ECM COMPONENTS AND COMPONENT FUNCTION

                                                                                             SENSOR FUNCTIONS (INPUT)



INTAKE AIR TEMPERATURE SENSOR (IAT)                     IGNITION REFERENCE SIGNALS                                                                   MAGNETIC VSS

THROTTLE POSITION SENSOR (TPS)                                Distributorless Ignition System                                                                           Digital Ratio Axle Controller (DRAC) module.

MANIFOLD ABSOLUTE PRESSURE MAP)                        LT1 OPTISPARK 1992-1997                                                                                PARK-NEUTRAL SWITCH

MASS AIR FLOW SENSOR (MAF)                                       CRANK POSITION  SENSOR (CKP)                                                                  AUTOMATIC TRANS. GEAR SWITCHES

 OXYGEN SENSOR (O2)                                                        CAMSHAFT POSITION SENSOR (CMP)                                                          A/C REQUEST SWITCHES AND SENSORS

                                                                                                POWER STEERING PRESSURE SWITCH (PSPS)




The coolant sensor could be either a two or three wire type that threads into the water jacket on the engine in direct contact with the coolant. The sensor is a thermistor that changes resistance in response to changing temperature.  The three wire CTS uses the third wire to provide feedback to the instrument panel temperature gauge. 

The ecm uses the CTS to make necessary calculations for:




The intake ( or manifold) temperature sensor is a two wire sensor that is mounted in the intake tract to measure the temperature of the intake air. Like the CTS, it is also a thermistor that changes resistance based on temperature.

The ecm uses this value to do the following:




The throttle position sends a variable voltage signal to the ecm depending on the angle to the throttle blade relative to its idle stop position. At idle or near idle, the output voltage is low, typically around .2 to .5V. When the throttle is wide open (WOT) the output voltage signal will be closer to 5V. 

The information from the TPS is used by the ECM for:

On the some of the engines, the TPS is adjustable, following shop manual procedures. The majority are not adjustable and is auto-zeroing every time the key is turned on.



The MAP sensor is a three wire sensor that has a signal input to the ecm, as well as a Reference voltage and ground. The sensor is connected to intake manifold. The information from this sensor is used to indicate engine load in order to calculate fuel and spark timing. Commonly the MAP sensor is used on fuel injection systems known as "Speed Density." It does not use a Mass Air Flow (MAF) sensor. 

The ecm uses:

Manifold Absolute Pressure is the opposite of manifold vacuum. When MAP is low, vacuum is high (during closed throttle) and MAP is high when vacuum is low, as during acceleration or WOT operation. When the engine is not running, the MAP sensor is registering atmospheric pressure or  (barometric pressure=14.7 lbs/sq. in at sea level).

The MAP sensor is constructed of a flexible silicon wafer that when it is stretch will change resistance and the voltage output associated with it. This flexible wafer is mounted in a sealed housing, which side is connected to the manifold. The other side is sealed with vacuum above the chip, so the atmospheric pressure is below it. 

When manifold pressure is low, such at idle, the sensor voltage will be low, in the order of around 1V. During acceleration or WOT operation, the manifold pressure will be high and the output voltage will be around 4.5 to 5V.




The MAF sensor is positioned in the air intake duct or manifold and measures the density and volume of the incoming air. The sensor is able due to its engineered design, able to take temperature, humidity and density and use this information to determine the "mass" or weight of the incoming air at that precise moment. The output voltage or sine wave signal sent to the ecm is processed to calibrate the fuel delivery requirements much more precisely than a speed density based system. 

Although GM used two different types of MAF sensors, the principles are the same with both of them. Some of the early ones used a heated film, most of them used a heated wire design. The conductor in both of them are maintained at a precise calibrated temperature. The heated film MAF was kept at 75 C above the incoming air. The heated wire was kept at 100C. As air passed over this heated conductor, it carried away the heat from this sensor conductor. Changes in this current caused by the cooling effect of the passing air caused the monitoring circuits to raise the current to try to maintain this fixed temperature of the wire. The changes in the current is then translated into a voltage or sine wave (measured in cycle per second or HZ). This signal is then processed by the ecm to tell it how much to add or subtract from the fuel delivery through the injectors.

