Ford Aerostar 1986-1997 Repair Guide

Coolant Temperature Switch



See Figure 1

This component detects the temperature of the engine coolant and relays the information to the electronic control assembly. The sensor is located by the heater outlet fitting or in a cooling passage on the engine, depending upon the particular type vehicle. The function of the sensor is to modify ignition timing, control EGR flow and regulate the air/fuel mixture. On vehicles equipped with the electronic instrument cluster, the sensor is also used to control the coolant temperature indicator.

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Fig. Fig. 1: Coolant temperature sensor


Under correct operating conditions, the coolant switch is closed in the normal operating temperature range above 180°. The switch remains open below the 180° range. If a switch is defective, the electronic control module is able to determine this and alert the driver.

This switch can also be tested as follows;

  1. Remove the switch from the engine while the engine is cool.
  3. Using a multimeter, test for continuity at the switch. If the switch is functioning correctly, the meter will register open.
  5. Place the probe end of the switch into boiling water for at least two minutes. Use the meter and check for continuity. If the switch is function correctly, there should be continuity.


See Figure 2

When the battery cable has been disconnected and reconnected, some abnormal drive symptoms may occur while the powertrain control module relearns its adaptive strategy. The vehicle may need to be driven 10 miles (16 km) or more to relearn the adaptive strategy.

  1. Disconnect the negative battery cable.
  3. Unfasten the wire harness attached to the temperature switch.
  5. Place a suitable drip pan below the sensor to capture any coolant that may spill out during removal and installation.
  7. Using a suitable wrench or equivalent, remove the switch from the engine. Remove the washer from the switch and discard.

To install:
  1. Install the temperature switch, using a new washer and tighten to 15-18 ft. lbs. (19-23 Nm).

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Fig. Fig. 2: Coolant sensor at the top of the engine block

  1. Fasten the wire harness to the rear of the charge sensor.
  3. Connect the negative battery cable.