Owner's Problem Description
The owner reports that the low beam headlights on their 1991 Toyota Tercel are not functioning, while the high beams and running lights work properly. They have checked the fuses, which are intact, and suspect that the issue might be the headlight switch located in the steering column. The owner has replaced the switch, but the problem persists. They are now inquiring about the presence of a relay that might be causing the issue.
Professional Technical Analysis
The 1991 Toyota Tercel, equipped with a 1.5L engine and a 4-speed automatic transmission, is experiencing a failure of the low beam headlights while the high beams function correctly. This vehicle, typically with around 150,000 miles, has a common headlight switch assembly mounted in the steering column. The absence of low beam operation suggests a potential failure in the switch assembly, wiring integrity, or related components. Given that the owner has already verified the fuses and replaced the headlight switch, the next logical step is to investigate the relay system. Low beam headlights usually operate through a relay that receives input from the headlight switch. The relay for the low beams is often located in the fuse box area. Testing the relay for continuity and proper operation is crucial. If the relay is functioning, the next step would be to check the wiring harness for any signs of damage or corrosion, particularly near the headlight connectors, which can affect low beam functionality. Additional diagnostic steps should include checking the voltage supply at the low beam circuit with a multimeter to ensure that the switch is sending voltage to the relay and subsequently to the headlights. It's critical to ensure safety during this process, as working near the battery and electrical components poses risks.
Possible Causes
Most common causes (ordered by frequency):
- Headlight Switch Failure: The headlight switch is responsible for controlling the low beam operation. If the switch fails internally, it may not send voltage to the low beam circuit. Symptoms of failure may include intermittent operation or complete failure of the low beams. The switch should be tested for continuity using a multimeter. Typical voltage readings at the switch output should be around 12V when activated. Replacement part number for the OEM switch is 84140-19136, costing approximately $40-70. - High beams work, running lights operational, low beam circuit not receiving voltage from switch.
- Low Beam Relay Failure: The low beam headlights typically use a relay for operation. If the relay is malfunctioning, it will not provide power to the low beam circuit, causing them not to light up. Testing the relay for continuity and operation is essential. The relay part number for replacement is 90987-02021, costing around $15-25. It is crucial to check for any corrosion at the relay terminals which can impact performance. - Testing indicates no power at the low beam output from the relay; relay clicks but does not activate the low beams.
- Wiring Issues: Damaged or corroded wiring in the low beam circuit can lead to a lack of power reaching the headlights. Inspect the wiring harness for any visible damage or corrosion, especially at the connectors. Continuity testing of the wiring should show low resistance (ideally below 5 ohms). Any breaks or high resistance points can prevent the low beams from functioning. Repair or replace damaged wiring as necessary. - Visual inspection reveals corrosion at headlight connectors; continuity testing shows inconsistent readings.
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 - Visual Inspection: Begin with a thorough visual inspection of the headlight assembly and wiring harness. Look for any signs of damage or corrosion, especially at the headlight connectors. Ensure that all grounds are secure and clean. This initial step is critical as it can often reveal obvious issues that are easy to fix.
- STEP 2 - Relay Testing: Locate the low beam relay and perform a continuity test using a multimeter. Check for proper operation by applying voltage to the relay coil and observing if the contacts close. If the relay fails this test, replace it with the part number 90987-02021. Confirm that the relay is receiving power from the fuse box.
- STEP 3 - Headlight Switch Testing: Using a multimeter, check the output voltage from the headlight switch while activating the low beam. The output should read approximately 12V. If no voltage is present, further investigation into the switch assembly is required. If the switch is faulty, replace it with OEM part number 84140-19136.
- STEP 4 - Wiring Continuity Testing: Conduct a continuity test on the wiring leading to the low beam circuit. This should demonstrate low resistance throughout the circuit. Repair any damaged wiring found during this inspection. Ensure all connections are secure and free of corrosion.
