Other Articles - July - 2021

Well Shaken – Transmission Shudder that is not Transmission Shudder

We’re all aware that the complexity of today’s vehicles is continuing to accelerate. The problems that we see today may look like the issues of yesterday, but the solution to those issues may be quite different. Technician frustration with drivability issues that appear to the customer to be related to the transmission is nothing new.

In the 1990’s we saw a major uptick in the number of issues that seemed to be related to the transmission but ultimately had nothing to do with the transmission. An example of this was the advent of the high voltage switch (HVS), which replaced the old HEI distributor on GM vehicles. Most people thought it was a distributor, but it was nothing more than a rotor/cap that provided a spark to the wrong cylinders. The only electronics in the HVS was a camshaft position sensor. Rotating the HVS position did not affect ignition timing as it was not a distributor. When the system first came out, we had a lot of issues with dealer techs not understanding the function of the HVS. As you rotated the HVS, a couple of things happened: you retarded or advanced the cam signal that the ECM received, and you changed the position of the rotor in relation to the cap\plug wire terminals. The cam signal is, of course, used as an input for the ignition system and fuel delivery. We told the technicians to install the HVS and rotate it until the “Cam Offset” value on their scan read 0 degrees. At that point, the HVS should be locked into position.

At that time, we had engine misfire issues that techs were having a hard time nailing down. We discovered that if the cam sensor position was not correct, misfire/shudder/surging could occur. I remember getting into a long discussion with a GM design/ release engineer for the HVS regarding misfire/shudder/surge caused by an incorrectly positioned HVS. I was trying to get a bulletin written on the issue, but he would have no part of it. He indicated that what I was proposing was impossible. We left our discussion with a challenge from me for him to set the cam offset at 8 degrees in one of the proving ground vehicles and take it for a drive. The next day I received a call, and sure enough, he found the shudder I was telling him about.

A few years ago, variable valve timing came out (which we’ve covered at multiple ATRA seminars), and we saw that a faulty camshaft actuator will cause issues with transmission operation. Today these systems are still causing problems, but it may not be the camshaft actuator. In the past few months, I have seen some 5.3L applications that exhibit shudder-related issues, but the issue was not the transmission nor the cam actuator but rather the cam position sensor.

The camshaft position sensor is a 3-wire hall-effect design that provides a digital signal to the ECM based on the rotation and position of the camshaft (figure 1). The camshaft sensor is mounted in the front of the block on the V-6/V8 GM applications and utilizes a trigger wheel, very much like a typical vehicle speed sensor. The sensor’s tonewheel has wide and narrow slots, so the duty cycle of the signal output to the ECM will vary based on the position of the camshaft. Comparing the camshaft position sensor signal to the crankshaft position signal allows the ECM to determine the exact position of the valve train and the location of each piston in their respective cylinders. This information determines when the fuel should be injected and when ignition should occur for each cylinder.

And like vehicles equipped with an HVS, the newer design engines are suspectable to an incorrect cam position signal. For example, if the ECM thinks the camshaft position is in one position, but it’s actually in another, then the spark timing and fuel delivery calculations will be incorrect.

Since the camshaft sensor is an electronic signal generator, the sensor can output a good quality signal, but that signal can occur at the wrong time. This can cause the ECM to miscalculate the actual camshaft position leading to the surge/shudder/ miss. One would think that an issue like this would set a DTC P0340 (Cam sensor circuit) or a P0341 (Cam sensor performance), but they usually don’t. A P0340 code sets if an electrical issue (such as open or short to ground) occurs. A P0341 code sets if the ECM sees too few or too many pulses from the sensor within a specific number of crankshaft rotations. Since our issue is that the delivery timing of the signal is shifted, you will likely not get a DTC.

The diagnosis of this issue is pretty straightforward. Just like when you deal with the cam sensor on an HVS application, your scan tool will provide you with a parameter displayed as “Cam Position Variance” to help you diagnose the issue. Monitor the cam position variance value on your scan tool and notice the variance range. It should be 0-4 degrees. If you are outside these values, there is a very good chance that the sensor is faulty or there is a problem with the trigger wheel assembly (figure 2).

As you can see, a good scanner and some basic knowledge of how the vehicle is supposed to work are your friends. Until next time remember, “A dream without ambition is like a car without gas. It’s not going anywhere.”