ASE 8 - Engine Performancefaculty.ccbcmd.edu/~smacadof/Books/A8StudentWorkBooks161/SWB_… · Performance Module 11 - Powertrain Control – Input 11-8 Circuit Operation Student Workbook

ASE 8 - Engine Performance

Module 11Powertrain Control Input

AcknowledgementsGeneral Motors, the IAGMASEP Association Board of Directors, and Raytheon ProfessionalServices, GM's training partner for GM's Service Technical College wish to thank all of thepeople who contributed to the GM ASEP/BSEP curriculum development project 2002-3. Thisproject would not have been possible without the tireless efforts of many people. Weacknowledge:

• The IAGMASEP Association members for agreeing to tackle this large project to createthe curriculum for the GM ASEP/BSEP schools.

• The IAGMASEP Curriculum team for leading the members to a single vision andimplementation.

• Direct contributors within Raytheon Professional Services for their support of translatinga good idea into reality. Specifically, we thank:

– Chris Mason and Vince Williams, for their leadership, guidance, and support.– Media and Graphics department under Mary McClain and in particular, Cheryl

Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & NancyDeSantis.

– For their help on the Engine Performance curriculum volume, Subject MatterExperts, John Beggs and Stephen Scrivner, for their wealth of knowledge.

Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEPColleges for their contribution for reformatting existing General Motors training material, addingcritical technical content and the sharing of their expertise in the GM product. Separatecommittees worked on each of the eight curriculum areas. For the work on this volume, wethank the members of the Engine Performance committee:

– Jamie Decato, New Hampshire Community Technical College– Lorenza Dickerson, J. Sargeant Reynolds Community College– Marvin Johnson, Brookhaven College– Jeff Rehkopf, Florida Community College at Jacksonville– David Rodriguez, College of Southern Idaho– Paul Tucker, Brookdale Community College– Kelly Smith, University of Alaska– Ray Winiecki, Oklahoma State University - Okmulgee

ContentsModule 11 – Powertrain Control Module – InputsAcknowledgements .......................................................................................... 2Introduction ...................................................................................................... 4Objectives ........................................................................................................ 4

General Operation .......................................................................................................... 5Signal Types ................................................................................................................... 6Circuit Construction......................................................................................................... 7Circuit Operation ............................................................................................................. 8PCM Inputs ................................................................................................................... 13Heated Oxygen Sensor (HO2S) Circuit ........................................................................ 16Intake Air Flow Measurement ....................................................................................... 19Ignition Reference Signals ............................................................................................ 22A/C System Signal Inputs ............................................................................................. 27Power Steering Pressure (PSP) Switch ........................................................................ 29Automatic Transmission Inputs ..................................................................................... 30Traction Control Desired Torque Request ..................................................................... 31Theft Deterrent Fuel Enable ......................................................................................... 31

© 2002 General Motors CorporationAll Rights Reserved

ASE 8 - EnginePerformance

Module 11 -Powertrain Control– Input

11-4

Student WorkbookIntroductionNATEF Standards VIII. Engine PerformanceB. Computerized Engine Controls Diagnosis and Repair

6. Inspect and test computerized engine control system sensors,powertrain control module (PCM), actuators, and circuits using agraphing multimeter (GMM)/digital storage oscilloscope (DSO);perform necessary action. P-1

7. Obtain and interpret scan tool data. P-18. Access and use service information to perform step-by-step

diagnosis. P-1

STC StandardsALL Competencies for Electrical Stage 3 18043.03 W

B. Automotive Computers4 Identify types of computer input signals5 Identify automotive data input sensors6 Identify cautions to be observed when testing sensors

A-8 Competencies for GM Powertrain Performance 16044.10 W/D/HF. PCM Engine Control Management

1 Identify and list the sensors that provide PCM inputs2 Describe each sensors that provides PCM inputs

ObjectivesUpon successful completion of engine performance module 11, the ASEPstudent will be able to:• Describe sensor signal types• Explain sensor circuit construction• Explain sensor circuit operation• Explain PCM operating parameter operation• Verify sensor circuit operation




11-5

Student WorkbookGeneral OperationIn order to make operating decisions, the PCM depends on informationfrom a network of sensors, switches, and other modules locatedthroughout the vehicle. The information from these items is consideredinputs to the PCM.

