Clearing and Relearning Procedures for Scan Tools

22-09 Examine VVT repair and relearn procedures.

After a VVT repair, clearing and relearning procedures may be required. Remember that any scan tool may list functions that are not available for the vehicle being repaired. When faced with this situation, refer to service information to confirm which procedures are required or allowed.

The following lists the typical available functions on the scan tool:

Variable Camshaft Timing Information

When testing a VCT system for faults, there are several pieces of information required for proper and thorough diagnosis:

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On some VCT systems (GM does this across its line), when a correlation code is stored, the scan tool will default to zero. On other systems, such as Chrysler and Hyundai, the default position on the scan tool is the lobe centerline angle. For example, a Chrysler 3.6 L Pentastar will display 117.0 for the exhaust cam and 128.0 for the intake camshaft. Another trick that may help in determining the default/base camshaft position is checking VCT PIDs with KOEO.

Variable Camshaft Timing Problems

VCT problems fall into three categories: electrical, mechanical, and hydraulic.

Hydraulic
Low Oil Level
Electrical
Mechanical
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If camshaft timing is out during the crank-to-run transition, it is typically a fault in the sprocket or chain. If the camshaft timing is out after the engine starts, suspect a solenoid fault.

The Five-Step Variable Camshaft Timing Test Procedures
  1. Check oil level, pressure, and condition:
    • Always begin diagnosis here after completing a visual inspection. Inspect the oil filter and oil pan for visible damage.
    • Address any fault in the oil system before performing any other testing.
    • Measure EOP manually and compare to specification.
    • Use a scan tool to check the useful oil life PID parameter, which may prove useful in diagnosis or when informing the customer of necessary maintenance.
    • Perform a quick check by disconnecting the VVT solenoid for the camshaft actuator at hand. Most systems default to their home/base timing position. When unplugging the solenoid, monitor the cam position. Use the actual PID for an indication if base timing is correct.
    • When checking oil pressure, the manufacturer-supplied specification may include checking at one or more engine temperatures and rpm. A recommendation to aid in diagnosis is always checking oil pressure when the engine is cold and hot, as well as at a variety of engine speeds, including idle and 2000–3000 rpm.
    • Most systems require a minimum of 7–10 psi (48–69 kPa) to operate correctly. Do not base the oil pressure “check” from the oil pressure light illumination. While considered low oil pressure in this range, it will typically not cause engine damage, keeping the oil pressure light off.
    • Oil pressure may be available as a PID, depending on manufacturer. If PID data are not available, use a manual gauge to check oil pressure. If the PID data show low pressure, confirm sensor operation by checking pressure given against a manual gauge reading to verify a sensor or an actual low-oil-pressure concern.
    • Check for filters in the oil supply to the camshaft phasers if the base EOP is within specification. The phasers may still fail to move if the in-line filter/bucket is restricted or plugged.
  2. Use a scan tool:

    A major asset to diagnosing VVT concerns is the use and interpretation of scan tool PID data. The majority of manufacturers provide scan tool information for desired and actual cam phasing, error (deviation), and cam solenoid status/command. This information is typically provided for each camshaft that uses a camshaft actuator or phaser. Some manufacturers provide “ideal” operating parameters in their service information for comparison during diagnosis.

    The information provided by the scan tool indicates what the PCM/ECM is asking for, the command, and the actual result, such as commanded and actual camshaft timing. Provided information can also include the error/deviation that is resulting in the camshaft timing as well as the duty-cycle control of the solenoid command by the PCM. Output tests verify solenoid.

    • Read codes: Current, pending, permanent, or history codes may be present.
    • If a code is present, refer to freeze-frame data for operating conditions that were present when the DTC was set.

      •  Freeze-frame data help when attempting to replicate a fault, especially intermittent concerns.

      •  Use freeze-frame data to verify a completed repair by operating the vehicle under conditions similar to those that set the code.

    • A typical diagnostic plan is to record codes and freeze-frame data. Clear codes and retest.

      •  If the DTC resets during KOEO testing, it is a circuit fault.

    • If the code does not set during KOEO, drive the vehicle in a manner reproduces the freeze-frame data.

      •  If the code resets by replicating the freeze-frame operating conditions, it is a performance fault.

    • If the code and concern are intermittent, perform wiggle and tap tests to attempt to replicate it.

      •  When performing a wiggle test, move small sections of the wiring harness to help isolate the area of the fault if the concern can be replicated.

      •  Another test useful for finding intermittent faults is the tap test.

