Study guide
11.8 PCM Programming and Network Diagnosis
ASE T1 — Gasoline Engines Medium/Heavy Truck
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1 Introduction & ASE T1 Exam Overview
1.1 ASE T1 Exam Format, Scoring, Fees & Certification Requirements 1.2 Class 4–8 Gasoline & LPG Truck Engines and ASE Question Strategy
2 Chapter 1: General Engine Diagnosis — Complaints, Leaks, Noises & Exhaust
2.1 Concern Verification and Service Information 2.2 Fluid Leak Isolation and Gauge Verification 2.3 Lower-End Noise and Cylinder Cut-Out Evidence 2.4 Valvetrain, Accessory and Vibration Diagnosis 2.5 Oil Consumption and Blue Exhaust Smoke 2.6 Coolant Vapor, Rich Mixture and Exhaust Sounds
3 Chapter 2: General Engine Diagnosis — Vacuum, Balance, Compression & Leak-Down Testing
3.1 Vacuum Principles and Controlled Dynamic Tests 3.2 Vacuum Patterns and Manifold Pressure Diagnosis 3.3 Cylinder Power Balance and Contribution Testing 3.4 Dry, Wet and Running Compression 3.5 Cylinder Leakage (Leak-Down) and Combustion-Leak Block Testing
4 Chapter 3: General Engine Diagnosis — Electrical, Starting, Charging & Lab Tools
4.1 DMM, Scope and Scan-Tool Evidence 4.2 Harness, Connector and Schematic Diagnosis 4.3 Battery Condition, Parasitic Draw & Starting System Diagnosis 4.4 Charging System Performance, Voltage Drop & Belt Drive Service
5 Chapter 4: Cylinder Head & Valve Train Diagnosis and Repair
5.1 Head Disassembly, Cracks, Threads and Plugs 5.2 Head Metrology and Gasket Installation 5.3 Valve and Guide Inspection 5.4 Valve Seats, Springs and Stem Seals 5.5 Pushrods, Rockers and Valve-Train Harnesses 5.6 Lifter Preload, Valve Lash and OHC Service
6 Chapter 5: Engine Block Diagnosis and Repair
6.1 Pans, Covers, Seals and Sealants 6.2 Block Inspection and Cylinder Bore Metrology 6.3 Cam Bearings, Lobe Lift and Endplay 6.4 In-Block Lifters and Timing Drive 6.5 Crankshaft Inspection and Bearing Wear Evidence 6.6 Bearing Fit, Clearance and Crankshaft Endplay 6.7 Pistons, Ring Fits and Installation 6.8 Rods, Vibration Damper and Flywheel Inspection
7 Chapter 6: Lubrication & Cooling Systems Diagnosis and Repair
7.1 Oil Pressure and Lubrication Fault Diagnosis 7.2 Oil Pump, Cooler and Oil/Filter Service 7.3 Overheating, Overcooling and Thermostats 7.4 Coolant Testing and Water Pump Service 7.5 Radiators, Caps and Recovery Systems 7.6 Fans, Belts and Warning Indicators
8 Chapter 7: Ignition System Diagnosis and Repair
8.1 Ignition Symptoms and Combustion Knock 8.2 Ignition DTCs and Misfire Data 8.3 Primary Ignition Circuits and Coil Testing 8.4 Spark Plugs and Secondary Waveforms 8.5 CKP/CMP Sensor and Correlation Tests 8.6 Ignition Modules and Profile Learning
9 Chapter 8: Fuel, Air Induction & Exhaust Systems Diagnosis and Repair
9.1 Fuel Trim and Driveability Diagnosis 9.2 Fuel Tanks, Lines and Fuel Quality 9.3 Fuel Pump Circuits and Current Waveforms 9.4 Fuel Pressure, Volume and LPG Applications 9.5 Injector and Fuel Rail Testing 9.6 Electronic Throttle Testing and Learning 9.7 Air Filtration and Intake Leak Diagnosis 9.8 Exhaust Leaks and Restriction Testing 9.9 Gasoline Direct Injection Diagnosis and Service
10 Chapter 9: Emissions Control Systems Diagnosis and Repair
10.1 PCV Flow and Crankcase Pressure 10.2 EGR Flow, Feedback and Service 10.3 Secondary AIR Operation and Diagnosis 10.4 Catalyst Oxygen Storage and Five-Gas Analysis 10.5 EVAP Flow, Purge and Refueling Faults 10.6 EVAP Monitors and Leak-Test Procedures
11 Chapter 10: Computerized Engine Controls Diagnosis and Repair
11.1 DTC States and MIL Operation 11.2 Readiness, Freeze Frame and Repair Verification 11.3 Service Literature and Diagnosis with DTCs 11.4 No-DTC Driveability Diagnosis 11.5 Sensor Circuits and Shared References 11.6 Actuator and Low-Side Driver Testing 11.7 PCM Power Feeds and Loaded Grounds 11.8 PCM Programming and Network Diagnosis 11.9 Scan Data and Freeze-Frame Interpretation
11.8 PCM Programming and Network Diagnosis
Key Takeaways Explain This Topic AI
Use the OEM programming supply voltage, current capacity and recovery procedure.
