Circuit/System Testing

22-04 Interpret electrical faults in VVT systems.

Electrical concerns with the voltage supply, ground connection, wiring harness, poor pin fit at the connector, or a damaged/corroded connector to the OCV solenoid(s) can all prevent the PCM from controlling oil flow. If the solenoid fails to open and close when commanded by the PCM, oil pressure will fail to reach the cam phaser, setting a code for a CMP error.

After performing basic system tests, dynamically check the OCV. Dynamically testing the system is the most efficient procedure to replicate faults. Solenoid testing is performed either on the vehicle or with the solenoid removed from the engine (preferred). Actuate the OCV solenoid to complete this test. Raise rpm to approximately 1500–2000 rpm, and either using a scan tool or a set of fused jumper wires to actuate the solenoid (FIGURE 22-11). During activation, the engine should respond with a noticeable drop in engine rpm (in V6 or V8 engines) or stall (in V4 engines) on an intake camshaft. If the solenoid fails to click, ensure that the wiring is OK and replace the solenoid. If the solenoid clicks but engine rpm do not change, suspect an OCV concern. Remove the OCV and retest, watching for movement from the spool valve. If the valve fails to move, replace the solenoid. If the solenoid moves, inspect the oil passages for debris and sludge. If any sludge is present, flush the engine with a high-quality engine flush and follow with an oil and filter change, which may help. But the condition may repeat, in which case it will require extensive repairs. In this situation, notify the customer of all possibilities before attempting a repair.

FIGURE 22-11 When checking the actuation of the VVT solenoid, check all the wires under a load to help determine whether one part of the circuit is failing when it is being operated. This test is a direction test in that it is not a definitive test but still helps to direct the technician to the right portion of the circuit.

At times, repeatedly cycling the solenoid (a test procedure that applies to any solenoid) to increase the stress on the solenoid windings may be required to replicate a failure. Verifying movement visually, with the solenoid removed while operating, can shorten diagnostic time, avoiding the need for more intrusive testing. Remember, regardless of where the solenoid is tested, it is duty-cycle controlled. Do not apply continuous power for more than a few seconds. Leaving constant power applied to the solenoid may cause it to overheat, damaging a perfectly good solenoid. Repeated and rapid cycling will not damage the solenoid.

If the solenoid fails the dynamic testing, it requires additional testing to isolate the control of the solenoid and actuator. Circuit system testing requires that the control circuits be properly loaded. Voltmeters check available voltage but fail to stress the circuit(s) when simply checking OCV. Resistance testing a component with an ohmmeter also fails to stress the wiring to the solenoid or the solenoid windings. Most manufacturers’ flow charts test for wiring faults with OCV testing or resistance testing. Ohmmeters and OCV testing normally fail to find unwanted resistance in a circuit.

To test the circuits and PCM with the solenoid disconnected, use the correct terminal probe and a scan tool to activate the solenoid control circuit. Using a scan tool, activate the solenoid either by increasing duty cycle to 100% or by turning an on/off system on. Install the test light at the solenoid connector across both pins, using the proper terminal adapters. When activated, the test light should illuminate. When using a test lamp, verify that the circuit’s intended resistance and that of the test light are close. Using a test light with the incorrect resistance will produce inaccurate results. Low voltage indicates an open circuit, high-circuit resistance, a short-to-ground concern in either the power or ground circuits, or a failure of the PCM. Performing individual circuit testing on both branches of the circuit will indicate which is at fault. To begin circuit testing, disconnect the solenoid and PCM, and load test the circuit(s) independently, applying power to one end of the circuit and the test light to the other. If the light fails to light, a circuit fault is present. If the test light illuminates normally, suspect a PCM concern and perform additional testing to verify before replacing the PCM (FIGURE 22-12).

FIGURE 22-12 To test the camshaft VVT solenoid, use a scan tool to actuate the solenoid. If the scan tool doesn’t allow for solenoid actuation pulsing, the solenoid may need to be manually activated to test the operation of the system.

Another option to test the circuit involves moving the test light between the control circuits by back probing directly at the PCM connector with the circuit activated. If the test light illuminates, this eliminates the PCM as the problem and indicates that a wiring concern is present. If the test light fails to light at the PCM or if both circuits test correctly when tested individually, suspect a PCM concern. Additional testing of the PCM’s power and ground circuits is required before replacing it.

