Other Articles - June - 2022

Shake Me Up – Shudder Complaints Due to Variable Valve Timing Issues

This week, I spent time in 15 different shops in the Atlanta area. Without exception, the shops were packed with work, and the mix of vehicle/transmission types that were in for repair mainly were newer trucks and cars varying from 1 to 8 years old. For the most part, gone were the 4L60E and the 4R70, which were the staples of the industry for decades. Instead, shops were working on 6L80s, 9T50s, 10R60s, 10R140s, 8F35s, and 8L90s. The presence of these new units sparked numerous discussions with the technicians at the various shops, especially regarding diagnostic procedures. They also discussed the issues they were experiencing with these units coming in the door. Like the earlier transmission models, everyone has experienced a few burnt clutches, but more common than transmission failures, customer complaints regarding transmission operation were even more prevalent. Concerns such as shift flares, hard shifts, and TCC shudder were concerns that many of the shops had experienced.

As we have discussed at the ATRA seminars for years, a transmission is no longer a transmission. Instead, it is part of a complex power train system that works hand in hand with other systems on the vehicle. With this in mind, you need to understand what those systems are and how they function and interface with transmission operation for success in this profession. Today, we will look at the Variable Valve Timing system issues.

Variable Valve Timing (VVT) has been around for decades. Every manufacturer has engines equipped with some mechanism that repositions the camshaft to allow it to be advanced and retarded depending on the engine’s needs. The systems improve fuel economy as well as reduce emissions. GM equips many of its engines with a VVT feature like other companies. GM V6 or V8 gasoline engines started introducing VVT systems in the early 2000s.

Engines such as the 4.3L (LV3), 5.3L (L82 or L84), or the 6.2L (L87) are all equipped with VVT.

These engine applications are equipped with a VVT system, which utilizes a camshaft actuator and camshaft position sensor. The ECM controls the cam position via a PWM-controlled camshaft actuator solenoid. The solenoid controls engine oil pressure fed to one of two passages, cam advance or cam retard. The control oil pressure changes the camshaft position relative to the crankshaft position through a cam gear actuator. This camshaft-driven gear controls the advancement and retard of the camshaft timing. The gear can move clockwise or counterclockwise relative to the crankshaft.

The camshaft position sensor monitors the camshaft position. By correlating the camshaft and the crankshaft position, the ECM can determine the exact position of both. Based on the ECM command, the camshaft will advance or retard its position when the system is operating correctly. The ECM monitors the variance in the camshaft position (CMP) actuator system by monitoring the desired position versus the actual position of the camshaft through a 4X signal generated by the cam sensor.

Excessive variance in desired versus actual camshaft position causes a surge/shudder condition that may be misinterpreted as a transmission-related shudder issue. The typical school of thought is that if something is to go wrong, you should have a DTC set for the fault. Although most manufacturers have electrical DTCs for the systems, some manufacturers may/ may not have DTCs related to mechanical issues with the system. This concern has fooled many technicians working on 6L, 8L, and 10L applications as it mimics a TCC shudder condition.

Using your scan tool, monitor the variance value via engine data when the condition occurs (figure 1). Engineering indicates that a 4° or more variance may cause a surge/shudder-related issue. Variance should be 0°, but 0° – 3° is common. The vast majority of engines I have looked at will have the variance running typically 0, 1, or 2 degrees depending on RPM and load.

Fuel and spark delivery will be affected enough to cause issues if the variance gets too high. Therefore, engineering recommends replacing the camshaft sensor magnet if the variance is too great. GM calls this component a magnet, which most refer to as the cam sensor. The magnet is integral to the sensor and cannot be serviced separately. The cam sensor is typically mounted on the engine timing chain cover (figure 2). The actuator is part of the cam-driven gear.

Keep in mind that this system uses engine oil to control the cam actuator. If a customer fails to service their engine oil properly, sludge can develop, leading to cam actuator issues (figure 3). Remember proper viscosity of oil, oil level, and oil pressure are essential to the proper operation of these systems. Running the wrong viscosity oil in an engine today can cause a lot of concerns that, frankly, you do not want to face.

Issues with the sensor or the actuator may cause shudder/surge-related issues that may be misdiagnosed as transmission issues. In addition, the condition may worsen with TCC applied and/or when the system is running in reduced cylinder modes.

So, the next time you get a transmission issue that has you scratching your head, it just may be something outside of the transmission that may be causing it. Until next time remember, “Some people dream of success while others wake up and work.”