Two directly connected 120-ohm terminations measure 60 ohms in parallel; three measure 40 ohms.
A normal termination reading does not prove every branch, module or signal is healthy.
Last updated: October 2026
Note
PCM Programming and Network Diagnosis uses stated teaching examples. Actual pin assignments, dimensions, thresholds and procedures must come from the identified engine, component and diagnostic tool.
PCM Replacement, SAE J2534 Pass-Thru Reprogramming & Relearns (Task H.9)
Whenever a PCM is replaced or reprogrammed via SAE J2534 Pass-Thru Flashing to resolve an OEM Technical Service Bulletin ( TSB ), emissions recall, or false-DTC software bug, technicians must follow a strict pre-flashing protocol:
Connect a Dedicated Programming Power Supply ( PPS / Battery Maintainer) : Never connect a standard shop battery charger, jump-starter, or roll-around booster pack during module flashing! Standard chargers output unfiltered AC ripple ( >300–800 mV AC ) and uncontrolled voltage spikes ( >15.5 V ) that corrupt flash data. Instead, connect a clean, regulated Programming Power Supply ( PPS ) with <100 mV AC ripple set to the OEM-prescribed voltage and current capacity; 13.2–13.8 V is only an example. If battery voltage sags below 12.0 V while the PCM is erasing and writing its flash memory blocks, the bootloader can be corrupted, potentially requiring OEM recovery ( "bricking" the PCM ).
Eliminate Parasitic Loads & Network Interruptions : Turn off headlamps, HVAC blowers, interior lights, and radios; pull the electric cooling fan fuse/relay if specified by the OEM (since fans often cycle on at 100% when PCM communication pauses during flashing); plug the laptop into AC wall power with sleep/screensaver modes disabled; and use a hardwired USB or Ethernet cable to the J2534 pass-thru interface.
Mandatory Post-Reprogramming / Post-Replacement Initialization & Relearns
Check which of the following initializations the identified replacement or programming operation requires:
VIN & Vehicle Identification ( VID ) Block Configuration : Writing the 17-character VIN , tire size ( revolutions per mile ) , rear axle gear ratio , transmission type, and fuel tank capacity.
Security / Immobilizer Parameter Reset & Key Handshake : Synchronizing the PCM encrypted security code with the BCM /immobilizer so the engine is permitted to crank and inject fuel.
Crankshaft Position ( CKP ) System Variation / Tooth Profile Relearn ( CASE Learn) : When required, use the exact scan-tool or road profile-learning procedure. Some GM-style procedures use a controlled fuel-cutoff event; Ford procedures can use specified decelerations. Do not substitute a generic 4,000-RPM maneuver. If the specific application requires profile learning and it is incomplete, P0315 or unreliable misfire monitoring can result.
Electronic Throttle Control ( ETC ) Closed-Throttle & Idle Air Volume Relearn : Teaches the PCM the mechanical lower stop of the throttle blade and compensates for throttle bore airflow.
Caution
Never Unplug the DLC Cable or Turn Off the Key If a J2534 Flash Stalls: Interrupting power or data during an active EEPROM erase/write cycle destroys the module's primary operating system. Always follow the OEM J2534 recovery procedure while keeping the regulated 13.5 V Programming Power Supply connected.
