1. The Speed Sensor (VSS):
* Location: The Vehicle Speed Sensor (VSS) is usually located on the top of the transmission. It's a relatively small sensor with a single wire connector. You'll need to consult a repair manual specific to your 1993 S10 Blazer to pinpoint its exact location.
* Function: The VSS sends a signal to the PCM (Powertrain Control Module) which controls the transmission shifting and speedometer. A faulty VSS will prevent accurate speed readings and can lead to erratic or improper shifting.
* Testing:
* Visual Inspection: Check the VSS for any physical damage, corrosion, or loose connections.
* Connector Check: Inspect the connector for broken wires or corrosion. Clean the connector terminals with electrical contact cleaner.
* Resistance Test: Using a multimeter, check the resistance of the VSS according to the specifications in your repair manual. The resistance value will vary depending on the sensor type. A value outside of the specified range indicates a faulty sensor.
* Continuity Test: Test for continuity between the sensor and the PCM connector. A broken wire in the harness will prevent the signal from reaching the PCM.
* Replacement: If the VSS is faulty, replace it with a new one from a reputable parts supplier. Ensure you get the correct part number for your specific year and transmission type.
2. Transmission Wiring:
* Harness Inspection: Carefully inspect the wiring harness leading to and from the transmission, paying close attention to areas near the VSS and the transmission connector. Look for broken wires, frayed insulation, or chafing against the transmission case. Repair or replace any damaged wiring.
3. PCM (Powertrain Control Module):
* Less Likely Culprit: While less likely, a faulty PCM could also cause these symptoms. However, PCM issues are usually more widespread than just a single gear and speedometer problem.
* Diagnosis: Diagnosing a PCM problem usually requires specialized tools and knowledge, often best handled by a professional mechanic.
4. Transmission Issues (Less Likely):
* Internal Transmission Problems: While less probable given the speedometer issue, internal transmission problems like worn clutch packs or valve body issues *could* contribute to shifting problems. This is much more complex to diagnose and repair, requiring a thorough transmission inspection and potentially a rebuild or replacement.
Troubleshooting Steps:
1. Start with the VSS: This is the most likely culprit given both your speedometer and transmission problems. Replace it if it's faulty.
2. Inspect the Wiring: Look for broken or damaged wires in the harness leading to and from the transmission.
3. Check the Transmission Fluid: Ensure the transmission fluid level is correct and that the fluid is clean and free of debris. Low fluid can cause shifting problems.
4. Consult a Repair Manual: A repair manual specific to your 1993 S10 Blazer will provide detailed diagrams, specifications, and troubleshooting steps. This is crucial for accurate diagnosis.
5. Seek Professional Help: If you are not comfortable working on your vehicle's electrical system or transmission, consult a qualified mechanic.
Important Note: Before attempting any repairs, disconnect the negative battery terminal to prevent electrical shocks and accidental damage. Always refer to a repair manual for specific procedures and safety precautions.
