Intermittent Electrical & Drivability Diagnostic Strategies - Potentiometers

Common Faults
Reference voltage and ground circuit faults can occur, but they are not common faults in potentiometer circuits. Additionally, most potentiometers share a reference voltage and/or ground circuit with other sensors, so these faults would cause problems with multiple sensors.

Sensor Calibration
A common potentiometer fault is that it performs perfectly but the range is shifted due to maladjustment or internal mechanical wear. Circuit or connector resistance faults can also shift the sensor range, but this is much less common. Adjust the sensor if a procedure is available from the manufacturer, and verify that there are no reference voltage or ground faults before replacing the component.

Note: Sensor calibration faults can cause problems in all operating ranges because the fault is present throughout the sensor range. This includes emissions, shifting, and power problems.

Resistor Wear & ‘Dead Spots’
Another common potentiometer fault is physical resistor wear. The potentiometer arm drags across the resistor repeatedly, and problems can develop. Any wear that causes an open in the sensor will cause the signal to the module to drop to 0 Volts. This is often a momentary dropout, so you have to set your tool up to look for momentary changes (i.e. using the Min/Max feature on a DVOM).

Note: Resistor wear faults cause intermittent symptoms under specific operating conditions, but do not affect other operating conditions. In other words, potentiometer faults are rarely intermittent. Rather, the operating conditions intermittently expose the fault.

NOTE: On some applications, (i.e. Chrysler), there is a high resistance bridge (a ‘short’) inside of the module between the reference and signal circuits. During normal operation, the normal path through the sensor has much lower resistance than the bridge, so the bridge does not affect the signal. However, if the sensor, reference voltage, or sense circuits are open, no voltage can drop across the bridge, so full reference voltage pressure is felt at both sides of the bridge (voltage can’t drop and current doesn’t flow). The end result is that when the sensor is disconnected, you’ll see full reference voltage on the signal wire as well as the reference voltage wire. This is often mistaken for a short to reference voltage or faulty module.

These excerpts are from our 200-350 page manuals and may possibly refer to information from other sections of the manual or graphics / pictures. Our manuals contain in-depth procedures that require screen shots of parts, charts and various graphs - as well as detailed steps that guide you through troubleshooting real-world automotive repair issues methodically for a quicker resolution.

For additional information on how to obtain the manuals that these tips come from, please refer to the products page on our website: 
www.atgtraining.com - you will also find trouble code handbooks as well as DVD's on our more popular subjects.