- January 26, 2004
- Reaction score
- City, State
- Mechanicsville, Virginia
- Year, Model & Trim Level
- 2004 Acura TL
The use of computers on automobiles has expanded to include control and operation of several functions including fuel delivery, ignition systems, emission systems, climate control, lighting circuits, cruise control, anti-lock braking, electronic suspension systems, and electronic shift transmissions. Some of these functions are the responsiblity of the power train control module (PCM), while others are functions of what is known as a body computer module (BCM). Most recent vehciles have computers/modules for just about everthing (a seperate computer for the engine, seperate one for the transmission, sepereate one for the ABS, and so on).
A computer processes the physical conditions that represent information (data). The operation of the compute is divided into four basic functions:
1. Input - a voltage signal sent from an input device. This device can be a sensor or a switch activated by the driver or technician.
2. Processing - The computer uses the input information and compares it to programmed instructions. The logic circuits process the input signals into output demands.
3. Storage - Program instructions are stored in an electronic memory. Some of the input signals are also stored for later processing. A "keep-alive memory" voltage is always constant to the computer to keep the memory intact.
4. Output - After the computer has processed the sensor input and check its programmed instructions, it will put out control commands to various output devices. These output devices may be the instrument panel display or a system actuator. The output of one computer can also be used as an input to another computer/module.
Analog and Digital
The computer is capable of reading only voltage signals. The program used by the computer is "burned" itno ignition control (IC) chips using a series of numbers (binary numbers 0's and 1's). An analog voltage signal is continuously variable within a defined range. For example, engine coolant temperature sensosr do not change abruptly. The temperature varies in steps from low to high. The same is true for other inputs such as engine speed, vechile speed, fuel flow, and etc.
Compared to analog voltage, digital voltage patterns are square shaped because the transition from one voltage level to another is very abrupt. The simplest form/generator of a digital signal is a switch.
An Analog Signal
An Digital Signal
A Common Binary Code Chart
ROM, RAM, PROM = Memory
Read Only Memory (ROM): Contains a fixed pattern of 1s and 0s that represent permanent stored information. This information is used to instruct the computer on what to do in response to input data. The CPU reads the information contained in ROM, but it cannont write to it or change it. ROM is permanent memory. ROM contains the basic operating parameters for the vehicle.
Random Access Memory (RAM): Constructed from flip-flop circuits formed into the chip. The RAM will store temporary information that can be read from or written to by the CPU. RAM stores information that is waiting to be acted upon, and it stores output signals that are waiting to be sent to an output device. RAM can be designed as volatile or nonvolatile. Volatile RAM, the dta will be retained as long as current flows through the memory.
Keep Alive Memory (KAM): KAM is a version of RAM, but is connected directly to the battery through circuit protection devices (fuses, etc.). The Microprocessor can delete KAM information, but once the ignition key is turned off, the KAM retains information. KAM will be lost when the battery is disconnected, if the battery drains to low, or if the circuit opens (blown fuse). Nonvolatile RAM will retain its information if current is removed.
Programmable Read Only Memory (PROM): Contains specific data that pertains to the exact vehicle in which the computer is installed. The information may be used to inform the CPU of the accessories that are equipped on the vehicle (4wd, a/t, a/c, etc.). The information in PROM is used to define or adjust the operating perimeters held in the ROM. PROM
can be removed and replaced with an updated PROM.
A PROM Chip
Erasable PROM (EPROM): similar to PROM except that its contents can be erased to allow new date to be installed.
Electrically Erasable PROM (EE-PROM): Allows changing the information electronically one bit at a time. The flash EEPROM is an IC Chip inside the computer. It is possible to erase and reprogram the EEPROM without removing this chip from the computer. Seen in most cars today. Here is an example: Reprogramming
Hope this is somewhat informational for some people. Anyone else want to put in a few words, please do. To read information on how to TALK to the computers, check out this thread: Scanners . Also, to see what codes computers send out, go to here: OBD II DTCs
Glacier991 has written a thread on Oxygen Sensors. This thread is a wonderful thread with a lot of great information. I suggest everyone read it before replacing a single O2 sensor. http://www.explorerforum.com/forums/showthread.php?t=149917