Here's a breakdown of what to do:
1. Immediate Action:
* Safety First: Do NOT drive the truck until you've diagnosed the issue. Driving with a malfunctioning speedometer and a complete dash light illumination is extremely dangerous.
2. Diagnostics:
* Check the Battery and Alternator: A weak battery or failing alternator can cause erratic electrical behavior. Test the battery voltage (should be around 12.6V when not running, and around 13.5-14.5V when running) and have the alternator tested at an auto parts store.
* Check Fuses and Relays: Inspect the fuse box (under the hood and possibly inside the cabin) for blown fuses, especially those related to the instrument cluster and PCM. Also check any relevant relays. A visual inspection might reveal a blown fuse (a broken filament).
* OBD-II Scanner: This is crucial. A code reader will pull diagnostic trouble codes (DTCs) from the PCM. These codes will give you valuable clues as to the source of the problem. Even a cheap OBD-II scanner from an auto parts store will help. The codes will pinpoint whether the problem is with the VSS, PCM, or another module.
* Visual Inspection: Carefully examine all wiring harnesses, especially those near the PCM and instrument cluster. Look for any signs of damage, corrosion, or loose connections. Pay particular attention to the wiring around the VSS (located near the transmission).
3. Potential Problems and Solutions:
* Vehicle Speed Sensor (VSS): A faulty VSS is a common cause of speedometer failure and can trigger other electrical issues. Replacement is relatively straightforward but requires some mechanical skills.
* Powertrain Control Module (PCM): A failing PCM can cause a wide range of problems, including the symptoms you're experiencing. This is a more serious issue and often requires professional diagnosis and repair or replacement. PCM issues can be costly.
* Instrument Cluster: While less likely to be the root cause given your other symptoms, a faulty instrument cluster can cause the lights to illuminate and the speedometer to fail.
* Wiring Harness Problems: Damage or corrosion in the wiring harness, particularly between the PCM, the instrument cluster, and the VSS, is another possibility. Tracing and repairing this can be time-consuming.
* Data Bus Issues: The data bus is the communication network within your vehicle. A problem here can manifest as widespread electrical issues.
4. Seeking Professional Help:
If you're not comfortable performing these diagnostics yourself, take your truck to a qualified mechanic or auto electrician. They have the tools and expertise to accurately diagnose and repair the problem safely. Attempting more advanced troubleshooting without the proper knowledge could potentially cause further damage.
Remember, driving with these symptoms is risky. Getting it towed is the safest course of action until you determine the root cause.