SensorsA sensor provides an electrical output that can be calibrated. Informationis supplied to the PCM by sensors that monitor the operating environmentand vehicle conditions. These sensors include Engine CoolantTemperature sensor, Vehicle Speed sensor, Intake Air Temperaturesensor, Throttle Position sensor, and others. The PCM feeds theinformation provided by the sensors to the microprocessor. Themicroprocessor then uses this information and the vehicle specificinformation in the PROM to calculate desired powertrain operation. Theelectrical outputs from the PCM command devices (fuel injectors, sparktiming, canister purge valve, EGR valve, etc.) to change operatingconditions.

Figure 11-1, PCM Input Parameters (Typical)




11-6

Student WorkbookSignal TypesSensors can be categorized in a variety of ways. One method is by thetype of signal the sensor produces. There are three types of sensorsignals.

Time Base SignalSome signals must be correlated with time to have a meaning. Themicroprocessor has a high-speed clock input for time measurement.Some of the signals that require a time reference are obvious: enginespeed (rpm) and vehicle speed (VSS). However, one that is not soobvious is the oxygen sensor.

Analog SignalAnalog signal have continuouslyvarying voltage. Since the PCM isa digital computer it cannot makecalculation with analoginformation; it must first beconverted to digital information.To do this the PCM passes allanalog input signals through ananalog-to-digital converter (A to Dconverter) before being sent tothe microprocessor.

Figure 11-2, Analog Signal

Digital SignalDigital signals consist of twoconditions, HI and LO. A LOsignal is 0 volts and a HI signalcan be 5 or 12 volts, dependingon the circuit. Themicroprocessor can use thisinformation directly.

Figure 11-3, Digital Signal




11-7

Student WorkbookCircuit ConstructionAnother method of categorizing sensors is by the location of the circuit'ssource voltage and ground. Circuit voltage and ground can be internal orexternal to the PCM.The microprocessor determines the state of a sensor input by measuringthe input voltage. To understand a PCM circuit you should think of themicroprocessor as a voltmeter measuring the sensor input. Schematics inService Information (eSI) show sensor inputs connected to either avoltage source or to a ground; either case, the schematics also show aresistor in the PCM. The voltmeter measures the voltage drop across thisresistor.If the sensor input to PCM is connected to the voltage source for thesensor, the PCM reads the voltage source minus the amount that thesensor has reduced the voltage.Pull-DownA "pull down" circuit is providedwith a reference voltage signalfrom the PCM. The power sourcefor the circuit is internal to the PCMand the signal voltage is pulled lowby the sensor to an externalground.

Figure 11-4, Pull-Down Circuit

Pull-UpA "pull-up" or "push-up" circuit hasa power source outside the PCM.The PCM does not provide thereference voltage signal. The PCMprovides the circuit ground throughan internal resistor.

Figure 11-5, Pull-Up Circuit




11-8

Student WorkbookCircuit OperationThe final method of categorizing sensors is sensor circuit operation.There are five methods of circuit operation. These five methods are:discrete input, temperature input, position/pressure input, voltagegenerator, and signal generator. Although each of these circuits provide avoltage signal to the PCM the difference is the kind of information thecircuit is providing and how the PCM interprets it.

Discrete InputThe simplest kind of signal the PCM receives is known as a "switched"input. A switched input is either a HI or LO signal depending on whetherthe switch is open or closed and whether it is a push-up or a pull-downcircuit.In a "pull-down" circuit the power source for the circuit is internal to thePCM. When the switch is closed; signal voltage is pulled low to anexternal ground. The PCM registers a low voltage reference signal. Whenthe switch is open, the PCM registers a high reference signal.

Figure 11-6, Pull-Down Switch Circuit

A "pull-up" circuit has a power source outside the PCM. When the switchis closed, external source voltage generates a high reference signal to thePCM. An open switch, on the other hand, generates a low referencesignal.