      •  To perform the test, tap lightly on the component (exercise caution not to damage a part that does not have a fault) by using the handle of a screwdriver while attempting to replicate the fault.

    • For hard-to-replicate concerns, perform the tests under a variety of ambient temperature situations, cold and hot.
    • Use a spray bottle on electrical components, connectors, and wiring during all testing, to identify faults.
    • Monitor system operation by using PIDs.

      •  Unfortunately, due to manufacturer variations in naming, the PIDs offered by a manufacturer, as well as system design and operation, there is not an exact PID list for all makes and models.

    • There are several typical PIDs that should be used:

      •  Use desired and actual cam timing.

      •  These are the best PIDs for diagnosis, and if offered, they should be selected.

      •  While driving in graph form, check for slow, limited, or delayed responses by accelerating and decelerating often to ensure the VVT system is operating correctly.

      •  Use VVT error percentage if available.

      •  Use CMP, CKP, and sync PID if available.

      •  Use vacuum (MAP sensor if available).

      •  Graph over time: at idle, expect 30–35 kPa, and at wide-open throttle (WOT), expect typically 100–105 kPa.

      •  Use kPa because it is more sensitive than inches of mercury (inHg).

      •  If a large fluctuation exists in the MAP sensor reading, an internal engine fault is the cause of the drivability concern.

      •  Use short-term fuel trim (STFT) and long-term fuel trim (LTFT).

      •  Pay particular attention to skewed trim readings.

      •  Use MAF.

      •  Use calculated load.

      •  Determine how cam position is displayed.

      •  Use solenoid/actuator duty-cycle commands.

    • Check for bidirectional control ability (output tests) offered by most manufacturers to command the solenoid on. This control option may be by sending a varied duty cycle; an alternative is simply an on/off control. Operating the system with bidirectional controls verifies that the PCM driver, circuit, and solenoid are able to operate the phaser. If output controls are not available, use fused jumpers to control VCT operation during diagnosis by applying either power or ground to the OCV, depending on its control circuit design.

      •  The diagnostic flow chart may not provide this information, but it’s very helpful to verify system operation.

      •  With the engine running, activate the test to advance the camshaft (or use fused jumper wires). If the engine does not respond, four cylinders will typically stall at idle; for other engines, a noticeable change in rpm should be present if the system is operating correctly.

      •  If there is no change in engine operation, perform a KOEO check of the solenoid operation or remove and verify solenoid operation by watching it while commanding it to move.

      •  If the solenoid fails to move, check that the power and ground circuits are present at the solenoid; if both pass, replace the solenoid.

      •  If the solenoid does move, verify oil pressure (which should have already been performed), and if it’s OK, an actuator is the likely cause, assuming that the oil delivery system to the solenoid is not restricted or damaged.

  3. Check the cam phase angle:
    • Command each camshaft through its full range of motion.
    • Using scan tool or fused jumper wires, increase engine speed and command full phase angle.

      •  Raise the engine rpm to increase oil pressure for system operation and to prevent stalling.

      •  While operating the camshaft throughout its range, a change in engine rpm, combined with a noticeable change in engine smoothness, should be noticeable.

      •  If the engine rpm is not raised, then assuming the camshaft will move during testing, it may stall a four- or six-cylinder engine when the intake camshaft is fully advanced.

    • Check the range of motion, either by using scan tool to view the CMP for camshaft being tested or by using a scope to view the change in the cam-to-crank relationship.
    • If the camshaft’s desired and actual PIDs are available, select both on the scan tool and operate the camshaft through its full range of phase (motion), comparing the two PIDs in graph form to verify that the camshaft is responding correctly.
    • If the cam timing is stuck in either the advanced (intake) or retarded (exhaust) state, use the desired and actual PIDs to help diagnose the fault.

      •  If the PIDs are normal after staring but the difference begins to increase as the engine runs, this typically indicates a sticking solenoid spool valve.

      •  If the PIDs are incorrect when starting but the actuator is stuck, then the engine has jumped time or the engine was assembled incorrectly.

      •  If the actual PID always follows the desired PID but is slow to respond, suspect a concern with the engine oil, such as low pressure, incorrect viscosity, an internal leak, or plugged/restricted screens or passages.

      •  If the PIDs typically match but intermittently while driving and the actual PID fails to follow the desired PID, then the actuator or the solenoid may be sticking.

      •  If the actual PID is overly advanced (intake), overly retarded (exhaust), or very erratic after starting but then begins to follow the desired command after a few seconds, then the actuator pin is typically inoperative, not locking the phaser together as an assembly.