If the air was more humid, or cooler it would absorbed more of the heat from the sensing wire, requiring more current to maintain the temperature of the sensor.  The screen on the input of the sensor is there to straighten the air flow to insure a more precise measurement of the air density. 

The heated wire design, also known as a "hot wire" also has a special burnoff function to eliminate any contaminants from the sensor wire. If this was not done, over time this accumulation of dirt would affect the sensitivity and precision of the MAF sensor. After the engine is shut down, the wire is heated briefly to 1000C to burn off the contamination. This burn off cycle is controlled by the ECM. 

The signal from hot wire design is around .4V at idle to around 4.5V at WOT.


Bosch hotwire MAF sensor used on the TPI engines 1985 to 1989




One of the most important sensors in the inputs to the ecm is the O2 sensor. It is used to monitor the amount of oxygen in the exhaust stream and provide feedback to the ecm to control the air/fuel ratio more precisely.  There are three styles of O2 sensors that have been used over the years. The first is a simple single wire design, which used the exhaust manifold as the ground return. It generally required more time to reach operating temperature before the ecm went to closed loop operation. The second type had three wires, although the sensor itself still depended on the exhaust manifold for the sensor ground return. The other extra wires was for the heating element which brought the sensor up to operating temperature much quicker.  The third type of sensor is a four wire design. It had the O2 sensor input and ground returning back to the ecm, instead of depending on the manifold for the electrical return path. The other two wires supplied the heater element with power and ground. The later ECM's, the heating element return is switched on and off during operation by the ecm itself (see picture below). 

The ecm applies a bias or line voltage of 450mv to the O2 sensor. This voltage is compared to the voltage generated by the O2 sensor when it is in operation.  The amount of electricity generated by the O2 sensor is proportional to the amount of oxygen present in the exhaust stream.

With a rich mixture, there is almost no oxygen in the exhaust, and with the large difference between the outside air and the exhaust oxygen level, the O2 sensor voltage swings above 450mv. With a leaner mixture, the percentage of oxygen present in the exhaust is higher and the the O2 sensor voltage swings below the 450V threshold to a lower value. The ecm is constantly trying to maintain a 14.7 to 1 ratio and the feedback from the O2 sensor constantly changes as well responding to the ecm adjusting the fuel calibration.  This mode is known as closed loop due to the nature of the feedback between the ecm and O2 sensor.  In a normally operating mode, the O2 sensor will swing between 100 to 900mv. The ecm is also cross checks this fluctuation between voltage and time span and if it fails outside the required parameters, will set a code and alert the driver through the check engine lamp.




Engines that are calibrated for improved performance and operate with adding timing advance may exhibit spark knock. To control this undesirable condition, the engines are equipped with Electronic Spark Control (ESC).  The early engines may or may not have a ESC module integrated with the knock sensor, late model engines may have two knock sensors.

With engines that are equipped with a ESC module, this module sends a constant 8-10 volts to the ECM.  When the knock sensor detects a vibration due to spark knock, the ESC will pull the signal low and the ECM will see a voltage close to 0 volts. When the engine is running above 900 rpm, the ECM will retard the ignition timing until the spark knock disappears. It must be noted here, that when the engine is first started, the ECM will purposely advance the timing momentarily until a spark knock is detected (this knock is not even noticed by the operator) and then retard the timing. This is a self test mechanism to insure the electronic spark control is functional. 


ESC module from a 1988 350 TBI V8                Knock sensor on a 1988 350 engine, located on the passenger side.                          Knock sensor module for a 1994 LT1 V8

The integrated ESC has the signal line going directly to the ECM itself. The ECM provides a 5V signal to the sensor and under normal conditions the internal circuitry of the sensor pulls the voltage down to a 2.5V reference.  When knock is detected, the sensor produces a variable (AC) signal that is imposed on the DC voltage reference. The amplitude and frequency depends on the severity of the knock and the ECM will retard timing until the knock is eliminated.  Just like the ESC module equipped system, the integrated system also goes through a self test on start up.