Figure 11-7, Pull-Up Switch Circuit




11-9

Student WorkbookTemperature InputA temperature sensor is an inverse temperature coefficient thermistor. Theresistance of the temperature sensor varies predictably with temperaturechange. At low temperature it has high resistance (100,700 ohms @ -40deg F/C) and at high temperature it has low resistance(177 ohms @ 212 deg F/100 deg C).The PCM supplies a 5 volt reference and ground for the temperaturesensor. The sensor input is measured across the resistor at the voltagesource. With temperature low, the thermistor's resistance is high and verylittle current flows through the thermistor; the sensor input close to thereference voltage. When the temperature is high, the thermistor'sresistance is lower; most of the reference voltage is dropped across thePCM's internal resistor and the sensor input voltage is about 1.5 to 2 volts.

Figure 11-8, Temperature Sensor Circuit




11-10

Student WorkbookPosition/Pressure InputAlthough a Position sensor and a Pressure sensor are internallyconstructed differently, both have identical circuits that operate similarly.The three-wire sensor (potentiometer) has a 5 volt reference, a groundcircuit back to the PCM and a signal voltage wire. Depending on theposition/pressure, the signal voltage at the PCM varies between a lowvoltage (0.5v) and a high voltage (4.5v).

Figure 11-9, Position/Pressure Sensor Circuit




11-11

Student WorkbookVoltage GeneratorVoltage generator sensors are sensors that produce a voltage signal. ThePCM is looking at the quantity or voltage level of the signal. The PCMusually looks at the voltage with regard to a reference level. The Oxygensensor is an example of this kind of senor.

Figure 11-10, Voltage Generator Circuit




11-12

Student WorkbookSignal GeneratorSignal generator sensors are sensors that generate a timed voltagesignal. The PCM is looking at the frequency or timing of the signal. TheMass Air Flow sensor is an example of this type of sensor.

Figure 11-11, Signal Generator Circuit




11-13

Student WorkbookPCM InputsEngine Coolant Temperature (ECT) SensorThe Engine Coolant Temperature sensor, or ECT, is a two-wire sensor. Itis threaded into the engine coolant jacket, in direct contact with the enginecoolant. The coolant sensor contains a thermistor and provides the PCMwith an engine coolant temperature reading. The PCM provides a five-voltsignal to the ECT sensor through a dropping resistor. When cold, thesensor provides high resistance, which the PCM detects as a high signalvoltage. As the engine warms up, the sensor resistance becomes lower,and the signal voltage drops. Approximate resistance values are shown inthe accompanying chart. At normal engine operating temperature, 85oCCelsius to 105oC, the signal voltage is in the range of 1.0v to 2.0v.The PCM uses information about coolant temperature to make thenecessary calculations for fuel delivery, ignition control, knock sensorsystem, idle speed, torque converter clutch application, canister purge,exhaust gas recirculation, and cooling fan operation in some applications.

Figure 11-12, Engine Coolant Temperature (ECT) Sensor




11-14

Student WorkbookIntake Air Temperature (IAT) SensorThe Intake Air Temperature sensor, or IAT, is a two-wire sensor positionedin the engine air intake to register the temperature of incoming air.Like the coolant temperature sensor, the IAT sensor is a thermistor device,which provides a varying voltage signal depending on resistance. Itsresistance decreases as temperature increases. The PCM supplies a five-volt signal to the IAT through a dropping resistor. Sensor resistance andthe resulting sensor voltage are high when the sensor is cold. Astemperature rises, resistance and sensor voltage go down.Air temperature readings are of particular importance during open loop, orcold engine operation. A reading of manifold, or intake air temperature isneeded by the PCM to:• Adjust the air fuel ratio in accordance with air density• Modify spark advance and acceleration enrichment• Determine when to enable EGR on some applications.

Figure 11-13, IAT Sensor Circuit




11-15

Student WorkbookThrottle Position (TP) SensorThe Throttle Position, or TP sensor, is a three-wire, variable resistormounted to the throttle body and operated by the throttle valve shaft.When the throttle is closed, the PCM reads a low voltage signal. When thethrottle is wide open, the PCM reads a high voltage signal. The voltagesignal changes relative to the throttle position, about 0.5v at idle and about4.5v at wide open throttle.Information from the T P sensor concerning throttle plate angle is oneparameter used by the PCM to calculate fuel delivery, ignition timing, andtransmission shifting schedule, EGR, torque converter clutch application,upshift light operation, and the evaporative emission control system.