      The following use advanced diagnostic techniques requiring somewhat expensive equipment and experience to perform them. These tests can be added to the diagnostic arsenal later. The steps listed before will find the majority of VVT faults. Additional testing capabilities, such as in-cylinder pressure testing, may shorten diagnostic time and/or provide information on a difficult diagnosis that “standard testing” procedures fail to detect.

  4. Check scope cam-to-crank sync:
    • Determine the trigger from Coil 1, and compare it to the crank sensor and cam sensor(s), using an overlay that can be downloaded for the scope.
    • Check for proper cam-to-crank relationship to help determine belt or chain alignment and stretch.
    • Depending on the scan tool being used and the manufacturer, the tool may offer a cam-to-crank sync PID; if the sync is correct, the PID may display Yes, OK, or True, among others.
  5. Use a scope to check in-cylinder pressure to determine the following:
    • Perform cranking and running compression checks.

      •  Cranking compression will increase (MAP sensor output will increase also) with the throttle held at WOT over cranking compression with a closed throttle, since air can more easily enter the engine.

      •  Advanced intake camshaft timing will increase running compression.

    • The scope determines the true top dead center (TDC) of the crank sensor signal.
    • It provides insight into the actual cylinder conditions while running, compression, sealing ability, and the cam-to-crank relationship (timing) without engine disassembly.
    • To determine crank sensor reluctor “degrees per tooth,” check service information for the CKP sensor description.

      •  Typically given as a number of slots/teeth of the target wheel/reluctor divide number of teeth by 360 degrees (complete circle) to get the degrees per tooth.

      •  For example, a reluctor with 60 teeth, the degrees per tooth is 6 degrees. There are 360 degrees in a complete circle, so divide that by the number of teeth on the reluctor to yield the result, which is 6 degrees (360 degrees ÷ 60 teeth = 6 degrees per tooth).

      •  Be aware that some reluctors may have a designed missing tooth so that the PCM can identify Cylinder 1.

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During diagnosis, using PIDs or other testing via a scope, ammeter, or voltmeter to confirm and monitor OCV circuit operation (duty cycle) verifies that the PCM is attempting to move the camshaft. Sticking solenoids and actuators will result in an increase in OCV duty cycle; the PCM continues to increase the duty cycle to achieve the desired CMP. While this is important, other PIDs offer information that is more informative. The desired and actual timing PIDs provide the best insight into system operation. If desired and actual PIDs match, typically within 2%–3%, there is no fault in the VVT system. A significant variation between the desired and actual PIDs will offer information that is more conclusive. If the variation is excessive, further and more intrusive diagnosis will be required.

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Using fuel trims on dual bank engines can uncover a variety of drivability concerns, including cam timing. Use both LTFT and STFT for diagnosis. On a MAF-equipped vehicle, if one bank is positive and the other bank is negative (known as skewed fuel trims), either the exhaust is clogged or cam timing has jumped. To help isolate the cause, remember that skewed fuel trims at both idle and 2500 rpm typically indicates a cam timing fault. Skewed fuel trims while driving but that are usually normal at idle (unless the exhaust is badly restricted) usually indicate an exhaust restriction. In the case of an exhaust restriction, the side with the negative fuel trim is always the side with the restriction. If the skew has been present for a long time, the PCM may have adjusted the fuel trims so that the skew split is only in LTFT while the STFT shows zero.

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When diagnosing a VVT fault, it is normal during rapid acceleration or deceleration to see a difference greater than ±3% that lasts for a few seconds when monitoring desired and actual camshaft position or deviation/error or percentage PIDs. The difference occurs since the mechanical operation of the system does not respond as quickly as the electrical command from the PCM. After a few seconds, the discrepancy between the two PIDs should return to specification. If the variation between the two PIDs does not return to normal (±3%), a problem exists in the phaser or OCV solenoid. Consequently, additional diagnosis will be required.

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If a code is present in the system, bidirectional control testing may not be available until after the DTC has been cleared. If the code appears immediately after cycling the ignition from off to KOEO, there is a circuit fault. Address any circuit failure first, then continue testing. Scan tools are a tremendous asset in diagnosis; however, some issues may arise during their use. Bidirectional controls allow for bypassing enabling conditions once “normal” vehicle operating conditions have been present. Some scan tools may list bidirectional (functional) controls that are not active on the current vehicle. Incorrect listings of PIDs and functional tests are normal on both aftermarket and OEM scanners. If a bidirectional control test fails to work without a DTC, check for a possible software update for the scan tool. If updates are not available at the time, submit a request to the manufacturer and check again later. Scan tools may also fail to provide the required information, such as required vehicle conditions, to perform a bidirectional test. If this situation arises during testing, attempt the bidirectional test again with a different manufacturer’s scanner and determine whether the issue repeats.