If load testing is not an option or if it provides inaccurate results due to the test light selection for the circuit tested, perform voltage drop testing of the circuits during operation to verify proper operation. Voltage drop testing, similar to load testing, will find unwanted resistance in a circuit that normal OCV testing or resistance testing may not find, and it’s a dynamic test. Check voltage drop across the load or by testing the entire circuit. To check across the load, set the multimeter to DC volts then place the leads on both sides of the OCV, with the solenoid activated (current flowing) by using a scan tool at 100% duty cycle or fused jumper wires. The voltage dropped across the load (OCV) should be within 0.3 volt of available voltage to the load. Excessive voltage drop prevents the load from operating correctly. If the load is not accessible or if excessive voltage drop is present across the load, then voltage drop both OCV circuits. To test the circuits for excessive voltage drop, set the multimeter to DC volts and then connect the leads to two points in the circuit that have the same polarity. For example, to test the supply voltage, connect the negative lead of the multimeter to the positive battery cable, and connect the other lead to the supply-side voltage at the OCV connector (back probe at the connector or pierce connect the wire near the connector). Verify that the OCV is operating at 100% duty cycle by using a scan tool or fused jumper wires. Any voltage displayed on the multimeter indicates voltage drop: the amount of voltage not available to the OCV (load) for proper operation. Maximum voltage drop in a low-current circuit typically should not exceed 0.3 volt. To test the ground side, simply move the leads between the negative circuit at the connector and the negative battery terminal. Use the same 0.3-volt specification for pass/fail when checking voltage drop of the ground circuit.

Resistance Testing

As with any solenoid, an electromagnetic function must take place in order for a mechanical function to occur. This is important to understand to ensure that both functions (electric and mechanical) are tested on all solenoids. Failure to check proper operation of both the electric and the mechanical operation of a solenoid during diagnosis can lead to a failed repair. All electrical checks may pass resistance, scope waveform, current waveform, and parameter identification (PID) display test. However, debris in the solenoid may prevent the mechanical portion from operating correctly.

Resistance tests on a solenoid are a legitimate test and are typically found in manufacturer service information. Solenoids, however, may fail mechanically or electronically while still passing a resistance test. Ohmmeters fail to stress the windings and do not move the solenoid mechanically. To catch intermittent or hard-to-replicate concerns, perform a dynamic test. Dynamic testing (when solenoid operation is controlled with a scan tool) allows the entire control circuit and the solenoid to be accurately tested under normal operating conditions, when the fault occurs.

Most manufactures provide a resistance test of the solenoid in their service information. While it may be effective in finding severely shorted or open solenoid windings, it fails to test solenoid operation under normal operating conditions, when most parts fail. Simply passing a resistance test does not confirm that the solenoid will operate correctly mechanically or that it will operate correctly under temperature extremes or after multiple cycles.

To test the solenoid with an ohmmeter, disconnect the solenoid, removing it from the circuit—as always with ohmmeter testing, Verify lead and ohmmeter operation by touching the test leads together and noting the display on the digital multimeter (DMM); this verifies lead and meter operation. Zero the ohmmeter to remove the lead resistance. Touch one lead to each terminal of the solenoid (FIGURE 22-13). Refer to service information for the correct resistance reading. At room temperature, the majority of solenoids are 8–15 ohms (Ω). Remember that resistance changes with the temperature of the solenoid windings. As temperature increases, resistance increases.

FIGURE 22-13 Using an ohmmeter to test the VVT solenoid will work to determine whether there is an open in the solenoid. This test is not a definitive test, because it doesn’t test the solenoid in operational mode.

Testing Operating Voltage

Testing a duty-cycle-controlled circuit for available voltage with a digital volt-ohmmeter (DVOM) requires an understanding of duty-cycle control and its effects on voltage and on DVOM operation. During operation, the PCM supplies either power or ground for the solenoid, causing current to flow. Remember that when current is flowing, work is being performed; if no current is flowing, the circuit is not active, and thus work is not being performed. Although there are better methods for diagnosing VVT systems, technicians still need a quick overview of operating voltage checks.

When checking voltage on a duty-cycle-controlled component, the DVOM displays an average of the available voltage (FIGURE 22-14). Circuit activation time (time during which the circuit is on) will affect the voltage displayed on the multimeter. As duty cycle increases, noted by a higher number (increased on time), the average voltage displayed on the DVOM increases. Using a DVOM to read a voltage on any duty-cycle-controlled component requires some math, an understanding of how a duty-cycle signal works, and knowing where to install the DVOM in relation to circuit construction.

FIGURE 22-14 When back probing the power side of a duty-cycle-controlled VVT solenoid with a duty-cycle-controlled solenoid, the power that is displayed is relative to the on time of the component. Duty cycle quickly cycles the power or ground to increase the longevity of the electrical component.

When the system is operational, the PCM either applies power (high-side controlled) or pulls one circuit to ground (ground side controlled). On a ground-side-controlled system, when checking supply voltage, it should show ±0.2 volt from system voltage. Any voltage drop that is higher that this indicates excessive circuit resistance and requires additional testing to locate the cause. Typically, expect the ground side to show 0 volt on an on/off solenoid because the load uses all the voltage available to perform its function. When a component is duty-cycle controlled and a DVOM is used, this is not the case. Remember duty-cycle control varies the amount of on time compared to the amount of off time, and DVOMs average cycling signals. The voltage measured by the DVOM varies based on supply voltage and the amount of time the circuit is on. The longer the circuit is on, the higher the voltage measured by the meter. Increasing supply-side voltage, from battery voltage to charging system voltage will also increase the voltage measured.

