Fluid pressure switch assembly (PSA) sensor:
Mounted in valve body 
Five switches respond to manual valve position 
Different combinations inform PCM which PRNDL range is selected 
Transmission fluid temperature (TFT) sensor:
Mounted in PSA or part of harness 
PCM uses TFT sensor signal to: 
Help control TCC apply 
Control line pressure 
 
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 
EVAP canister purge 

A/C request: 
A/C switch does not control A/C compressor 
A/C control head signals PCM to: 
Delay A/C clutch engagement 
Adjust idle RPM to compensate for extra load 
Disengage A/C clutch 
Other A/C system switches must be closed 
 
A/C system sensors:
Refrigerant pressure sensor: 
Three-wire sensor 
Responds to changes in system high side pressure 
Identifies pressure increase 
Determines IAC valve position for idle speed control
Evaporator temperature sensor: 
Three-wire sensor 
Cycles A/C clutch for optimum cooling 
Disables A/C clutch  

Pulse-width modulation (PWM): 
Pulse is turning on and off
Width is amount of time voltage is on
Modulation means circuit is controlled over an operating range

PCM output drivers: 
Turn components on and off

Operate devices of engine management system
Quad driver:
Electronic switch within PCM 
One of a group of four 
Switches ground side of solenoid and relay coils 

Quad driver (QDR): 
Fusible driver 
Unprotected 
Quad driver II (QDRII): 
Protected 
Limits circuit current 

Quad driver module (QDM): 
Incorporates protection against circuit faults
Contains fault line to provide feedback signal to PCM 

Output driver module (ODM): 
Controls seven outputs of varying currents
Offers circuit protection
Communicates with central processor, specifying output faults 

Malfunction indicator lamp (MIL): 
Displays as ignition key is turned to on position without engine running
Newer systems shut off light after 3 seconds
Off during normal engine operation
Turns on when trouble code stores or PCM goes into backup mode 

To determine upshift light activation, information is received from: 
ECT sensor
TP sensor
VSS
MAP or MAF sensor
Engine RPM 

TEMP warning light: 
Controlled through PCM
PCM provides ground path
On some models, light is illuminated when coolant temperature exceeds 253°F (113°C) without DTC being set 

Exhaust gas recirculation (EGR) system: 
Used to lower oxides of nitrogen (NOx)
EGR valve:
Feeds exhaust gas back into combustion chamber 
Activated during warm engine operation and above idle speeds
PCM calculates EGR flow using the following readings: 
ECT
TP
MAP or MAF
Ignition reference
Vehicle speed 
Linear exhaust gas recirculation (EGR) valve: 
Used to lower combustion temperatures
Helps limit NOX emissions 
PCM: 
Monitors position of EGR using feedback signal
Supplies 5-volt reference to EGR pintle position sensor
Supplies voltage and ground to EGR valve 
Linear EGR valve: 
Supplies EGR independent of intake manifold vacuum
Controls EGR flow from exhaust to intake manifold through pintle:
PCM controls pintle position 
PCM monitors pintle position using feedback signal
Response time is 10 times faster than vacuum-operated EGRs 
Digital EGR: 
Control is entirely electronic
No vacuum
Has either 2 or 3 PCM-controlled solenoids:
Operate separate valves 
Share common power supply 
Each has separate ground at PCM driver
Ports have different diameters
Either entirely open or entirely closed
Can be operated together or separately

Idle air control (IAC) valve: 
Located in throttle body of both TBI and MFI systems
Contains movable pintle driven by stepper motor
Controls idle RPM
Stepper motor: 
Moves in exact, measured amounts called steps
Currents can be reversed in stators:
Change stator polarity 
Cause rotor to rotate 
To determine desired position of IAC pintle at idle or during deceleration, PCM refers to the following inputs: 
Battery voltage 
ECT 
TP sensor 
Engine RPM 
Vehicle speed 
Engine load 
AC Compressor 
PSP Switch and PNP Switch 
During ignition key cycle: 
PCM commands IAC valve to seat itself (extend)
PCM commands IAC valve to move away from the seat (retract)
Establishes correct reference for operation when engine is restarted
Provides exact amount of air for start-up  
Idle air control (IAC) valve pintles: 
Single taper
Dual taper
Blunt tip

Fuel pump relay: 
Provides system voltage to fuel pump 
When on, PCM energizes relay 
PCM powers relay circuit as long as it receives ignition reference pulses  

Oil pressure switch provides backup circuit (some models): 
If relay fails, it receives power from this circuit 
May result in extended crank times 

Injector pulse width (on time) is determined by: 
Engine temperature
Intake air temperature
Engine RPM
Throttle position
Manifold pressure/mass airflow
Oxygen sensor
Engine load
System voltage