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Correct diagnosis of any VVT system requires a thorough understanding of the cause-and-effect relationship between oil pressure and camshaft timing. In the event of a base engine mechanical fault, the reaction torque from the valve train always pushes the camshaft actuator and camshaft in the retard/late direction. During normal operation, oil pressure overcomes this torque by moving the intake camshaft actuator in the opposite direction, the advanced/early direction. Camshaft timing that is always advanced is a result of an incorrectly installed timing component or a stuck VVT solenoid or phaser.

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A camshaft phaser containing a return spring is more difficult to move since oil pressure must overpower spring pressure to move the phaser. Therefore, low oil pressure/low oil level will affect a camshaft with a return spring first and may set a performance code.

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When presented with a lack of information on the system’s operation, reference a wiring schematic for the system. If the PCM is on the power or feed side, the voltage will be duty cycled to the solenoid during operation. If the PCM is on the ground side, then duty cycling the ground will operate the solenoid.

Camshaft Sensor and Crankshaft Sensor Testing

CMP sensors and CKP sensors send accurate CMP and CKP monitor information and provide feedback to the PCM. These sensors can be either variable reluctance (magnetic) or Hall effect. Variable reluctance sensors produce an alternating current (AC) sine wave and are two-wire sensors. Hall-effect sensors generate a digital square wave signal and are two- or three-wire sensors.

Due to the importance of accurate output signals from these sensors, shielding the wires enhances the signal sent to the PCM. Interference from ignition coils, failed alternator diodes, and faulty grounds can affect the CMP or CKP sensor output. If a sensor fails and sends either an incorrect signal or no signal, a CMP code can set (FIGURE 22-43).

FIGURE 22-43 The CMP sensor must have a reluctor in good shape to create the necessary waveform for the PCM to time the injection and ignition events.

To test either of these sensors, perform a quick visual check of the sensor and its associated wiring. Disconnect the sensor(s) and inspect for oil contamination or corrosion in the electrical cavity. Variable reluctance sensors, also known as permanent magnet (PM) generators, produce an AC sine wave. To verify the operation of a variable reluctance sensor, with the engine at idle, expect to see a minimum of 20 millivolts (0.020 V). Monitor sensor output on a graphing scan tool, DMM, or preferably a lab scope, which is much more accurate for low voltage signals. Raising the rpm should increase the amplitude (voltage output) and frequency of the sensor.

When checking AC voltage output or sensor resistance, most technicians use a DMM. A high-quality DMM or DVOM may be able to check a variable reluctance sensor AC voltage output voltage or sensor frequency (Hz). Using a DMM for voltage checks may require using min/max settings due to the low sensor output when cranking. The majority of manufacturers’ diagnostic procedures instruct the technician to test the sensor for shorts and opens using an ohmmeter. Disconnect the sensor, and check the value of the sensor against manufacturer specifications, typically 500–900 ohms.

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If a wiring repair or connector pigtail replacement is to be performed on any shielded wire, ground only one end of the shield to prevent current loops.

Hall-effect sensors should produce a digital on/off square waveform. Test the Hall-effect sensor using a graphing scan tool, lab scope, or DMM. The scan tool may also provide a PID for CMP position PIDs. Only the output frequency of the sensor should change with load and rpm. On V-type engines that have more than one intake or exhaust camshaft, the sensors can be compared against each other to verify the signal output when using a scan tool or lab scope. A swap test is also a valid test if a code is present for a single CMP on a multi-cam engine. Swap the CMP from like cam to like cam to determine whether the fault follows the sensor or stays with the camshaft in question. If the fault follows the sensor, replace the CMP and retest. If the fault remains with the camshaft in question, additional testing will be required to determine whether the fault is electrical, mechanical, or hydraulic.

