Here's a breakdown of likely causes and troubleshooting steps:
* Vehicle Speed Sensor (VSS) Failure: This is the most likely culprit. The sensor itself could be faulty, the wiring harness to the sensor could be damaged (broken wires, corrosion, poor connection), or the connector at the sensor could be bad. The sensor is usually located on the transmission, sometimes near the output shaft.
* Wiring Problems: Check the wiring harness leading from the VSS to the PCM and the instrument cluster. Look for broken wires, chafing, corrosion, or loose connections. A visual inspection is crucial, and a multimeter can help test continuity.
* PCM (Powertrain Control Module) Issue: Though less likely, a faulty PCM can cause both the check engine light and speedometer malfunction. This is a more complex issue requiring diagnostic tools and potentially professional repair.
* Instrument Cluster Malfunction: While less probable given the check engine light, a problem within the instrument cluster itself could affect the speedometer. This usually manifests as other gauges failing as well.
Troubleshooting Steps:
1. Check Engine Light Codes: Use a code reader (OBD1 for your 1991 Honda) to retrieve any diagnostic trouble codes (DTCs). This will provide valuable clues to the problem's source. Many auto parts stores will read codes for free.
2. Inspect the VSS: Locate the VSS (consult your repair manual for the precise location) and visually inspect it for damage. Check the wiring connector for corrosion or loose connections. Carefully unplug and inspect the connector pins.
3. Check the VSS Wiring: Using a multimeter, check for continuity in the wiring harness leading to and from the VSS. Compare your readings to a wiring diagram (found in a repair manual). Look for broken wires or shorts.
4. Test the VSS (If possible): Testing the VSS requires a multimeter and possibly a specialized tool to simulate the wheel's rotation. This is best done with a repair manual that provides specific instructions.
5. Check the Instrument Cluster (if other gauges fail): If other gauges are malfunctioning, the instrument cluster may need repair or replacement.
Important Considerations:
* Repair Manual: A repair manual specific to your 1991 Honda model is invaluable. It provides diagrams, specifications, and troubleshooting procedures.
* Safety: Always disconnect the negative battery terminal before working on any electrical components.
* Professional Help: If you're not comfortable working on your car's electrical system, it's best to take it to a qualified mechanic.
The VSS is the most probable cause, but a systematic approach using the steps above will help pinpoint the exact problem. Remember to check for diagnostic trouble codes first – they're your best starting point.
