Reading Diagnostic Trouble Codes

The check engine lamp on the instrument panel will illuminate if the logic module or SMEC receives an incorrect signal or no signal from the coolant temperature sensor, charge temperature sensor, MAP sensor, throttle position sensor or battery voltage sensor input depending on vehicle application. This lamp indicates that the logic module or SMEC has gone into the ``limp in mode'' to keep the system operational, and indicates an immediate need for service.

The following is a list of engine sensors that can turn on the Check Engine Lmap if they fail:

1. M.A.P. (MANIFOLD ABSOLUT PRESSURE) SENSOR - When the signal from this sensor is too low, too high or not changing, one of a series of MAP failure messages is set into the Controller's memory and the Check Engine Lamp is switched on. At this time the Throttle Position and Engine Speed Signals are used by the Engine Controller to generate a "modified" MAP signal in place of the one that failed.
2. THROTTLE POSITION SENSOR - If this signal is too high or too low, a TPS message is stored in memory. The Check Engine lamp is turned on, and a "modified" MAP signal is generated to replace the failed Throttle Position Sensor.
3. COOLANT TEMPERATURE SENSOR - If this signal is out of range, a coolant error message is stored and the Check Engine lmap is turned on. At this time the Engine Controller will operate at a default temperature of 104 degrees F.
4. BATTERY VOLTAGE SENSE - If this signal drops below 4 volts for one minute after the engine starts a "Battery Input Sense" message is stored in memory and the Check Engine Lamp is turned on. At this time the Engine Controller will operate the Charging System at a fixed rate.
5. BATTERY VOLTAGE HIGH - If the charging voltage rises 1 volt above desired control voltage this message is set in memory and the Check Engine Lamp is turned on.
6. INJ PEAK CURRENT - Caused by a high resistance in control circuit any time engine is running.
7. INJ CONTROL CKT - If the engine controller does not sense injectors switching on and off when a control is switched. Check during any engine running.

If these sensor signals return to within specifications while the engine is running, the system will return to normal operation, but the Check Engine Lamp will remain on until the engine is turned off and re-started. The fault message will remain in the Controller's memory for evaluation by the technician but will be cleared after 50 engine start-ups if the fault does not happen again.