The check engine light is a warning light that comes on if the computerized engine control system detects an engine performance or emissions problem. Also called the "malfunction indicator lamp" (MIL) or "Service Engine soon". It can come on for any of hundreds of different causes from any of the systems that the computer monitors. Some are very important and some not as serious.
Your first step is to have the computer scanned to retrieve the trouble codes that were set when the check engine light came on. Those codes will get you pointed in the right direction to find your problem.
Here is a way to manually retreive codes from your truck.
Once you have retreived and recorded them, disconnect the battery to clear them and run the truck again to see what codes if any reset.
Diagnostic Trouble Codes - 1994
|13 ||Open Oxygen Sensor Circuit |
|14 ||Coolant Temperature Sensor (High Temp Indicated) |
|15 ||Coolant Temperature Sensor (Low Temp Indicated) |
|21 ||Throttle Position Sensor (Signal Voltage High) |
|22 ||Throttle Position Sensor (Signal Voltage Low) |
|23 ||Manifold Air Temperature (MAT) Low Temp Indicated |
|24 ||Vehicle Speed Sensor (VSS) Circuit |
|25 ||Manifold Air Temperature (MAT) High Temp Indicated |
|32 ||Exhaust Gas Recirculation (EGR) Circuit |
|33 ||Manifold Absolute Pressure Sensor Circuit (High Sig.) |
|34 ||Manifold Absolute Pressure Sensor Circuit (Low Sig.) |
|35 ||Idle Air Control (IAC) |
|41 ||Cylinder Select Error |
|42 ||Ignition Control (IC) Circuit |
|43 ||Knock Sensor (KS) Circuit |
|44 ||Oxygen Sensor Circuit (Lean Exhaust Indicated) |
|45 ||Oxygen Sensor Circuit (Rich Exhaust Indicated) |
|51 ||EEPROM or ECM Failure |
|53 ||System Voltage Overcharge |
|54 ||Fuel Pump Circuit (Low Voltage) |
|55 ||ECM Error |
|61 ||Degraded Oxygen Sensor |
|62 ||Transaxle Gear Switch Signal Circuit |
|63 ||A/C Refrigerant Pressure Sensor Circuit |
To extract a trouble from the computer for diagnostic purposes, the Assembly Line Communication Link (ALDL) connector is used.
Ground terminals A and B with the ignition "ON" and engine not running. The system will enter the diagnostic mode. In this mode, the computer will display a 12, three consecutive times, by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" or light. A 12 consists of one flash, a short pause, then two flashes in rapid succession.
After 12 is displayed, any stored trouble s will be displayed by flashing the "CHECK ENGINE", "SERVICE ENGINE SOON" or "MALFUNCTION INDICATOR LAMP" light. All s are displayed 3 times each then a 12 will repeat. In the diagnostic mode, the computer will also energize all computer controlled relays and solenoids.
ALDL Connector (Located Underdash)
The ALDL diagnostic connector is located in the passenger compartment.