Fuel injector pulse width: 
PCM determines how long to leave injector open
Injector ON time determines how rich or lean air/fuel mixture is
Duration of on time is called pulse width
Two principal types of injector drivers: 
Saturated switch 
Peak-and-hold 

Saturated switch driver: 
Used with injectors having high resistance (12 to 16 ohms)
Circuit resistance limits maximum current
Injector coil takes longer to build and collapse magnetic field 

Peak-and-hold driver: 
Used in conjunction with low resistance injectors (1 to 2 ohms)
Incorporates limiting device to prevent overheating of injector coil: 
Monitors current flow through injectors 
When current reaches maximum level, it is reduced 
Current is maintained to hold valve off seat during pulse
Injector opens and closes more quickly
Used in TBI and MFI injectors 

EVAP on OBD II vehicles: 
Evaporative system canister 
Fuel tank pressure sensor 
Canister purge valve and vent valve 
Fuel level sensor, tank, and cap 
Service port 
EVAP canister purge occurs when: 
Engine runs for specified time 
Coolant temperature above specified value 
Vehicle speed above specified mph 
Throttle is off-idle 

Purge valve: 
Allows fuel vapor to flow from canister to engine
Pulse-width modulated by PCM
Opened during enhanced EVAP diagnostic tests
Closed to seal system 
Vent valve: 
Replaces fresh air vent used on past canisters
Allows fresh outside air to canister during purge modes
Creates vacuum on fuel tank by closing 
 
Secondary air injection (AIR) system: 
Air pump
Control valves
Check valves
Necessary plumbing 
AIR system will shut off when: 
System goes into closed loop 
Pump has been on for a set time 
Air/fuel mixture is too rich 
Catalytic converter is over temperature (PCM recognizes problem and sets DTC) 

Throttle actuator control (TAC): 
Throttle by wire system 
Uses actuator to move throttle blade in unison with accelerator pedal 
Works together with PCM via serial data line
Throttle actuator control system: 
APP is input to TAC module 
PCM requests TAC module to reposition throttle blade via control motor 
TP signals TAC when throttle position is reached and PCM verifies 
Note: Some TAC systems may have minor operating differences. For example, the 2002 4.2L engine has the TAC module integrated into the PCM. Please refer to service information for your specific application. 

Accelerator pedal position (APP) sensor: 
Mounted on accelerator pedal
Three separate sensors in one housing
Three separate circuits: signal, ground, and 5-volt reference
Works with TPS and TAC module to input to PCM for requested accelerator pedal position and throttle angle 

Torque converter clutch (TCC):
Located inside converter
Reduced slippage in converter
PCM evaluates inputs to activate TCC:
ECT sensor 
VSS 
MAP/MAF sensors 
Gear selector 
Transmission fluid temperature sensor 

PCM engages electric cooling fan after receiving information from the following: 
Engine coolant temperature (ECT) sensor 
Vehicle speed sensor (VSS) 
A/C system request 
A/C on, and vehicle below a specified RPM 

A/C clutch:
At idle, PCM first increases engine idle, then grounds A/C relay 
PCM will interrupt A/C operation: 
At wide-open throttle 
When power steering pressure is high at low engine speed 
When engine cooling system is overheating

PCM uses information from the following components to operate cruise control:
Cruise control engagement switch 
Brake switch 
VSS 
TP sensor 
Servo position sensor

Transmission shift solenoids: 
On/off design 
When energized (on), solenoids are closed 
When de-energized (off), solenoids are open 

3-2 downshift control solenoid: 
Found on 4L60 E electronic automatic transmission 
Pulse-width modulated 
Operates 3-2 downshift control solenoid at 50 Hz 
Driver feels smoother shift transition 

Pressure control solenoid (PCS): 
Modulates fluid in torque signal hydraulic circuit
PCM looks at throttle position to determine PCS duty cycle
Minimum throttle: 
Maximum duty cycle is 40% 
Torque signal fluid pressure and line pressure minimized
High throttle openings: 
Near 0% duty cycle 
Maximum signal fluid and line pressure
Part of adaptive learning function 

Short-term/long-term fuel trim:
Read with scan tool 
Used when diagnosing engine performance 
Reflection of PCM changing fuel injector pulse-width 
Short-term - current changes of short duration 
Long-term - learned changes over long period of time 

Short term fuel trim: 
Helps make temporary corrections to air-fuel mixture in closed loop mode 
Monitors voltage from oxygen sensor 
Helps to maintain air-fuel mixture of 14.7:1 
Fuel trim numbers based on range from -20% to 0 to 20% 
PCM adjusts for lean or rich conditions 

Long-term fuel trim: 
Shows PCM learned fuel correction 
Indicates trend in short-term fuel trim 
Makes significant fuel delivery changes 