Figure 11-14, Throttle Position Sensor Circuit




11-16

Student WorkbookHeated Oxygen Sensor (HO2S) CircuitThe Heated Oxygen Sensor, or HO2S, is unique among the engine controlsystem sensors because it acts like a battery and is able to generate itsown low voltage signal. It is located in the exhaust system and monitorsthe amount of oxygen in the exhaust stream. It provides feedback to thePCM, which uses this information to manage fuel delivery. The electricallyheated oxygen sensor warms up quickly and remains hot, even at idlewhen the exhaust manifold may cool down.

Figure 11-15, Oxygen Sensor

ConstructionThe HO2S has a center elementmade of a ceramic materialcalled Zirconia. There are twoplatinum electrodes, which makeup the inner and outer surfacesof the center element. The innersurface of the sensor is exposedto outside air. This surface formsthe positive terminal of theHO2S circuit.The platinum coating on theoutside of the sensor element isexposed to exhaust gases. Thegases heat up the HO2S andkeep it at the correct operatingtemperature of 600 degreesFahrenheit. The outer surfaceforms the negative terminal ofthe sensor circuit.The HO2S generates anelectrical signal as the result ofthe interaction of outside air, theinner surface of the element,exhaust gases, and the outersurface of the element.




11-17

Student Workbook

Figure 11-16, Different Oxygen Levels

OperationThe PCM applies a referencevoltage, also known as biasvoltage, of 450 mv to the HO2S.The PCM compares thisreference voltage with thevoltage generated by the HO2S.The amount of voltage the HO2Sgenerates is proportionate to thedifference between the amountof oxygen in the outside air andin the exhaust gases.The atmosphere contains about21% oxygen. The exhaust from arich air fuel ratio contains almostno oxygen. With a largedifference between the amountsof oxygen contacting the twosurfaces, the sensor is able togenerate more voltage. Whenthe exhaust gas is rich, below14.7:1, the voltage output ishigh, above 450 mv.

The exhaust with a lean air fuel ratio has about 2% oxygen. With a smallerdifference between the amounts of oxygen on the two surfaces, thesensor generates less voltage. When the exhaust gas is lean, above14.7:1, air fuel ratio, the sensor's voltage output is low, below 450 mv.In either case, the PCM reads the difference and adjusts injector operationto make the air fuel ratio richer or leaner as required.




11-18

Student WorkbookIn a normally operating engine, the HO2S output voltage constantlyfluctuates up and down between 100 mv and 900 mv. This fluctuationreflects the changes in the air fuel ratio. The PCM adjusts injector pulsewidth in response to the changing HO2S signals. This data is used todetermine short term and long term fuel trim.

Figure 11-17, HO2S Voltages

HO2S output voltage constantly fluctuates up and down and is used for:• Adjusting injector operation• Determining short term and long term fuel trim• Open loop/closed loop criteria• EGR diagnostics• Monitoring catalyst efficiency• Secondary air and EVAP diagnostics




11-19

Student WorkbookIntake Air Flow MeasurementThere are two methods of sensing incoming engine air flow: speed densityand mass air flow.The PCM uses intake air flow measurement information for:• Barometric pressure readings• Fuel delivery (enrichment, enleanment, fuel cut-off)• Spark calculations• Diagnostics

Speed DensitySpeed Density is a system of measuring intake air flow by sensingchanges in intake manifold pressures which result form engine load andspeed changes. The PCM uses a MAP sensor to read manifold absolutepressure. The PCM combines MAP along with temperature, RPM,estimates of volumetric efficiency an EGR to calculate mass air flow.As manifold pressure increases, air density increases as well andadditional fuel is required. The PCM increases injector pulse width to meetthis requirement based on a "calculated" air flow.

