354 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. be used to take readings with the wiring harness con- nected and the ignition switch in the on position. Testing procedures are similar to those for checking a resistor- type sensor. Some throttle position sensors can be adjusted if they are out of range. However, if the throttle position sensor gives erratic readings, it is defective and should be replaced. Checking Speed Sensors The simplest way to check a speed sensor is to use the proper scan tool. If the sensor is part of the distribu- tor pickup assembly, the scan tool will be able to detect the ignition speed signal. Scan tools can also check the opera- tion of speed sensors installed at the engine crankshaft or camshaft, as well as those located on the transmission or transaxle input shaft. Modern scan tools can read the speed sensor output as miles per hour. Comparing this to the actual speed of the vehicle will immediately tell the techni- cian whether a vehicle speed sensor is operating properly. Some scan tools are designed to display speed sensor read- ings as drive shaft revolutions. As was discussed in Chapter 12, speed sensors produce an alternating current (ac) output. Therefore, the operation of most vehicle speed sensors can be checked by measuring the output in ac volts. To make this test, raise the vehicle and disconnect the speed sensor at the trans mission or transaxle. Note: Vehicles with anti-lock brakes (ABS) or traction control may have speed sensors at each wheel. Consult the service manual to determine which speed sensors provide input to the transmission or transaxle. Next, set the multimeter to the proper ac voltage range. With the drive wheels off the ground, shift the transmission or transaxle into drive and accelerate. The multimeter should read increasing ac voltage as the engine speed is increased. Some manufacturers call for check- ing the resistance of the speed sensor winding as shown in Figure 14-22. This should generally be done only after other tests have indicated a sensor problem. Before condemning the speed sensor, check the tip of the sensor for a buildup of metal shavings. Often the sen- sor will operate properly when the shavings are removed. Because they are magnetized, both internal and external sensors can collect metal particles. Testing Pressure Sensors Since most pressure sensors are on-off devices, they can be easily tested with an ohmmeter. A typical procedure for testing a pressure sensor is shown in Figure 14-23. With no pressure supplied, the sensor will be in its normal posi- tion, either normally open (NO) or normally closed (NC). If a normally closed sensor reads infinite resistance, it is defec- tive. If a normally open sensor has a low resis tance reading, it too is defective. However, even if a pressure sensor has the proper ohmmeter readings with no pressure applied, it may still be defective. It is always possible for a pressure sensor to fail and remain stuck in its normal (no pressure) position. If you suspect that a pressure sensor is not operating prop- erly, the best way to check it is with a scan tool. Use the scan tool to access the pressure sensor signal. For example, if a pressure sensor should be closed in a particular gear but the scan tool does not indicate a closed signal when the unit is in that gear, the sensor is probably defective. Figure 14-21. Manual lever position sensors can usually be checked with an ohmmeter. One manufacturer’s resistance specifications for various manual lever position sensor. Figure 14-22. Some speed sensors can be tested with an ohm- meter. You must have access to the proper resistance specifica- tions. Other speed sensors can only be checked with a scan tool or an oscilloscope. (Nissan) 1 2 3 Transmission manual lever position Resistance (ohms) Min Max P 3770 4607 R 1304 1593 N 660 807 D 361 442 2 190 232 1 78 95