Dave Hobbs Figure 10: Voltage drop testing using a substitute load and dedicated power and ground as shown can be applied to virtually any circuit on the vehicle provided solid state devices (module and sensor) are disconnected and the substitute load chosen for the test does not draw more current than the wiring under test can handle. With the sensor wiring in the harness isolated from the sensor and module, circuitry suffering from corrosion and loose connections often displays voltage drops on one or both of the circuits that are greater than the .1 volt per connection / .5 volt total drop universal specification using the setup shown in the diagram. Some sensors use fairly small wiring due to the fact they rarely draw more than a few milliamps. However, the wiring to them should be able to easily withstand 2-3 amps. In this case that means the substitute load's resistance should be somewhere between 4-6 ohms. (12 volts / 3 amps = 4 ohms) Many techs use a light bulb as their substitute load. Keep in mind that light bulb filaments get so hot when lit that their resistance changes as soon as they illuminate. Therefore, always check your substitute load's actual current draw with a bench test prior to integrating it into an on car voltage drop test. You want to load the circuit to the max it can handle without burning it up! to the sensor are loaded with a substitute electrical load that is low enough in resistance to 'work the circuit hard' but not so low the circuit under test melts! This test is accompanied with a voltage drop test. (Fig. 10) Crazy System Interactions General Motors In addition to DTCs and Defaults, sometimes the sensor is not being displayed properly. GM, for example, has problems with select models with up level radios (Fig. 11) showing 2 dashes instead of an outside temp reading. The sensor being unplugged is the most common scenario. Because AAT / OAT now play a multi area role throughout the vehicle's electronic systems, it's more important than ever to look at the vehicle as a whole, rather than just one system. Consider the 2015 Cadillac CTS partial schematic in Figure 12. It shows the ambient temp sensor connected to the IPC. That pretty much makes sense - clusters show temperature on the vast majority of vehicle's today. You can be fairly certain that the ambient temp val- March 2017 Figure 11: DTC B0158 (Ambient Air Temp Sensor Circuit) can set if the sensor has problems or someone left the sensor disconnected. Most of the time the display will show dashes ( - - ) instead of an actual value. The inside temp selection readings (lower portion of screen) can also default to degrees Celsius when there is an open in the ambient temp sensor circuit. When the sensor is reconnected, the display may still show ( - - ) dashes even when the key is switched off and back on with some GM models. Simply turn the ignition off and back on once more. This will initiate an "Engine Run Flag" message to be sent on the data bus in order to get the modules talking once again. 6 MACS Service Reports