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Detailed Sensor Descriptions


  • Detailed Sensor Descriptions


    Knock Sensor (KS)

    This sensor is screwed into the block and detects detonation. If knocking or pinging is sensed the ECM will retard the ignition timing to prevent serious engine damage. Depending on the strength of the knock the ECM will pull a set amount of timing very fast and then slowly reduce the knock retard back to 0 or more knock is encountered, whichever happens first.

    The circuitry in the knock sensor pulls the +5V input voltage down to 2.5V, the knock sensor then produces an AC voltage that rides the 2.5V DC voltage. A knock will cause a voltage spike in AC voltage that oscillates about the 2.5V bias, if the spike is above 3V it is considered knock.

    Exhaust Gas Recirculation Valve (EGR)

    There are 3 types of EGR's: vacuum, digital, and PWM. Vacuum is vacuum so I'm not talking about that one. The purpose of the EGR is to reduce oxides of nitrogen (NOx) emissions.

    From AllData: "The atmosphere is made up of mostly Nitrogen, with a smaller percentage of oxygen, and a mixture of other gases. Oxygen and Nitrogen do not normally combine except at very high temperatures and pressures, conditions which are present in the combustion chamber especially during hard acceleration. When the engine is under load, the EGR valve admits a small amount of exhaust gas into the intake manifold to mix with the air/fuel charge. The exhaust gas is essentially inert (contains no fuel or oxidizer) and reduces peak combustion temperatures and pressures by absorbing some of the heat of combustion without participating in the actual burn. Greater amounts of exhaust gas are metered in as engine speed and load are increased."

    The digital EGR uses 3 different sized solonoids, think of it as a low/meduim/high setting and the combonation of the 3 solonoids activating can vary how much exhaust is let into the intake.

    The new design is the Pulse Width Modulation (PWM) solonoid which is basically infinate in its adustability while the digital is somewhat stair-stepped.

    Coolant Temperature Sensor (CTS)

    The CTS is usually located in the lower intake somewhat close to the thermostat. Since the coolant temp is usually the same or higher then then temp of the lower intake the sensor is fairly accurate. on most body platforms there is seperate CTS sensor that runs the dash coolant temp guage.

    The sensor is a thermosistor, as the coolant temp gets higher the resistance drops. The ECM supplies the sensor +5V and measures the voltage drop through the thermosistor to determine temperature. Some resistance vs temperature values follow:

    F Ohms
    210 177
    158 467
    104 1459
    68 3520
    32 9420
    -4 28680
    -40 100,700

    The sensor is used for open/closed loop operation and is used in the calculation for fuel and ignition.

    Throttle Position Sensor (TPS)

    This is actually just a potentiometer (variable resistor, sometimes called a rheostat), by turning the throttle body plate it rotates a shaft in the sensor causing different voltages to output. There are 3 wires: ground, +5V, and the sensor output. The different resistances caused by the rotating shaft vary the voltage output o*n the sensor output wire, the ECM then measures this voltage to determine how far the throttle is open. The sensor is located o*n the throttle body opposite the throttle lever.

    0% throttle is usually about .5V and 100% throttle is around 4.5V

    Intake Air Control Valve (IAC)

    The IAC is located in the throttle body and controls idle speed and prevents stalling do to varying engine load. It controls the amount of air that is bypassed around the throttle plate, more air the idle increases, less air the idle decreases. The IAC has a conical shaped tip that it moves in and out to block/open the bypass air passage. The IAC is moved in small increments called "counts" and can be read by most scan tools

    A stuck IAC will cause a high idle, low idle, or perhaps correct idle but it won't change if you turn the A/C on (idle increases with A/C).


    There is a vent on the gas tank that goes into a charcoal canister so that gasoline vapors do not vent to atmosphere, the CPC solonoid is hooked up between the charcoal canister and engine manifold. The solonoid is a Pulse Width Modulation (PWM, variable output) solonoid, turned on it blocks flow, turned OFF it allows. If the engine is warm, has been running for a set time, above a speed, and throttle is above a set point the ECM turns the solonoid OFF allowing the engine vacuum to suck the gasoline vapors out of the charcoal canister.

    Oxygen Sensor (O2)

    The oxygen sensor measures the amount of oxygen in the exhaust system to determine if the engine needs more or less fuel. A regular oxygen sensor is sometimes referred to a narrow band o2 sensor (NBO2) because it is only accurate at stoich (14.7 air fuel ratio (AFR)) which is where an engine will produce the least emissions. As you can see by the image below, a NBO2 is only good at telling you if your are rich or lean but never how rich or how lean you are.


