As stated in July's column, many sensors correlate physical changes in the position of vehicle components to changes in the resistance of the sensor itself, there by changing the amount of signal voltage the sensor circuit outputs to the ECU. Knowing this, we can use a simple potentiometer style variable resistor to substitute for sensor outputs during our testing procedures. First, we need to have the potentiometers and a good understanding of their uses at hand. We will use the next several columns to cover some of the potentiometer testing procedures used on different vehicle systems.
I recommend purchasing at least three different potentiometers (pots) to cover the resistance ranges on the majority of sensors on today's vehicles. A 0-25Ω pot for testing airbag systems, a 0-5KΩ pot for some sensors and a 0-250KΩ for others as shown in Figure 1. When you go to the electronics supply store, ask for pots with a linear taper, which means the resistance changes in direct proportion to the amount of adjustment shaft rotation through out its range. These simply are easier to dial in desired values than audio taper pots.You can add spade or banana connectors to the ends of the test leads so you can use your favorite brand of terminal specific test probes to make a damage free connection to the vehicle harness.
The reason I prefer to use a three-contact variable potentiometer rather than a simple two-contact variable resistor is that we can use the potentiometer as a voltage divider tool. With a voltage divider tool, we can output a range of specific voltages to the vehicle harness as a substitute for complex three wire sensors, like a MAF sensor, that create an output voltage by more complex means than a simple change in resistance. We will cover the voltage divider technique in a succeeding month's column.
This month we will cover using a potentiometer as a substitute for some simple resistance type sensors. Connecting the pot to the vehicle circuit for the tests we will be describing below are shown in Figure 2. Of course, make sure the vehicle harness and voltage levels are normal by using the test methods described in the June 2010 Scope & Scan column "The Highs and Lows of Three Wire Sensor Testing."The first time most technicians used a pot as a test tool was as a replacement for the fuel gauge sending unit in the fuel tank. You could use a pot wired in at a convenient connector in place of the tank sending unit so that you could test the ability of the fuel gauge — and nowadays one or more ECUs — to read and display the full and proper gauge range without filling and draining the fuel tank. We still can use a pot to do this test today with one caution. Some fuel gauge systems, like newer GM cars, will default to a reading of empty if the fuel level circuit shows a fault. Be sure to make all your test connections, clear DTCs and then cycle the ignition key off to on before you perform your test.
We can use the pot as a replacement for a three wire Throttle Position Sensor (TPS) if we need to test the effect of static TPS readings on ECU operating strategies. An example of this is on some Honda products that will not allow actuation of the transmission shift lock solenoid while the TPS is greater than 0.49V or zero to 6 percent.
These Honda TPS sensors are not adjustable and must be purchased with the entire throttle body. They also have a habit of going out of that range. It would be a good idea to use the pot to dial in a normal reading before purchasing an entire throttle body only to find out the problem was elsewhere. We can also use the pot to test for base idle strategy on many vehicles without having to get at hard to reach or non-existent TPS adjustment screws.
Wondering if that analog style MAP sensor on a speed density based vehicle is out of calibration and causing a mixture concern? Substitute the MAP sensor with a pot and dial in a reading that the fuel system likes before purchasing a new MAP sensor. And, of course, be sure the MAP sensor has a normal vacuum supply and level to it before condemning the sensor.
Three wire potentiometers also can be used to substitute for Accelerator Pedal Position (APP) sensors. Some APP are six-wire dual sensors that have separate reference voltage, signal and ground connections. Other APPs are four-wire and shared common reference voltage and ground connections. Consult your vehicle's specific wiring diagram to connect your pot(s) test tool appropriately.
Automatic HVAC door position sensors, some of which are no fun to get to, can be diagnosed with pots also. Next month we'll cover more systems to test with pots.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.