Figure 11-18, Map Sensor, Speed Density System




11-20

Student WorkbookManifold Absolute Pressure (MAP) SensorThe Manifold Absolute Pressure (MAP) sensor is a three wire sensorlocated in the engine compartment. The MAP sensor measures changesin intake manifold air pressure.MAP is low when vacuum is high, and MAP is high when vacuum is low.When the engine is not running, the manifold is at atmospheric pressure,and the MAP sensor is registering barometric, or BARO, pressure. BAROreadings are used for fuel delivery calculations at start up, and fuel andspark calculations when the engine is running. The PCM updates itsBARO reading when the ignition is turned on and when the throttle is wideopen.The MAP sensor currently being used on GM vehicles is a Strain Gaugetype. This sensor contains a silicon chip, approximately three millimeterssquare. It is placed in a sealed housing, which is connected to themanifold. A fixed pressure is sealed above the silicon chip with manifoldpressure below it. When the engine is running and manifold vacuum iscreated, the pressure below the chip drops, creating a change inresistance.During operation, constantly varying vacuum from the intake manifold isapplied to the sensor housing. Any change in applied vacuum causes acorresponding change in the sensor's resistance. Electrically, whenmanifold pressure is low, the sensor voltage is low. When manifoldpressure is high, sensor voltage is high.

Figure 11-19, MAP Sensor Circuit




11-21

Student WorkbookMass Air FlowA Mass Air Flow (MAF) sensor is positioned in the intake air duct ormanifold. It measures the volume and density of the incoming air. TheMAF sensor is able to take the temperature, density, and humidity of theair into account. All of these variables together determine the mass of theincoming air. The PCM reads actual mass airflow to calculate fuelrequirements.

Mass Air Flow (MAF) SensorGM has used several types of MAF sensors. All use the same operatingprinciple: the resistance of a conductor varies with temperature. In thecase of the MAF sensor, the conductor is maintained at a constantcalibrated temperature. As a greater volume of air passes the heatedconductor, the passing air carries heat away. More current is required tomaintain the constant temperature of the conductor. In a similar manner, ifthe air is more humid, denser, or cooler, it will absorb more heat from thesensor, requiring more current to maintain the temperature of the sensor.This current then translates into a voltage signal, telling the PCM howmuch airflow there is, so that the PCM can make fuel delivery and sparktiming calculations.

Figure 11-20, Hot Wire Mass Air Flow Sensor Circuit




11-22

Student WorkbookIgnition Reference SignalsDistributor Ignition (DI) or Electronic Ignition (EI) reference signals are anindication from the ignition system of engine speed. These signals aresent on the reference circuit from the ignition module to the PCM. Onengines with an HEI distributor, the DI module receives signals from thepickup coil assembly in the distributor and sends them on to the PCM onthe RPM reference wire.On distributorless engines, the ignition module receives signals from thecrankshaft position sensor located either in the engine block or on thefront of the engine, depending on application. This signal is then sent tothe PCM as a 5-volt digital signal.The PCM requires ignition reference pulses in order to control:• Spark timing• Triggering and synchronization of fuel injectors• Idle Air Control (IAC) valve operation• Fuel pump relay• EGR• Canister purge (EVAP)

Figure 11-21, Ignition Reference Circuit




11-23

Student Workbook

Figure 11-22, 4.3L Ignition System

Crankshaft Position SensorThe crankshaft position sensor (CKP), is the most critical input for theignition system. It identifies cylinder pairs at top-dead-center. The PCMuses the camshaft (CMP), sensor to identify which cylinder is on thecompression stroke and which is on exhaust.

Camshaft Position SensorWhile the CKP sensor identifies cylinder pairs at top dead center, the CMPsensor identifies cylinder stroke. The CMP sends a signal to the PCM,which uses it as a sync pulse to trigger the injectors in proper sequence.The PCM uses the CMP signal to indicate the position of the number onepiston during its intake stroke. This allows the PCM to synchronize theignition system and calculate true Sequential Fuel Injection (SFI).




11-24

Student WorkbookVehicle Speed Sensor (VSS)The vehicle speed sensor (VSS), provides vehicle speed information tothe PCM.The PCM needs information about vehicle speed to operate:• The idle air control valve• Canister purge• Torque converter clutch• Cruise control• Transmission shift solenoids• Electric cooling fans.The magnetic VSS consists of a permanent magnet generator, whichproduces an AC voltage whenever vehicle speed is over three miles perhour. The AC voltage level and the number of pulses increase with vehiclespeed. Since the VSS output is AC voltage, which cannot be directly usedby digital electronic components like the PCM, the AC voltage is convertedinto a digital signal by the VSS buffer.

