274 Automatic Transmissions and Transaxles Copyright by Goodheart-Willcox Co., Inc. Oxygen sensors Figure 12-13. Oxygen sensors may be located on the exhaust manifold or in the exhaust pipes. This illustration shows an OBD II system with sensors installed both before and after the catalytic converter. (General Motors) Powertrain operation Sensor inputs Output solenoids ECM Figure 12-14. The input sensors, computer, and output actuators form a continuous loop of inputs, ECM operation, outputs and power train operation. Note that the operation of the engine and transmission or transaxle is part of the loop. a chemical reaction. The materials in the oxygen sensor gen- erate a small voltage because of the difference in the amount of oxygen in the exhaust gases and the outside air. The greater the difference in the oxygen content between the exhaust and the outside air, the greater the voltage produced. This voltage corresponds to the air-fuel ratio of the engine—the richer the mixture, the greater the voltage. The ECM reads this voltage as a rich or lean mixture. A few vehicles use a resistor-type oxygen sensor. This type of sensor changes resistance as the oxygen content of the exhaust gases changes. Most modern engines use several oxygen sensors, Figure 12-13. Other Vehicle Inputs Other vehicle inputs to the ECM include the charging system voltage, which the ECM reads directly from a wire connection at the alternator. On some vehicles, the ECM monitors the operation of the ABS or traction control system, as well as the air conditioning system. These inputs are taken directly from the unit as it operates. There is a hard-wired connection to the ECM. The presence of voltage at this wire tells the ECM that the device is operating. ECM The ECM is composed of various electronic devices, such as diodes, transistors, capacitors, and resistors. These devices are combined onto a complex electronic circuit by etching the circuitry on small pieces of semiconductor material. A circuit made in this manner is called a chip, or a microprocessor. These devices can precisely control transmission operation through solenoids, replacing the hydraulic and mechanical components previously used. Main ECM Sections As discussed in Chapter 7, all ECMs contain two main sections: the central processing unit, or CPU, and the memory. The CPU and memory are built into the ECM, and the entire ECM must be replaced if either is defective. The CPU receives the input sensor information, compares this information with the information stored in memory, performs calculations, and makes output decisions. There are two basic types of memory: read only memory (ROM) and random access memory (RAM). ROM contains the per- manently installed programs and operating standards that tell the ECM what to do under various operating conditions. Disconnecting the battery or removing the system fuse will not erase ROM. Portions of the ROM are permanent, while other parts can be reprogrammed or changed. On late-model vehicles, special tools can be used to access ROM for repro- gramming. In older vehicles, a separate chip called a PROM (programmable ROM) can be replaced. RAM is a temporary storage place for data from the input sensors. Information received from the input sensors is temporarily stored in RAM, overwriting old information. When a trouble code (discussed later) is generated by the system, it is stored in RAM. The RAM has a temporary, or volatile, memory. On some older systems, RAM memory will be lost if the system fuse is removed or the battery is disconnected. Newer systems retain memory even when voltage is removed from the ECM. Control Loops The microprocessors that make up a modern ECM control transmission operation by processing information from the input sensors and issuing output commands. This ability allows them to operate as part of a control loop, Figure 12-14. A control loop can be thought of as an endless circle of causes and effects that is used to operate part of the vehicle, such as the automatic transmission. When the control loop is operating, the input sensors furnish infor- mation to the computer, which makes output decisions and sends commands to the output devices. The operation of the output devices affects the operation of the vehicle system, causing changes in the readings furnished by the input sen- sors. There is a continuous loop of information from the