In the example that follows, the ground side is being cycled on and off (duty-cycle) by the PCM. The DVOM averages the amount of on time compared to the amount of off time. The voltage measured by the DVOM varies based on supply voltage and the amount of time the circuit is on. The longer the circuit is on the higher the voltage that measured by the meter. If reading voltage on a ground- side controlled circuit, with a duty cycle of 50% and 14 volts are available, the DVOM would read close to 7 volts at the ground side of the connector, since the component is equally on and off. If the ground-side duty cycle was 25% and the same 14 volts were available, the DVOM would indicate approximately 9 volts as the solenoid would be on 75% of the time.

If the technician is ever unsure about which side is controlled by the PCM, they should refer to the system wiring schematic. The wire leading to the PCM is the control side. If the component receives its power from a fuse or ignition feed and the ground wire attaches to the PCM, the circuit is ground-side controlled. If the routing of the ground wire is directly to ground and the voltage supply wire is from the PCM, the circuit is high-side controlled. If both wires connect to the PCM, manufacturers typically control the high side. A connector pinout may also provide this information when labeled. This is important when reading voltage or duty cycle.

If the system is ground-side controlled by the PCM, the following holds true. During operation, when testing voltage supply at the connector, the available system voltage—as with all circuits and connectors—should be within ±0.3 volt. If the voltage available is less than the specification given, the next step is to find the excessive resistance in the voltage supply circuit. When back probing, to check voltage at the ground side of the connector, instead of 0 volts being found, as would be normally expected on a completed circuit, it should read 6 volts due to the cycling on and off of the ground by the PCM (duty cycle). The DVOM averages the voltage of the cycling signals between on and off. The longer the circuit is on (higher duty cycle), the higher the average voltage reading displayed on the DVOM. As duty cycle decreases, the on time of the circuit also decreases and the voltage read by the DVOM will be lower.

For example, for a vehicle operating at 14 volts with a duty-cycle command of 50%, the DVOM will display approximately 7 volts since the circuit is on half the time. With a 75% on-time command (off 25% of the time), the voltage displayed would be 10.5 volts (14 V × 75% = 10.5 V). Checking the available operating voltage before testing the duty cycle of the OCV will help determine what the voltage range should be.

Scope Testing

The variable camshaft timing (VCT) solenoid is an actuator commonly supplied with system voltage and a ground provided by the PCM. Remember that either the system voltage or the ground can be duty cycled, and either can change depending on the OEM’s design of the system. Knowing how the system is controlled is important when selecting the scope settings.

When using a scope to diagnose, a valid known-good waveform is required. Several options are available to technicians who lack experience or do not have access to a known-good waveform. Some manufacturers will supply known-good waveform traces for comparison in their service information. The International Automotive Technicians Network (IATN) is another good resource. If the vehicle at hand has multiple camshafts that are controlled, the duty cycles of both camshaft actuators can be compared with each other and against the manufacturer’s information. When testing, manipulating the engine load may be required to load the engine enough to activate the duty cycle of the exhaust actuator (FIGURE 22-15). Power braking or driving under a steady throttle at 30–45 mph (48–72 kph) with a warm engine and under low load changes the engine operating parameters and EGR conditions.

FIGURE 22-15 Using a digital oscilloscope to graph the operation of the oil control solenoid can help diagnose an intermittent failure that can cause a performance issue.

Component Testing: Oil Control Solenoid

The following subsections describe the tests a technician should perform on an oil control solenoid.

Visual Test

Remove the camshaft solenoid and inspect for torn, missing, or restricted screens. Inspect the lands of the actuator for damage, including nicks and scoring; if present, replace the solenoid. Inspect the solenoid connector end for visible signs of oil seepage; if present, replace the solenoid and clean the oil from the wiring harness connector (FIGURE 22-16).

FIGURE 22-16 A solenoid that is allowing oil pressure to permeate the internal windings and through the wiring connector. This will not allow the solenoid to operate correctly, causing a malfunction indicator lamp (MIL) to illuminate and running issues.

Swap Test

Another testing option is swapping the solenoids from bank to bank, known as a swap test, if the solenoids and connectors are accessible. On a V-type engine, swap the solenoids from bank to bank and determine whether the fault follows the solenoid or remains with the camshaft. If only one solenoid is present on the engine, install a known-good solenoid and retest. Swap testing checks the mechanical operation of the solenoid after the electrical control system has been tested and passes those tests.