The actual relationship between the CMP and CKP is synchronization, or sync, and is an important test. A few degrees of CMP-to-CKP variance can result in drivability concerns and DTCs. Several test methods are available to verify synchronization, including scan tool, scope testing, or engine disassembly. On a scan tool, check for a PID that states whether the signals are in sync or not. A simple yes/true or no/false reading indicates a typical sync PID result. While readily available, the PID may not always be accurate to verify proper cam timing. Do not base an entire diagnosis for incorrect camshaft timing off of this PID alone. Use the PID as a quick reference only. The PID can be inaccurate due to the crankshaft reluctor design lacking the number of teeth required for the precise resolution capable of detecting a belt or chain that is off by a single tooth. For complete accuracy without engine disassembly, check the CMP-to-CKP synchronization with a digital storage oscilloscope (DSO). Sync testing with a DSO typically involves leading or falling edge signal comparisons between the CMP sensor and CKP sensor.

Using a DSO requires the technician to correctly understand the cam-to-crank relationship, how to install the scope properly, and what a known-good waveform looks like. Find the point of easiest access to scope the sensor(s). Using an overlay that displays the degrees of engine rotation can help verify proper camshaft timing. The overlays display the number of crankshaft degrees across the bottom of the DSO screen, showing the entire 720 degrees of crankshaft rotation. Overlays are readily available on technical information service websites, some OEM websites, and other sources of technical service information. A Google search will produce a variety of overlays available for installation on the scope. A CMP signal can be incorrect due to a jumped timing belt or chain, cam phaser sticking, improper tension on the timing chain or belt slipped cam/crank gear, incorrect assembly/incorrect part used during installation, or a cracked or bent reluctor trigger wheel(s).

The actual relationship between the CMP and CKP, known as synchronization or sync, is also an important PID/test. A few degrees of CMP-to-CKP variance can result in drivability concerns and DTCs (FIGURE 22-44). On a scan tool, a PID may be given that states whether the signals are in sync or not by a simple yes/true or no/false reading. While readily available, the PID may not always be accurate due to reluctor design lacking the number of teeth required for proper and accurate resolution capable of detecting a belt or chain off a single tooth. A dual-trace lab scope can be used here to heck this relationship with total accuracy. Lab scope sync testing typically involves leading or falling edge signal comparisons between the CMP sensor and the CKP sensor, and if used correctly, it can prevent the need for invasive engine disassembly. Using a scope requires the technician to correctly understand the relationship, how to properly install the scope, and what a known-good waveform looks like. Find the point of easiest access to scope the sensor(s). Using an overlay that displays the degrees of engine rotation can help can be of assistance. These are readily available on technical information service websites, on some OEM websites, and in service information. This signal can be incorrect due to a jumped timing belt or chain, cam phaser sticking, incorrect tension on the timing chain or belt, slipped cam/crank gear, incorrect assembly/incorrect part used during installation, or a cracked or bent reluctor trigger wheel(s) (FIGURE 22-45).

FIGURE 22-44 Using a scan tool to view the CKP-to-CMP relationship can allow the technician to home in on the area they will need to investigate further to determine whether there is a fault.

FIGURE 22-45 The CKP and CMP must be in sync for the VVT and VVL systems to operate correctly; if the timing is off by even one tooth, the operation of these components will be disabled until the timing issue has been resolved.

Other faults that can cause concerns with sensors that affect VVT operation include excessive camshaft or crankshaft endplay, allowing the sensor and the reluctor gap to be extreme or to contact, damaging either part. The accumulation of metal contamination on the magnetic end of the sensor, wiring and connector faults, and a cracked flexplate are all potential defects. Be aware when installing used or remanufactured engine components because there may be a difference in tone rings/reluctors. Manufacturers can and do change parts frequently, depending on the model year. Always perform a close visual inspection before installation, because if the part is incorrect, it can result in poor performance concerns, codes, or even a no-start.

During sensor installation, follow several precautions. Do not drop a sensor, because it can result in damage to either the magnet or internal circuitry of the sensor. Dropping a sensor requires replacing the sensor because internal damage is possible from the impact. A replacement sensor may come with a shim to set the correct air gap. On some sensors, it may be external and require removal prior to starting the engine. Other sensors may include a piece of paper or sticker on the end of the sensor that is removed by the engine when starting; if sensor depth is adjustable, refer to service information to ensure that the proper sensor depth is set. If a crankshaft sensor, flexplate or front engine dampener/reluctor assembly is being replaced, check and, if available, perform a crankshaft variation relearn procedure. Failure to perform the crank relearn may produce false misfire codes and poor engine performance if the PCM is basing the misfire on previously learned values.

After checking and verifying concerns but before servicing, always check for TSBs. TSBs may offer testing hints, updated parts, or PCM software revisions. Perform a crankshaft relearn procedure, if available, after a repair to a reluctor/trigger wheel, CKP sensor, or any major VVT or base engine repair.