It must be noted that the knock sensors are "tuned" to a specific engine displacement and fuel delivery system and are not interchangeable. They must match the application precisely. Failure to do so will result in poor performance and the ECM will recognize it as a fault and set a trouble code. 



The ECM needs to know the position and the speed at which the engine is running to be able to insure proper spark advance and fuel delivery. Also the ignition reference pulses are used to control the idle speed, and operation of the emission control devices. 

On distributor systems, the HEI (High Energy Ignition) module receives signals from the pick up coil. The pick up coil picks up its signal from the magnetic field from the pole piece, which has a stationary permanent magnet. The pole piece has teeth as well and when it rotates it builds up and collapses the magnetic field whenever the teeth pass each other. This collapsing magnetic field induces a current in the pick up coil and this signal is sent to the ignition module. The ECM controls the spark advance (or retard) through the module by modifying the timing of the trigger signal to the ignition coil. 

Some distributors will use a single magnetic switch, known a "Hall Effect" switch and a rotating non-magnetic pole piece with a number of slots that will match the number of cylinders. The slots passing through the window of the Hall Effect switch will also cause a build up and collapse of a magnetic field that triggers the switch to send out a reference signal to the ignition module. The ECM then controls the coil trigger signal in the same way as distributors equipped with a pole piece.

HEI pick up coil assembly shown. This is from a pre 1980 engine. Later ECM controlled distributors has a shield on the stationary pole piece. 


Distributorless Ignition System

The system does not use a conventional distributor and relies on a crank sensor (some engines has a crank and cam sensor) to trigger the ignition coil.  A sensor wheel or "reluctor" has teeth on it and the crank sensor in a sense is a "pick up coil".  The reluctor can be cast in place on the crankshaft, or part of the harmonic balancer or a "wheel" on the back of the crankshaft on the rear of the engine block. They all work in the same way. The crank sensor can induce its own signal through a permanent magnet and a shielded ground to keep it from picking up erroneous signals, or work as a Hall Effect design with a reference voltage, return and a signal input to the ECM. In the later case, the ECM provides the reference voltage.  The passing of the teeth over the sensor will cause a build up and collapsing of the magnetic field and induce the signal into the ignition module.  Again, the ECM modifies the spark advance or retard before triggering the coil.

DIS from a 1999 3.1L V6. Note the connections for the ignition module, the coils set on top of the module and has a 2 pin connection to connect it electrically to the module. 

As you see above, on a DIS system the coil is paired for two cylinders. This is because the DIS operates on the "waste spark" method. Regardless of the number of cylinders, one coil supplies the energy to a pair of spark plugs. Each end of the secondary coil is connected to one spark plug.  The resulting pairing of the cylinders to these plugs reach TDC at the same time. One piston will be on the top for the compression stroke, the other opposite cylinder at TDC for the exhaust stroke. 

Engine pairing:

One plug fires forward, from the center to the side electrode, and the other in reverse, from the side to the center electrode.  Since there is no compression on the exhaust stroke, less voltage is needed to fire the spark plug, so more than enough voltage is needed to fire the plug on the compression stroke. See diagram below. 




LT1 OPTISPARK 1992-1997

Another unique distributor in the GM family was known as the Optispark distributor ignition. It was not gear driven off the back of the engine like the conventional small block Chevy V8's were, but instead driven by a splined gear that mated it to the front of the block on the end of the camshaft. This direct drive reduced the spark scatter to virtually nothing while insuring tighter spark control and better performance. Although it was a good system, it was not without its faults, a coolant leak from the water pump above it could find its way into the housing and spell doom for it. Also the early design (1992-94) was not vented and ionized gases would build up and short it out. Later years were vented and actually did a good job to increase the service life. 


The first picture shows the optical sensor (arrow) that is used to detect the pulses. The signal is then sent to the ecm for reference and processing the spark angle. Then the ECM sends the pulse to a ignition module that is mounted on a heatsink platform. The ignition is triggered and the signal is sent to the coil and the resulting signal induces a field  in the ignition coil which expands and collapses to provide the voltage and current needed to fire the spark plug.


