Auto Potentiometer and Thermister Intergrity Checks

As mentioned in how to diagnose sensors on our blog, we ruled out this very important step of diagnosis because it must be well explained. Any mistake or misunderstanding of how to run this particular check-signal circuit integrity testing will damage your computers, be it PCM or TCM. 

My students always ask, why perform this check if my scan tool or multimeter reflects this varying voltage, indicating that my signal wire circuit is ok. This is a good question. But what about the situation where the signal wire voltage is fixed at 0v or 5volts? What if the circuit has substituted values?

This is not as simple as testing for the presence of power and grounds with a test light but as important as checking for voltage drops in a circuit. Like voltage drop testing, signal circuit integrity testing must be mastered. It always takes 3 important steps to complete this test. Now you know that the very reason why we perform signal circuit integrity testing is when we have found that the signal voltage is fixed at 0v or 5volts which should not be so. 

How?

STEP 1: Identifying the signal wire.

What makes this identification more complex is the various types of sensors and their wiring incorporated in a modern car circuit. A hall-effect sensor operation and testing are quite different from a thermistor like an engine temperature control sensor. Although their operations might be different their signal wire position is not different. Be it a two-wire or three-wire sensor, the signal wire is always the wire with the varying voltage. Formally we use to say that it's the only wire that goes from the sensor into the computer, but modern cars these days have some sensors with their feed and ground from the computer into the sensor. so it's not the only wire from the sensor into the computer.  Others might ask, what if the wire identified as the signal wire has a fixed voltage? Well that might be a fault or a bias voltage with sensor connector disconnected? Most cases its a fault. How do you differentiate that from power or grounds? Good question. The only answer is to confirm that with a circuit diagram. Once the signal wire is identified, performing the integrity test to identify the problem Is simple, but not easy.

STEP 2: Performing the integrity test. 

As mentioned earlier, not all sensors operate the same, likewise performing the signal circuit integrity test. In the running of this test, a simple jumper wire does the trick, forcing the computer to react based on the action taken, to confirm communication is intact and the wiring is good. Once again the jumper wires must be designed differently per sensor type or else hell will break loose. Let's first group the sensors into three - thermistors, pressure, and hall-effect sensors for better understanding and clarification. 

A thermistor sensor has two resistors wired in its circuit, one being a variable resistor and the other inside the computer a fixed resistor. They are positioned that way to create voltage drops on both resistors, but the variable resistor’s voltage drop is always used as the source for decision-making by the computer. These built-in resistors change the dynamics of the testing and prevent using a jumper wire with 5k resistor integration as a protector

But this 5k resistor jumper wire is very important when performing signal circuit integrity testing on potentiometers, pressure, and hall-effect sensors.   

Testing Potentiometers: Measure voltage on the signal wire with your DMM by back probing. Is the voltage fixed at 0v or 5v? Watch out for a sensor with 4 wires - (2 signal wires, a ground, and power)

If it's fixed at 0volts - unplug the sensor whiles you have already back-probed the connector signal wire. If the voltage jumps to 5volts signal circuit is good, if not and still at 0volts, jump the 5volts reference wire to the signal wire (not the ground wire) using a 5k resistance jumper wire to protect if there is a shot to the ground. Your scan tool PID voltage should read 5v, showing integrity is good.

If it's fixed at 5volts - unplug the sensor whiles you have already back-probed the connector's signal wire. Since you’re not expecting any change, jump the signal wire to the potentiometer ground wire, if the scan tool reads 0volts the signal circuit is good.

Let use an EGR valve position sensor (EVP) which is a potentiometer as a point in case. Since this sensor informs the PCM of the exact EGR valve position for feedback control and diagnostic purposes, the signal does not indicates actual EGR flow. But a potentiometer will always remain a POT. The only considering factor is how to turn the sensor on and off. This is why I made mention of understanding its function. Once again a bi-directional scan tool is an invaluable tool in this case. Plot the EVP sensor PID in voltage. Back probe the connector side of your sensor's signal wire. Then use a 5k resistor jumper wire to bridge the 5ref volts wire to the signal wire. watch out for changes in the scan tool PID or engine performance if you don’t have a scan tool. opening and closing the valve will affect the engine's performance.

Testing Thermistors: Check the voltage on the signal wire. Watch out for a 5v reference wire known as the signal, Thermistors 5v ref is never shared with any other inputs. When the thermistor is cold, the voltage is high, and when it's hot, the voltage is low. As mentioned earlier, since this circuit has two resistors, the pressure or voltage between the two resistors is dependent on the amount of resistance of each resistor and whether or not there is a complete circuit. Any opens in the circuit will create no current flow and therefore no voltage drop across the resistors. The circuit test will read 5v always. If the signal wire is short-to-ground or the sensor is internally shorting, all source voltage will drop across the first resistor and the test will read 0v always.

If it's fixed at 0volts - unplug the sensor whiles you have already back-probe the signal wire connected to a voltmeter. If the voltage jumps to 5volts the sensor is shorting.  If it remains at 0volts, there is a short to ground or an open in the signal wire. Using the scan tool to confirm PID voltage must read 5v, and temperature reading -40 degrees F. 

If it's fixed at 5volts - Check the sensor ground, if the sensor ground is good either the thermistor or connector is bad. Unplug the sensor and jump the signal wire to the thermistor ground wire, if the scan tool reads 0 volts or 260 - 400 degrees F the signal circuit is good. Replace the sensor or connector

Let us use Engine Coolant Temperature (ECT) as a case in point. Its the main input for cranking and warm-up curves, electrical cooling fans, climate control, and idle speed control, it stands as an authoritative sensor. Its presence is very critical or else other outputs like AC clutch, EGR control, AIR pump, closed loop, and spark timing will be affected. Find out more about ECT using the link above. The method testing is the same illustrated under thermistors.

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