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I recently had one of the camshaft position actuators on my '07 Pontiac G6 go bad. The symptom was a sort of "hard-shifting" that made me think something was wrong with the transmission. Luckily, it's a common problem with an easy solution that I was able to do myself.

My only question is Why would hard shifting be the symptom of non-ideal camshaft operation?? Obviously, this is why we have OBDII codes :), but there has to be some intuitive understanding of what's going on there...

Ready for my powertrain lesson...

  • Just to be clear; I take it's an automatic gearbox, or am I wrong? – Allman Sep 9 '15 at 11:03
  • That's correct. – mgalgs Sep 9 '15 at 15:23
  • Exactly, my answer is based on that premise :) – Ziezi Sep 9 '15 at 16:42
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Unfortunately there is no intuitive understanding. The general rule of thumb is always fix any engine problems before blaming the transmission. The sensor data that the transmission uses comes from the engine. This includes but not limited to load, temperature, throttle position, etc... If this sensor data misleads the transmission in it's calculations then strange shifting problems are to come.

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  • I think you're on the right track. If the sensor is lazy or accumulates "drift" the ECU might miscalculate the load and shift gears when things aren't so ideal. – Zaid Sep 9 '15 at 13:16
  • @vini_i there is no such thing as misleading signals: if the values of the signal are not in the established interval, then the device depending on them works on default values. An example would be: if the lambda sensor is not sending signal reflecting the combustion gases and indirectly inferring to the richness of the air-fuel ration, that doesn't mean that the ECU will be mislead, it will just start working on a default value (not taking into consideration the current conditions). – Ziezi Sep 9 '15 at 16:18
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    @simplicis veritatis a signal can easily be misleading if the signal is just off but never leaves the established interval. For example the Mazda CX7 had a problem with a wide band oxygen sensor (lamda). When a bad sensor got too hot the reading from it started to swing dramatically causing the vehicle to surge at cruising speed. The vehicle never set a code because the swing never left the established interval and the ECU adjusted for the reading in real time causing the surge. – vini_i Sep 9 '15 at 16:36
  • @vini_i I got your point, yes, you're right. I guess, I was referring more to the general case. – Ziezi Sep 9 '15 at 16:40
  • @simplicis veritatis in the case for this vehicle it is possible that the bad camshaft position actuator caused the load of the engine not to be calculated correctly. GM is known for using the MAP sensor for calculating load. If the engine vacuum is not correct the it's possible the ECU misinterprets the MAP and calculates the load incorrectly. If the load is too high for example the transmission will most likely shift harder then normal. – vini_i Sep 9 '15 at 16:45
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The most logical thing would be that the signals from the camshaft and crankshaft position actuators are used to synchronize the engine RPM with the gearbox, more specifically the throttle position. Thus, when you don't have synchronization it should be more difficult to shift, although not impossible.

As a matter of fact in the old trucks you need to push the clutch once to release the gear, then release it and push the throttle to increase RPM, so that it matches the RPM of the previous gear (which are higher), known as double-clutching.

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