 The second picture shows the optical disk. You will notice a set of very fine slots on the outer edge and eight more slots in the inside of these. The outer ring  has 360 evenly spaced slots, one for every degree of rotation, this is used to control spark timing, and since the camshaft rotates once per two revolution of the crankshaft, it is in reality seeing 720 pulses. The inner ring slots is used for cylinder identification. You will noticed  four of the slots are not the same length, while the other four evenly spaced slots are. Since the length of the slots are converted into degrees, the ecm times the time the optical module sees the slot "open" thus the cylinder it pertains to. This is used to measure engine rpm, provide injector pulses, calculate spark timing and energize the fuel pump. Like the outer ring, the ecm "sees" 16 pulses for each revolution of the camshaft, the ecm then determines if that cylinder is on the compression or exhaust stroke.

Third picture shows a ignition coil and module mounted on the heatsink bracket. The early style 1992-95 coil is shown, the later style ignition coil has a single 3 wire connector.  For more information on the ignition circuit diagrams for any particular year click here.



The crank position sensor is a magnetic sensor that picks up changes in the position of the crankshaft reluctor as it rotates. A predetermined number of notches corresponds to a fixed number of degrees of rotation.  This can be as little as 5  notches, 90 degrees apart, for a 4 cylinder (the extra notch is used by the computer to reference it as the number #1 cylinder), or it can be as much as 58 notches for a late model E38 based ecm. 

This output pulse to the ignition module and ecm tells it where in the firing in order to fire the ignition coils and injectors at just the precise moment.



The camshaft position sensor is also a magnetic sensor that is used to determine the position of the #1 cylinder on the camshaft. A notch in the camshaft rotates and the magnetic flux changes in the sensor produces a AC sine wave signal to the computer. The signal is used to determine initial start up position to improve start up times in conjunction with the crankshaft position sensor. The notched cam was used on earlier styles on engines, while the latest camshaft position sensors are positioned in the front of the engine, where the cam timing gear could have 1 or 4 notches for cam timing.

A faulty or missing camshaft signal will result in longer cranking times and a trouble code being stored in the diagnostics. 



Two different styles of VSS systems were available over the years. The first type used an optical sensor which was mounted in the speedometer head. The second type was magnetic and was mounted on the tailshaft of the transmission.

In the first type of VSS, an optical type used a reflector with two blades inside the speedometer head. An LED assembly consisting of a transmitting LED (emitter) and a receiving LED shown a light beam on the reflective blades as the speedometer cable turned.  the light reflected twice per revolution of the speedometer cable. The signal is then sent to a buffer circuit which turns the signal into a "on" and "off" signal the ECM can interpret as vehicle speed.    

VSS speedometer head showing the reflective blade. The optical sensor mounts on the opening.             VSS buffer (green box) and the VSS optical sensor.

The speedometer cable turns one thousand (1K) revolutions so the output from the buffer is 2K pulses per mile.




The Magnetic VSS uses a permanent magnet (PM) generator that is mounted in a housing that connects to the speedometer cable.  The first style of PM VSS the housing has a gear which meshes with the drive gear for the speedometer output. As this gear turns the PM generates a pulse that is also sent to the VSS buffer for processing into a digital output the ECM uses for speed calibrations. This style is designed to generate 4K pulses per mile.

 The second style of PM VSS does not use a mesh gear, but the PM magnet circuit works much like a Hall Effect switch and the trigger gear is a reluctor wheel with 40 teeth mounted on the transmission output shaft. As the shaft turns once, 40 pulses are generated, so in the course of mile, this VSS is a 40K pulse per mile output. 


       4K VSS (TOP)   40K VSS (BOT)                             40K VSS transmission, note to 40 tooth reluctor on the output shaft.                                                 4K VSS buffer for signal conditioning

Some of the 40K VSS, used on mid and full size trucks and vans, used a special buffer known as a Digital Ratio Axle Controller (DRAC) module. These were used if the vehicle was equipped with antilock brakes. The greater resolution is needed for the addition of anti-lock brakes which requires this pulse to be modified by the VSS buffer module to 128K pulse per mile. This equates to the vehicle moving less than 0.5 inches. Inside the module was corresponding jumpers that were used to set up the module base on the rear gear ratio and tire size to the ABS and speedometer, and cruise control ( if equipped) were calibrated properly for the proper speed output. See pictures below.