Fuel tank:
Made of terne-coated steel or high-density polyethylene 
Cap allows remaining pressure to escape gradually 
Fuel sender contains roll-over valve 

In-line fuel filter:
Removes particles as small as 10 to 20 microns 
Located in fuel supply line 
Should be replaced as needed 
Uses an O-ring seal to prevent fuel leaks, replace whenever you service

Fuel lines: 
Carry fuel from electric fuel pump to injectors 
Return fuel that flows past regulator back to fuel tank 
Couplings use O-ring seals to prevent leaks 
Evaporative emissions line: 
Routes fuel vapor from fuel tank to EVAP canister 
Made of steel or nylon 
Contains fuel-resistant rubber couplings at tank and canister 

Twin turbine pump:
Most common pump used for TBI systems
Supplies more fuel than engine requires at full load/maximum RPM
Fuel regulation between 9 and 13 psi
Check valve located in outlet side:
Prevents back flow of fuel into tank when ignition is off
Shortens cranking time

Turbine fuel pumps:
Introduced in 1994 (Gen II) 
1995 (Gen III) 
Provide fuel for higher pressure MFI systems 
Quiet operation at higher RPM 
Non-pulsating fuel flow 

Roller vane fuel pump:
Operates at 3,500 RPM
Impeller serves as vapor separator
Pressure relief valve regulates fuel pump pressure (maximum of 60-110 psi)
Check valve prevents back flow of fuel into tank 
Impeller generates pulses in fuel delivery system
Pulsator reduces pulses and prevents fuel pump noise
Sound insulator and isolator sleeve isolate mechanical noise and vibrations from tank assembly

Gerotor fuel pump:
First stage fill section at inlet of fuel pump: 
Fills modular reservoir 
Eliminates need for jet pump 
Part of modular fuel sender assembly 
Uses 33 micron filter

Fuel pump control module:
Used with 3800 VIN 1 (L67) engine
Located in trunk
Varies fuel pump output based on reference pulses
Normal duty cycle is set at 33%
Can be switched to 100% when needed 

Fuel sender assembly:
Pump mounted to fuel gauge sending assembly
Strainer is attached to prevent dirt/water from entering the system:
Self cleaning
Replaced at same time as fuel pump
Fuel blockage indicates abnormal amounts of sediment/water in fuel tank

Modular fuel sender: 
Incorporates several components:
Pump
Tank level sender
One strainer unit (two strainers)
Spring loaded to bottom of tank
Self-filling reservoir
Reduces pump noise transmitted to vehicle interior 

Returnless (demand) fuel system:
Introduced in 1998 on F-Car LS1 V8 engines
Only one fuel line comes from fuel filter to injectors
No return line routing fuel from rail back to tank
Reduces heating of fuel
Reduces evaporative hydrocarbon emissions  
Fuel pumped from fuel tank to T-connector 
Only fuel required for engine operation flows to engine 
Remainder of fuel returns to tank 
Fuel sender assembly: 
44 mm gerotor pump 
Pressure regulator 
Fuel level sensor  

New returnless on-demand system on 2003 ½ Corvette:
RPO code FFS
Two fuel tanks connected by crossover hose that contains: 
Auxiliary fuel feed 
Fuel return line
Electric turbine fuel pump located in fuel module reservoir assembly (MRA) inside left tank
Fuel drawn into MRA with venturi pump
Electric pump draws fuel from MRA
Pump sends high-pressure fuel through fuel filter and feed pump to injection system
Internal regulator maintains correct fuel pressure to injection system 
Fuel pump supplies pressurized fuel to venture jet tube 
Jet tube fills fuel sender assembly reservoir 
Pressurized fuel creates venturi action inside siphon jet pump, causing fuel to be drawn from right tank 
Fuel transfers from right to left tank through auxiliary fuel return pipe 
Anti siphon hole prevents fuel from siphoning from left to right tank
Secondary fuel pressure regulator:
Located in right fuel tank sender assembly 
Keeps fuel available to the siphon jet pump at regulated pressure 
Has lower set point than primary regulator to allow fuel flow to right-hand jet pump 
Pressure in feed lines drops to secondary set point at engine shut-off 
Equalizes left and right fuel tanks 
Pressurization reduces vaporization and boiling 

Fuel systems that cannot maintain constant fuel pressure may be leaking in one or more of the following areas:
Fuel pump check valve 
Fuel pump pulse damper 
Valve or valve seat within fuel pressure regulator 
Fuel injector 
Fuel lines, fittings, or connections 
Fuel filter

Static pressure and leak-down check for:
Faulty pump check valve 
Leaking line, coupling hose, or pulsator 
Pressure regulator stuck open 
Injector sticking open