Figure 11-23, Magnetic VSS Buffer Amplifies




11-25

Student WorkbookKnock Sensor SystemThe Knock Sensor system allows the PCM to control ignition timing for thebest possible performance while protecting it from potentially damagingdetonation. The Knock Sensor is used to detect engine detonation, orknock, and signal the PCM to retard ignition timing.The PCM supplies a five-volt reference signal to the knock sensor througha dropping resistor. Since the knock sensors have internal resistance, avoltage drop is created and read by the PCM to determine if the circuit isopen, shorted to ground, or shorted to voltage.The Knock Sensor sends the PCM an AC voltage signal when detonationoccurs. The PCM processes the signal and modifies the ignition timing tocontrol knock. When the Knock Sensor signal stops, the PCM begins toreturn the ignition timing in two to four degree increments back to normalIC advance.The Knock Sensor allows the PCM to advance ignition timing as much aspossible for the best performance and fuel economy. But perhaps moreimportant, the Knock Sensor controls ignition timing to protect the enginewhen low octane fuel is being used. High engine temperatures andpotentially damaging detonation are more likely with the use of low octanefuel. On some systems, the Knock Sensor system has a built-in self-testmode. Once per engine start-up, during certain engine conditions, theignition is advanced intentionally to induce a knock, which the KnockSensor should detect. This self-test is bypassed if a knock occurs and isdetected before the conditions are met to run the self-test.

Figure 11-24, Integrated KS System




11-26

Student Workbook

Figure 11-25, Ceramic Resistor CardFuel Sensor

Fuel Tank Pressure SensorThe fuel tank pressure sensor is used to detect leaks in the evaporativeemissions system. The sensor is a three-wire strain gauge sensor, muchlike the common MAP sensor. However, this sensor measures thedifference between the air pressure, or vacuum, in the fuel tank and theoutside air.The fuel tank pressure sensor mounts at the top of the fuel tank sendingunit and alters the reference voltage to create a signal voltage. Thesensor's seal at the sending unit is critical and should be inspectedwhenever the sensor is removed or serviced.

Ceramic Resistor Card Fuel Level SensorBeginning with some 1996 Enhanced EVAP equipped models, the fuellevel sending unit was switched from a wire wound 0-90 ohmpotentiometer to a ceramic cardresistor 40-250 ohm potentiometer.The improved resolution improvesfuel level sensing accuracy, neededwhen the PCM performs on-boarddiagnostic tests.The ceramic card assembly consistsof a:• Ceramic card• Wiper arm• Float arm assembly• Wire harness assembly.

The sensor is used to convert changes in the fuel tank level to a variableelectrical signal used to drive a gauge in the instrument cluster. Theceramic resistor card fuel level sensor attaches to the outside surface ofthe modular fuel sender assembly. An electrical harness attached to thefuel sender cover connects the ceramic resistor card to the vehicle wiringharness. Power to the sensor is received from the PCM.The float and float arm assembly work in conjunction with the resistor tomeasure fuel level. A full tank of fuel forces the float to the top position.With little or no fuel, the float moves to the bottom of the tank.The function of the ceramic resistor card is to vary the resistance of thesignal from the PCM, depending on the position of the float. Theresistance signal is determined by the wiper contact's position on theconductive bars of the ceramic resistor card. The fuel gauge converts thePCM signal into the fuel level reading on the instrument panel.




11-27

Student WorkbookA/C System Signal InputsThere is no specific sensor for the Air Conditioning (A/C) request. Instead,the input comes to the PCM from the A/C control head on the instrumentpanel.When the A/C system is turned on, the A/C compressor puts a suddenload on the engine. This could lead to driveability problems such asstalling, especially at idle. To prevent this, the A/C switch does not controlthe A/C compressor directly. Instead, the switch sends an A/C request tothe PCM. Depending on the engine control system and engine operatingconditions, the PCM does a number of things:• Including delaying A/C clutch engagement after A/C is requested• Adjusting idle RPM to compensate for the extra load• Disengaging the A/C clutch during wide-open throttle operation.There are a number of other A/C system switches that must be closed forthis to happen. These may include a high-pressure switch and or a low-pressure switch. These switches may be in series with the A/C request, orseparate inputs to the PCM.