    A narrow band O2 is a switching type, reading rich, lean, rich, etc. If you graph the voltage output it looks quite a bit like what siesmograph or lie detector needles sketch. When the ECM is using the O2 sensor to correct the fuel tables it measures how long it is lean and how long it is rich, if they are equal then the AFR is at 14.7 right where it should be. O2 "counts" is how many jumps back and forth it makes.

    Due to its nature a NBO2 is all but useless for PE (power enrichment, hard acceleration) where the most power is made with a richer then stoich. You can tell that you are rich but not how rich which leads to more difficult tuning. A wide band O2 sensor on the other hand is not a switching type, and will read accurately from a wider range (hence narrow and wide band).

    Starting in 94?, the NBO2 sensor got an upgrade to heated, which allows it to detect rich/lean counts much quicker than before. Mostly for emissions purposes, this is also required if you move your O2 sensor further away from the exhaust ports. WBO2 are all heated as far as I know. There are aftermarket setups that allow you to use a wide band for a gauge and/or datalogging, while also sending the narrowband signal to the ECM so it will know what is going on.

    Last reference, a NBO2 is 0-1 volt and a narrow band is 0-5 volt.

    Ignition Control Module (ICM)

    Also referred to as part of the Distributorless Ignition System (DIS). The ICM is the flat plate that the coils plug into and the ICM controls which coil is fired when, it is an independent system from the ECM. Through the CPS that is hooked to it it determins which coil to fire and sends a rpm/reference to the ECM so that the ECM can monitor engine speed. Below 400rpm the ICM controls spark advance by triggering the 3 coils at pre-determined intervals, above 400rpm the ECM will tell the ICM how much to advance the spark. It is a fairly decent and simple piggy back system that works quite well.

    Park/Neutral Switch

    The P/N switch is found on automatic transmission equipped cars only and serves multiple purposes.

    • power wire for starter is run through it, closed circuit (on) in park and neutral only
    • provide a park/neutral status to the ECM so that it can use special parameters for idle and rev limiter
    • trunk release safety switch, will only pop if car is in park (?)
    • reverse lights

    This switch has some adjustability so if for some reason your reverse lights don't come on in reverse but do slightly to one side of it or the other you might check into this.

    Power Steering Switch

    If the vehicle is equipped with it, the sensor is located on the steering rack. Under normal conditions it is an open circuit, no electricity flows through it. Under high pressure conditions that may be loading the motor (turning the wheel completely till you hear the pump straining) the switch closes.

    The ECM uses this switch to determine if it should raise idle speed to prevent possible stalling caused by the power steering excessively loading the engine. Due to its nature and the fact that it is a normally open circuit, this switch is really optional. Unplugging it will do nothing more then preventing the ECM to raise the idle slightly, but of course I RECOMMEND YOU LEAVE IT PLUGGED IN.

    Intake Air Temperature (IAT) / Mass Air Temperature (MAT) Sensor

    This sensor is is usually located in the airbox or on the airhose between the airbox and throttlebody that pushes into a rubber grommet. Some engines have a metal screw in type located in the engine plenum.

    The sensor consists of a thermosistor that is placed into the airstream, as it the air temp gets higher the resistance drops. The ECM supplies the sensor +5V and measures the voltage drop through the thermosistor to determine temperature. Some resistance vs temperature values follow:

    F Ohms
    210 185
    158 450
    104 1800
    68 3400
    32 7500
    -4 25,000
    -40 100,700

    GM IAT/MAT sensors are notoriously inacurate, they are succeptable to heat soak on the sensor itself and faster flowing air will cool the sensor off sometimes below actual air temp. The sensor is used for air density calculations that factor into fuel delivery.

    Crank Sensor (CS)

    The crank sensor is a component used to monitor the position or rotational speed of the crankshaft. This information is used by the ECU to control ignition system timing and other engine parameters.

    The crank sensor can be used in combination with a similar camshaft position sensor to monitor the relationship between the pistons and valves in the engine, which is particularly important in engines with variable valve timing. It is also commonly the primary source for the measurement of engine speed in revolutions per minute.

    Manifold Absolute Pressure Sensor (MAP)

    The manifold absolute pressure sensor provides instantaneous pressure information to the engine's electronic control unit (ECU). This is necessary to calculate air density and determine the engine's air mass flow rate, which in turn is used to calculate the appropriate fuel flow.