A DRAC from a S10 blazer with ABS brakes. The three letter code designates tire size and rear gear ratio.                                Inside the DRAC module showing the calibration jumpers to set for tire size and rear gear ratio

The ecm uses the VSS signal for the following operations:



The Park/Neutral switch is a 2 wire, on/off signal that tells the ecm whenever the transmission gear selected in any position but park or neutral. A switch operated by the shift lever mechanism, either on the steering column, gear shifter mechanism or on the transmission itself. Whenever the gear selector is in Park or Neutral, the switch is closed and provides a closed circuit signal to the ECM.  In any other gear, the switch is open. 

The ECM needs a signal indicating if the transmission is in park, neutral or in any other gear. The ECM  uses this information in several different ways, either for spark timing or to know when the vehicle is shifted from park or neutral to a drive gear to compensate for the load imposed on the transmission. 



Transmission gear switches are on off switches, that are located inside the transmission. These can be simple pressure switches that screw into the various fluid pressure passages or can be built as a pad with a integral wiring harness. Depending on the gear being engaged, the switch closes and sends a corresponding low voltage signal to the ecm. When the gear disengages, the pressure drops in that passage for that gear, and the switch signal goes high.  The gear switches are used by the transmission to know when to engage the torque converter or in the case of the later (1993 up) automatic transmissions that are fully electronic, in conjunction with transmission range switch mounted on the gear lever, tells the ecm that the proper gear being commanded has been done. Any mismatch in the gear being selected and pressure switches not being made up will set a trouble fault code and illuminate the check engine lamp. 

Early 3 speed automatic transmission switch assembly,  4 speed automatics will typically have switches for 3rd and 4th gear only.

In conjunction with the gear switches, the transmissions may also have an additional switch to monitor the fluid temperature. This switch known as a Transmission Fluid Temperature Sensor (TFT) is used to warn of an impending over-temperature fault, which may include setting a trouble code, check engine lamp, reduced engine power and even turning on the cooling fans to assist in attempting to lower the fluid temperature to keep from destroying the transmission. 




Over the years, the A/C controls and input systems have changed to meet the specific needs for the engines, as far as performance and to a degree, emission standards as well.  Typically the input to the ecm for A/C operation comes from the operators comfort panel (control head) in the instrument or dash cluster. The request typically has to go through a high pressure and low pressure switch in series with the signal on its way to the ecm. 

The high pressure switch protects the compressor in event of a dead head condition of the system that restricts or completely blocks the flow of refrigerant through the system. It will open under the condition of a over pressure condition and breaks continuity of the circuit.  There may be other high pressure switch in the circuit, generally on the back on the compressor, that is used to open the circuit to the fan signal. This signal goes directly to the ecm as a fan request, and will open under a high pressure condition to protect the compressor. 

The low pressure switch acts in the same manner to protect the system, however, it will open under a low pressure condition, where pressure and flow is not enough to keep the refrigerant oil, which acts as a lubricant to protect the moving metal parts in the compressor.  

The switches may be in series with each other or as individual inputs to the ecm, as in the case of late model vehicles. 

These sensors also assist the ecm in determining:

Typical late model A/C control system, 2005 Chevy truck shown here. Note the communication is through a serial data request to the ecm from the control head.\




The power steering pressure switch is a 2 wire  on/off switch located on the pressure (pump) side of the power steering line and is used to detect the presence of high system pressure. 

This switch is used to detect a condition, during low speed vehicle operation, such as parking. The added load to cause the engine to stall.  In response to the input signal, which could be a high or low voltage, is to slightly increase the idle speed. During the time this signal is active, the ecm may also turn off the A/C compressor as well. 

Power Steering Pressure Switch  (PSPS)





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