Owner's Problem Description
The owner reports that upon starting the truck, both the brake light and ABS light remain illuminated on the dashboard. Additionally, the speedometer does not function at all below 15 mph, but appears to read accurately at higher speeds. The transmission shifts normally across all speeds. A service station suggested a potential speed sensor issue. The owner seeks confirmation on whether it's the ABS rear wheel speed sensor located on the differential and inquires about checking the voltage at the connector to diagnose the sensor's condition.
Professional Technical Analysis
This 1998 Dodge Dakota, equipped with a 3.9L V6 engine and automatic transmission, has approximately 120,000 miles. The owner reports that the brake light and ABS light remain on after starting, and the speedometer fails to register below 15 mph, which is a common symptom associated with speed sensor failures. Given that the transmission shifts normally, the primary suspect is the speed sensor responsible for the vehicle's ABS and speedometer functions. It's important to note that the Dakota uses a combination of vehicle speed sensors: one for the speedometer located on the transmission and one or more ABS wheel speed sensors located at each wheel hub. In this case, further diagnostics are needed to determine if the issue lies with the ABS sensor at the rear differential or the speed sensor on the transmission. A thorough visual inspection of the ABS sensors and wiring, alongside a voltage check at the connector, is crucial. The absence of fault codes from a scanner is typical if the scanner is not compatible with ABS diagnostics, which often leads to oversight in identifying the root cause. As these vehicles age, common failure patterns include sensor degradation due to environmental exposure, corrosion at connectors, and wiring insulation breakdown. Therefore, both the ABS and speedometer sensor circuits should be evaluated for continuity and proper voltage levels during operation.
Possible Causes
Most common causes (ordered by frequency):
- Rear ABS Speed Sensor Failure: The rear ABS wheel speed sensor may be malfunctioning, which can lead to incorrect speed readings and the illumination of related warning lights. The sensor typically produces a voltage signal based on the rotation of the wheel; a failure can result in a complete lack of signal or a signal that is out of range. Testing the voltage at the sensor connector should yield a reading of approximately 0.5 to 2.5 volts while the vehicle is moving. If no voltage is present, the sensor is likely faulty, and it should be replaced. The OEM part number for the rear ABS sensor is 56041414AB. - Brake light and ABS light illuminated on the dashboard, speedometer non-functional below 15 mph, voltage testing at the sensor connector shows no signal.
- Transmission Output Speed Sensor Malfunction: The transmission output speed sensor may not be providing accurate readings to the PCM, which can also affect the speedometer reading. This sensor typically generates a signal that correlates with the vehicle's speed. If it fails, the speedometer may function erratically, only responding at higher speeds. Testing this sensor should yield a voltage signal of approximately 0.5 to 4.5 volts depending on the vehicle's speed. If the signal is absent, it indicates the need for replacement. The OEM part number for the transmission output speed sensor is 56041417AB. - Speedometer erratic behavior, voltage testing indicates absence of signal from the transmission output speed sensor.
- Wiring or Connector Issues: Corrosion or damage to the wiring or connectors associated with the ABS or speed sensors can lead to intermittent failures or complete sensor signal loss. Inspecting the wiring harness for fraying, corrosion, or disconnections is essential. Any damaged wires should be repaired or replaced, and connectors should be cleaned and secured. Resistance checks of the wiring should show less than 5 ohms from the connector to the sensor. - Visual inspection reveals corrosion on connectors, resistance measurements show high resistance in wiring.
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 - OBD-II and ABS Code Reading: Begin with a comprehensive OBD-II scan using a compatible scanner capable of reading ABS codes. This will confirm whether any codes are present that relate to the ABS system or speed sensors. If no ABS codes are detected, then the issue may be mechanical or related to wiring. If available, use GeekOBD APP for advanced diagnostics and real-time data monitoring.
- STEP 2 - Visual Inspection of Sensors and Wiring: Conduct a thorough visual inspection of the ABS sensors and wiring harnesses, focusing on the rear ABS sensor and the transmission speed sensor. Look for signs of damage, corrosion, and proper connector seating. Any frayed wires or corroded connectors should be addressed immediately.
- STEP 3 - Voltage Testing at Sensor Connectors: With the vehicle running, perform voltage tests at the ABS sensor and the transmission output speed sensor connectors. The voltage should be within specifications; for the ABS sensor, it should be around 0.5 to 2.5 volts, and for the transmission sensor, approximately 0.5 to 4.5 volts. Document the findings for further analysis.
- STEP 4 - Road Test for Speedometer Functionality: After addressing any found issues, conduct a road test to observe the speedometer's behavior. Monitor the speed readings at various speeds, particularly noting if the speedometer functions correctly below and above 15 mph. Use GeekOBD APP to log data during the test for later analysis.