    An engine control system that uses manifold absolute pressure to calculate air mass, is using the speed-density method. Engine speed (RPM) and air temperature are also necessary to complete the speed-density calculation. Not all fuel injected engines use a MAP sensor to infer mass air flow, some use a MAF sensor (mass air flow).

    Fuel Level Sensor

    The fuel gauge system consists of a sending unit, instrument voltage regulator and an electric fuel gauge. The sending unit is a variable resistor that is controlled by a float. Corresponding to actual fuel level, the float will rise or fall. When the ignition is turned to the On position, voltage is applied to the gauge through the voltage regulator, completing the gauge ground circuit through the sending unit.

    When the tank is full and the float is raised, maximum resistance (approximately 90 ohms) is produced by the sending unit, current flow through the gauge is decreased, and the gauge pointer moves slightly. As the tank empties and the float drops resistance in the sending unit decreases, current flow through the gauge increases and the gauge pointer moves toward empty.

    Most analog fuel gauges are of the free floating type, which means that the gauge pointer does not remain against the full stop when the ignition is off. Rather, the pointer floats to a mid-position when no voltage is applied to the gauge.

    Oil Pressure Sensor

    This oil pressure indicating system incorporates an instrument voltage regulator, electrical oil pressure gauge and a sending unit which are connected in series. The sending unit consists of a diaphragm, contact and a variable resistor.

    As oil pressure increases or decreases, the diaphragm actuated the contact on the variable resistor, in turn controlling current flow through the gauge. When oil pressure is low, the resistance of the variable resistor is high, restricting current flow to the gauge, in turn indicating low oil pressure. As oil pressure increases, the resistance of the variable resistor is lowered, permitting an increased current flow to the gauge, resulting in an increased gauge reading.

    Oil Level Sensor Circuit

    This lamp illuminates to warn the driver that the engine oil level is low. When the ignition switch is first moved to Run, the oil level indicator lights for about 1 1/2 seconds as a bulb check. The oil level detection circuit has two internal timers. The first timer records the amount of time the ignition has been Off. The second timer records the amount of time the ignition has been On before the ignition was shut Off. The instrument cluster uses this information to determine if the engine has been sitting long enough for the oil to have returned to the oil pan.

    The oil level monitoring circuits will check the oil level switch under the following conditions:

    1. Ignition has been turned Off for more than 30 minutes.
    2. Ignition has been Off for at least three minutes after ignition has been On for at least 12 minutes. If the oil level is low (oil level switch open), the "Check Oil" indicator will be turned On for the remainder of the ignition cycle.

    Vehicle Speed Sensor (VSS)

    The Vehicle Speed Sensor is a gear-driven Permanent Magnet Generator housed in the vehicle's transaxle. This sensor generates a sine wave output with a frequency proportional to vehicle speed. The ECM converts this signal to an output that is switched to ground at a frequency of 4000 pulses* per mile. This output is pulled up to 5 volts or greater by the components that use this speed signal as an input.

    *Pulses from the VSS vary for different transmissions. 4000ppm is for the 4T60E.

    • phonedawgz
      phonedawgz commented
      Editing a comment
      The resistance stated for the oil pressure sender is backwards. At 0 PSI the oil pressure sender has a low resistance (0 ohms) At full pressure the sender has a resistance of 90 ohms. If you disconnect the wire to the oil pressure sender, your oil pressure gauge will peg full right.

    • robertisaar
      robertisaar commented
      Editing a comment
      phonedawgz is correct, at least for a W-body. other platforms may vary.
    Posting comments is disabled.

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    Contents1 Knock Sensor (KS)2 Exhaust Gas Recirculation Valve (EGR) 3 Coolant Temperature Sensor (CTS) 4 Throttle Position Sensor (TPS) 5 Intake Air Control Valve (IAC) 6 CPC 7 Oxygen Sensor (O2) 8 Ignition Control Module (ICM) 9 Park/Neutral Switch 10 Power Steering Switch 11 Intake Air Temperature (IAT) / Mass Air Temperature (MAT) Sensor 12 Crank Sensor (CS) 13 Manifold Absolute Pressure Sensor (MAP) 14 Fuel Level Sensor 15 Oil Pressure Sensor 16 Oil Level Sensor Circuit 17 Vehicle Speed Sensor ...
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