ECM COMPONENTS AND COMPONENT FUNCTION


ECM Service Precautions

WARNING! Electrical damage is possible if precautions are not observed when performing any service on the ECM. The ignition switch must be in the "OFF" position. This must be heeded whenever connecting or disconnecting any energized component. This includes:




Random Access Memory (RAM)

The RAM in the ECM provides a temporary storage for the intermediate data from the systems sensors and switches as well as for the diagnostic trouble codes. Also certain calculations from the makes to control fuel delivery are also retained here. The information stored in the RAM is not necessarily stored in the order it was received. So, the last piece of information is available just as much as the first piece. 

Disconnecting power to the ECM will in effect erase all the memory stored here, including diagnostic codes. Drivability issues will occur in the first few minutes of driving because all the information that was used for fuel calculations will be lost and will have to be relearned during the driving cycle. 

Read Only Memory (ROM)

ROM is the permanent memory that is basically "hardwired" on this chip. These are the imbedded instructions that tells the ECM how, when and where to run specific functions. Information can be retrieved from the ROM, but neither added or deleted from it. Also, the information contained here kept, regardless if the power was disconnected from the ECM. 

Programmable Read Only Memory (PROM)

This is known in layman terms as the "chip" or "calibrator".  The PROM has the information programmed or "burned" or "flashed" in the case of the later ECM's, whose "chip" is non-removable. It contains such information such as engine size, transmission, vehicle weight and rear gear ratio and emissions calibration. It is specific to that particular vehicle and must match to insure no drivability issues. This is because with the vehicle specific information, the spark advance, volumetric efficiencies, fuel calibrations and transmission characteristics are programmed here. 

A prom ID code is displayed on the chip, which covers the quartz window used for erasing with a special lamp before reprogramming. 

Disconnecting power will not erase the onboard information programmed on the PROM, but in the case of the ecm being replaced (non-flashed chips-1993 and earlier), the prom must be swapped into the new ECM. A faulty or missing PROM will set a code and will not allow the vehicle to start.


Calibration Package (CAL-PAK)

Some of the  early mid generation ECM's (1984-1988) has a separate chip known as a CAL-PAK. This is part of the back up or "limp home" mode in case of a faulty ecm was unable to run the engine. It only requires throttle position and ignition reference pulses from the ignition module for calculations and will allow the fuel pump and injectors to keep the engine running.  The CAL-PAK, just like the PROM, needs to match the vehicle specifically and must be swapped to the new ECM, when replaced. 

A faulty or missing CAL-PAK will set a code and will not allow the vehicle to start.



The MEM-CAL contains both the PROM and CALPAK in one package. Just like the separate components on earlier ECM's, it is vehicle specific.

A faulty or missing MEM- CAL will set a code and will not allow the vehicle to start.





Calibration "chips" in their respective packages. Note the PROM ID codes. These non transparent stickers protect the exposed quartz programming window from accidental erasure from natural and artificial light. 


Flash memory was invented in 1984 by Toshiba's Dr. Fujio Masuoka, and while it took awhile for it to gain a foothold in the consumer world, it took the automotive world by storm and by 1994 it crept into the engine management computers and for good reason. The erasable programmable "ROMS" are expensive to produce with the quartz exposure windows to erase and reprogram for data corrections and calibration updates, which was time consuming in the dealer's servicing to correct drivability problems that necessitate a reprogramming of the current "chip".  With a flash memory chip on the ECM, the programming can be done through the diagnostic connector with no need to remove the ECM to gain removal of the chip. By 1994, virtually all GM ECM's were flash memory. 

The flash memory chip on a LS1B pcm 



On some ECM's up to around 1985 , the connectors are exposed on the edge of the circuit board, called an edgeboard connector. From 1985 up, the connectors are sealed with just exposed pins to mate to the harness connectors they were designed for. This new style is called a header connector and attaches solidly to the ECM case. The number of pins used are generally less than those available, but as the hardware and the more stringent emission compliance brought more sensors and device on line, the number of pins has grown. 

The harness connectors are specific to each header connector and must not be forced to fit. There is a keyed slot or a stepped edge on the connector that will only fit one way.

Header connectors on a circa 1986 ecm.



The solenoids and relay components are activated by switching the ground side of the circuit by use of an electronic (solid state) switch. There are four such circuits in one package, hence the name, quad drivers.  These are used by the ecm to switch the relay(s) for the cooling fan, fuel pump, emission solenoids such as the canister purge and the torque converter clutch solenoid. 

There was three different types of quad drivers used by GM since the early 1980's.  The first type being a fusible driver that was essentially unprotected and susceptible to shorts when exposed to a sudden high amperage draw and also would fail if the load current of the relay or solenoid was near the maximum limit.

The second type was known as a Quad Driver II (QDR II) and is self protected against high amperage draw.  If a shorted circuit limits the current draw to the solenoid or relay, thermal overload of the QDR II will shut down the entire driver, affecting all the components on that particular driver. 

The last type of driver, known as a Quad Driver Module, incorporates protection against circuit faults. This type of driver also contains a communication line to the ECM to help the technician pinpoint the error through a serial communication device or scanner.



The ecm constantly cross checks and updates information being received from the inputs, commands the proper activation of the output devices and receives feedback from the inputs to see if the commands are being implemented.  The program in the ECM has various tables that allow for a window of variance, either by time, voltage, current and these tables are constantly being accessed by the ECM to insure the fuel injector air to fuel ratio is correct via the feedback from the O2 sensors, the throttle position matches what the ecm should be seeing based on air flow, engine RPM, road speed, etc. If something is outside the window of acceptability, the ECM alerts the driver through the SES (service engine soon) or as it is known now is OBD II speak, MIL (malfunction indicator lamp). 

Early diagnostics were  limited to a small group of devices that gave a general idea of where the fault may be. For example; a code 44, lean O2 exhaust detected, may be caused by a faulty throttle position sensor or a small vacuum leak, or a burnt valve. It was up to technician to be aware that the fault was not necessarily a ECM component problem.  The OBD I diagnostics, up to 1995 allowed too much leeway in determining errors and this in turn lead to higher emission output. OBD II stricter emissions implemented for full compliance by 1996 and is standardized for all makes and models, domestic or import.

OBD II diagnostics has standardized protocols and in a sense, required specialized equipment to take advantage on its capabilities. 


Serial data is a string of data that is transmitted in sequence, one item at a time. The signal state changes from a high "1" to a low "0" and the number of pulses and the length of each pulse determines what message is being sent, and the priority that message has. The ECM  components are programmed to ignore the sequence of data meant for other components and vice versa. 


Early GM ECM's capacity was very small, on the order of 14K of memory for a 1985 TPI V8 and the PROM had a capacity of 8K, it ran at a whopping 160 bits per second.  By 1986 that baud rate was increased to 8,192 bits per second and 28K memory and the PROM capacity of 64K. Today's ECM's has memory in excess of 2M and processing speeds on par with a modern desktop computer. 



                                        PAGE 2          PAGE 3          PAGE 4          PAGE 5          PAGE 6          HOME