Oct 102016
 

1994 Chrysler LHSI recently replaced the ignition unit on my car after the engine would die while warming up and never start again until the engine would completely cool down again. Since replacing the unit the engine will die a couple of times but restart and continue to run after reaching full operating temperature. Can you tell me why this is happening and how to correct it completely?


RESPONSE

First thing to do would be to pull any engine trouble codes. This can be done without a scan tool .

The most common reason for this would be a failed Engine Coolant Temperature Sensor.

Coolant Temperature Sensor Operation

The coolant temperature sensor is a thermistor; a temperature controlled variable resistor with a range of −40°–265°F (5°–129°C) mounted in the engine coolant stream. The PCM uses the temperature input to adjust the fuel mixture slightly richer and raise the idle speed, until normal operating temperatures are reached. On all models, this sensor is also used to control the cooling (radiator) fan operation.

The PCM supplies 5.0 volts to the coolant temperature sensor. The sensor provides an input voltage to the PCM. As coolant temperature varies, the sensor resistance changes resulting in different input voltage to the PCM.

When the engine is cold, the PCM will demand slightly richer air/fuel mixtures and higher idle speeds until normal operating temperatures are reached.

The coolant temperature sensor changes resistance as the coolant temperature increases and decreases.

Coolant Temperature Sensor Testing

  • Remove the temperature sender from the engine.
  • Position the water temperature sending unit in such a way that the metal shaft (opposite end from the electrical connectors) is situated in a pot of water. Make sure that the electrical connector is not submerged and that only the tip of the sending unit’s body is in the water.
  • Heat the pot of water at a medium rate. While the water is warming, continue to measure the resistance of the terminal and the metal body of the sending unit:
    1. As the water warms up, the resistance exhibited by the ohmmeter goes down in a steady manner: the sending unit is good.
    2. As the water warms up, the resistance does not change or changes in erratic jumps: the sender is bad, replace it with a new one.
  • Install the good or new sending unit into the engine, then connect the negative battery cable.

 

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