Figure 11-26, A/C Systems Inputs




11-28

Student WorkbookThe PCM can have two additional inputs regarding the A/C system: TheRefrigerant pressure sensor, and Evaporator temperature sensor.The A/C refrigerant pressure sensor is a three-wire sensor that respondsto changes in system high side pressure. The PCM supplies a five-voltreference and ground. A signal circuit is monitored by the PCM. On mostsystems, the PCM uses the pressure sensor signal to identify a resultingpressure increase after the PCM has commanded the A/C compressorclutch to engage. On some other systems, this pressure signal is alsoused to determine IAC valve position for idle speed control.The evaporator temperature sensor, also a three-wire sensor, is used bythe PCM to cycle the A/C clutch for optimum cooling. Additionally, thePCM can help prevent evaporator freeze-up by disabling the A/C clutch.




11-29

Student WorkbookPower Steering Pressure (PSP) SwitchThe Power Steering Pressure (PSP), switch is a two-wire, ON/OFF switch,located in the power steering fluid pressure line. It is used to detect highsystem pressure. The PCM uses this information for:• IAC control• Spark retard during idle for improved idle stability• A/C compressor control.During low vehicle speed operation the power steering system pressuremay be high. The added load of the power steering pump could cause theengine to stall. The power steering pressure switch can be either normallyopen or closed, depending on design. When the calibrated pressure isreached, sensor circuit voltage switches. In response to this signal, thePCM operates the idle air control to increase engine speed slightly. If thevehicle is equipped with air conditioning, the PCM may also turn "OFF" thecompressor clutch relay when the PSP switch indicates high pressure.

Figure 11-27, Power Steering Pressure (PSP) Switch




11-30

Student WorkbookAutomatic Transmission InputsElectronically controlled transmission gear switches are ON/OFF switches,controlled inside the transmission transaxle. Some switches are normallyopen when the gear is not engaged, and closed when the correspondinggear is engaged. Other switches are normally closed when the gear is notengaged and open when the gear is engaged.Electronically controlled transmissions and transaxles have a FluidPressure Switch Assembly (PSA) mounted in the valve body. There arefive separate switches that respond to manual valve position. As theswitches in the PSA are exposed to the various fluid pressures of thedifferent gear ranges, different switch ON/OFF combinations occur. Threecircuits are monitored by the PCM; the different combinations inform thePCM which PRNDL range has been selected, based upon the manualvalve position.

Figure 11-28, Fluid Pressure Switch Assembly

The Transmission Fluid Temperature (TFT) Sensor is either mounted inthe PSA or is part of the harness. It is a thermistor, similar to the othertemperature sensors used for engine management, which is submersed inthe transmission fluid. The TFT sensor's resistance alters the five-voltreference signal sent by the PCM, which uses this signal to help controlTCC apply and to control line pressure. At higher temperatures, the PCMcan command TCC apply to reduce the temperatures generated by theconverter's fluid coupling.




11-31

Student WorkbookTraction Control Desired Torque RequestOn vehicles with traction control, there is constant communicationbetween the Electronic Brake Traction Control Module (EBTCM) and thePCM. The traction control desired torque request is a pwm signal thatranges from 0-100%. The EBTCM reduces the pulse width of the tractioncontrol desired torque request when a drive wheel slippage situation isdetected. The PCM monitors the traction control desired torque request. Ifa signal of less than 100% is seen, the PCM, depending on vehicleapplication, reduces wheel slippage by:• Retarding spark timing• Closing the throttle• Decreasing the boost solenoid pwm• Disabling fuel injectors

Theft Deterrent Fuel EnableThe Theft Deterrent Fuel Enable signal is an input from the Vehicle TheftDeterrent Module. It signals the PCM to enable the fuel injectors. If theVehicle Theft Deterrent Control Module does not send the correct TheftDeterrent Fuel Enable signal to the PCM, the fuel system may bedisabled.On some vehicles, this signal is a direct input to the PCM. Otherapplications use Class 2 serial data to transmit this message.

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ASE 8 - Engine Performancefaculty.ccbcmd.edu/~smacadof/Books/A8StudentWorkBooks161/SWB_… · Performance Module 11 - Powertrain Control – Input 11-8 Circuit Operation Student Workbook