable speed options, limiting operational flexibility.
Possible Causes
Most common causes (ordered by frequency):
- Blower Speed Controller (BSC) Failure: The BSC located on the evaporator case may fail, causing the blower motor to receive only a full voltage signal, leading to operation solely at high speed. The BSC converts control module signals into a variable ground output to regulate speed. If it fails internally, it can prevent any voltage modulation. Testing should include checking for voltage at the BSC connector, which should show 12V during operation. If there is no variance, the BSC is likely defective and needs replacement (OEM P/N F5VZ-19E624-AB, cost approximately $100-150). - Full speed operation only, no modulation of fan speed, voltage checks at BSC connector show constant 12V.
- Blower Motor Failure: The blower motor itself may have internal faults, such as a shorted winding or worn brushes, which restrict its operation to high speed. Testing should involve measuring the motor resistance, which should be between 2-6 ohms. If outside this range, the motor may need replacement (OEM P/N F5VZ-19805-AB, cost approximately $150-200). - Only operational at high speed, resistance measurements show values outside expected range.
- Wiring or Connector Issues: The wiring harness leading to the blower motor or BSC may have corroded connectors or damaged insulation, causing loss of signal or ground. Inspect connections for corrosion and continuity tests should verify proper voltage delivery. Any damaged wiring should be repaired or replaced as necessary. - Visual inspection shows signs of wear on connectors, voltage measurements indicate intermittent connectivity.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Visual Inspection: Begin with a thorough visual inspection of the blower motor and BSC connections. Look for any signs of corrosion or damage to the wiring harness and connectors. This step is crucial as it can reveal obvious issues that might cause the fan speed to malfunction.
- STEP 2 - Voltage Testing: Use a multimeter to measure the voltage at the BSC connector while the fan is set to different speeds. You should see varying voltage levels for different speeds. If the voltage remains constant at 12V regardless of the setting, the BSC is likely defective.
- STEP 3 - Blower Motor Resistance Check: Disconnect the blower motor and measure the resistance across its terminals. It should read between 2-6 ohms. If the resistance is outside this range, the motor may need replacement. Ensure all power is disconnected before performing this test.
- STEP 4 - Continuity Test: Perform a continuity test on the wiring harness leading to the blower motor and BSC. This will help identify any open circuits or shorts that may be affecting the operation. Repair or replace any damaged wiring as needed.
