Chrysler RAM50/D50/Arrow 1979-1993 Repair Guide

Idle Speed Control Motor Position Sensor



This procedure is only for 2.4L engines

The Idle Speed Control Motor (ISCM) position sensor is located at the mouth of the intake manifold plenum, near the TPS. The ISCM position sensor converts the position of the plunger in the ISCM to a voltage, which it then inputs to the ECM. The ECM takes the signal and controls the ISCM.

If the plunger in the ISCM moves from the contracted to the extended position, the resistance between the variable resistor terminal and the ground terminal of the ISCM position sensor increases in proportion to the amount of extension. Thus the voltage at the variable resistor terminal of the ISCM position sensor increases as the plunger is extended.

The ISCM position sensor is the most important sensor in control of the idle speed. In many cases where failures occur due to changes in the engine load, as when the air conditioning switch is turned on while the engine is idling, the fault lies with this sensor.

If the output voltage of the ISCM position sensor deviates from the standard value even though the results of the checks of the ISCM position sensor wire harness and individual components are normal, the problems may be that the basic idle speed adjustment is faulty, there are deposits on the throttle valve, air is being sucked into the intake manifold through gaps in a gasket, the EGR valve seat is sealed poorly, or the combustion in the cylinders is poor (spark plugs, ignition coil, injectors, compression pressure, etc.).


See Figure 1

  1. Unplug the ISCM position sensor harness connector from the sensor. Turn the ignition ON, and measure the power supply voltage from ISCM harness terminals 2 and 6. If the voltage is not 4.8-5.2 volts, repair the harness from ISCM terminals 2 and 6 to ECM terminals 13 and 23.
  3. Turn the ignition switch to OFF and check for continuity from ISCM harness terminal 3 to ground. If no continuity is detected, repair the harness from ISCM terminal 3 to ECM terminals 14 and 24.
  5. Unplug the ECM harness from the ECM, and check for an open-circuit (or, short-circuit) between the ECM and the ISCM position sensor. Check from ISCM terminal 5 to ECM terminal 17, if a short or open-circuit is found, repair the harness. If no short or open-circuit is found, the harness is good.

Click image to see an enlarged view

Fig. Fig. 1: The ISCM position sensor harness testing procedure-2.4L engines


See Figure 2

  1. Unplug the ISCM position sensor connector.
  3. Measure the resistance between terminals 2 and 3, which should be between 4-6 kilohms.
  5. Hook a 6V DC power source up between terminals 1 and 2 of the ISCM harness connector, and then measure the resistance between terminals 3 and 6 of the ISCM position sensor connector when the ISCM is activated (caused to expand and contract). The resistance should decrease smoothly as the ISCM plunger contracts.

Click image to see an enlarged view

Fig. Fig. 2: The testing of the ISCM position sensor component-2.4L engines

Apply only a 6V DC or lower voltage source. Application of higher voltages could cause a locking of the servo gears.

  1. If there is a deviation from the standard value, or if the change is not smooth, replace the ISCM assembly.


To remove the ISCM simply unplug the harness connector and remove the retaining screws. Make certain to remember which side was up. To install the ISCM, put the ISCM into place and tighten the retaining screws securely. Plug the harness connector onto the ISCM.