High-Speed CAN ( ISO 15765 ) & Heavy-Duty SAE J1939 Data Bus Diagnostics (Task H.10)
Medium- and heavy-duty gasoline trucks communicate across a twisted-pair differential serial data backbone —either High-Speed CAN ( ISO 15765 / SAE J2284 at 500 kbps on Pins 6 and 14 of the 16-pin OBD-II DLC ) or Heavy-Duty SAE J1939 ( 250 kbps or 500 kbps on Pin C [Yellow, J1939+] and Pin D [Green, J1939-] of the 9-pin Deutsch diagnostic connector) . Twisting the two wires at approximately the specified twist rate; 1 turn per inch is approximately 39.4 turns per meter helps ensure that external electromagnetic noise induces an identical voltage spike on both wires ( common-mode rejection ), leaving the voltage difference ( CAN High minus CAN Low ) uncorrupted.
The 60-Ohm Terminating Resistor Law & Voltage Signatures
A conventional directly connected two-termination high-speed CAN or SAE J1939 backbone has two 120-ohm terminating resistors located at the two physical ends of the main trunk cable (for example, one 120-ohm resistor inside the PCM at the front of the truck and a second 120-ohm resistor at the rear chassis frame or BCM / ABS module) to absorb high-frequency signal reflections. Because the two 120-ohm resistors are wired in parallel across CAN High and CAN Low ( 1 / R_total = 1 / 120 + 1 / 120 ), total loop resistance with the battery disconnected is approximately 60 ohms for this conventional directly accessible bus; a gateway or disconnected segment changes what the meter can see!
Test Mode & Parameter Normal Specification Fault Readings & Exact Diagnostic Root Cause
Total Bus Resistance ( CAN High to CAN Low , Battery Disconnected!) 60 ohms ( 55 to 65 ohms ) 120 ohms : One of the two 120-ohm terminating resistors is open/unplugged , OR one of the main trunk wires ( CAN High or CAN Low ) is open between the two ends of the bus. 0 to <5 ohms : CAN High and CAN Low are shorted together (or an internal transceiver is shorted). OL ( Infinity ) : Both trunk lines are cut or both terminating resistors are disconnected.
Recessive (Idle / Bit 1 ) Bus Voltage ( KOEO ) CAN High = 2.5 V , CAN Low = 2.5 V ( 0.0 V differential) Both wires rest at a 2.5 V bias when no node is transmitting a dominant bit.
Dominant (Active / Bit 0 ) Peak Voltage on DSO ( KOEO ) CAN High = ~3.5 V , CAN Low = ~1.5 V ( 2.0 V differential) Differential Signal Example : In the idealized example of a high-speed CAN / J1939 bus, CAN High and CAN Low are nominally complementary; common-mode voltage can shift, so inspect their difference as well ( 2.5 V + 2.5 V = 5.0 V idle; 3.5 V + 1.5 V = 5.0 V active). DMM average voltage reads ~2.6–2.8 V on CAN High and ~2.2–2.4 V on CAN Low .
Aftermarket ELD / Telematics / PTO Upfitter Intrusion Short drop stubs ( <1 m ) with zero extra terminating resistors Poorly spliced Electronic Logging Devices ( ELDs ) , GPS fleet trackers, or PTO controllers that add an unauthorized third 120-ohm resistor drop bus resistance to 40 ohms , or long un-terminated drop stubs cause signal ringing and U0100 ( Lost Communication with ECM/PCM ) / U0001 ( High Speed CAN Communication Bus ) codes.
Tip
Worked Diagnostic Scenario — Intermittent No-Crank and U0100 / U0101 Network Codes: A Class 6 gasoline rack truck exhibits an intermittent no-crank, wildly bouncing instrument cluster gauges, and U0100 ( Lost Communication with ECM/PCM ) and U0101 ( Lost Communication with TCM ) stored in the ABS and Body Control modules. The technician disconnects the negative battery cables , waits 2 minutes for all module capacitors to discharge, and measures resistance across Pin 6 (CAN High) and Pin 14 (CAN Low) at the 16-pin DLC . Instead of 60 ohms , the DMM reads 120.4 ohms ! Knowing that 120 ohms means the ohmmeter is seeing only one of the two parallel 120-ohm terminating resistors, the technician inspects the rear frame harness near the rear 120-ohm terminating resistor plug and finds the yellow CAN High trunk wire rubbed through and broken open against an aftermarket hydraulic liftgate bracket. Repairing the broken CAN High wire and restoring the 1 turn per inch twist brings the Pin 6-to-14 resistance right back to 60.1 ohms and permanently resolves all U-codes .