2008 Mazda CX-9 Auto Slipping and Shudder

If you shift in manual mode, you are asking the transmission to work more like a manual transmission than an automatic. In other words, when you pick 1st, 2nd, 3rd, etc., you are eliminating the torque converter’s spinup of the impeller blades. In turn, these blades begin to move the transmission fluid through the entire transmission.

Normally, the transmission fluid whooshes its way through the gears as the changing gear/clutch/band sets engage and disengage in response to valve body inputs as well as solenoid actions. However, when you are in manual gear, you essentially lock up each gear until you hit the next one. In this manner, there is no slippage — except for minor mechanical and built-in fluidic losses that will occur anyway. Instead, there is a hard connection. The proper settings are forced and the transmission works correctly.

What I think is happening in your transmission — you have pretty much confirmed it with your detailed description — is that your Mazda is down a bit on transmission fluid. What happens is that the level is just high enough to keep things working but not without problems. In this case, the transmission has to work harder to provide enough pressure to make sure that the transmission parts work correctly. And, since there are times when there just may not be enough fluid available, as in a turn when you are accelerating, then the gears/clutches/bands that were engaged correctly, lose their connection because the fluid level drops too far.

Check the tranny fluid level first. I think you’ll find the transmission is down about a quart.

When you drained out the fluid, did you remember to drop the pan cover? If not, you likely have ended up with more fluid in the transmission, probably a quart or so. If you don’t remove the tranny cover to take out the bit of fluid — actually a quart or so — it remains locked up and when you refill it you automatically end up with more fluid. Check the level again as soon as you can and you may find this is the case.

Are you also saying that the transmission is performing as it did with old fluid/filter? I think your analysis of the transmission’s performance was done before the change out, right? If so, how is it performing with the changed fluid/filter?

Leaving aside the other mechanical components in the transmission, for now, your next step should be to a transmission shop with OBD-II diagnostic scanner. Have the shop guys perform a check of your car’s engine control system to see if any codes have been thrown and collected by the system memory. At this level, you will find any error codes that may have been generated by a failed electronic or electrical part, or even a mechanical part, if the software covers it, is quite general. However, it will point you in the direction of the issue. You may possibly be able to fine-tune it by checking the transmission control module, looking for transmission-specific error codes.

Until I have a bit more information in hand, I can only speculate the you are likely looking at a key sensor failure such as the Mass AirFlow (MAF) sensor or the Mass Air Pressure sensor. If either one goes down, your transmission could easily perform in the manner you suggest. The same is true if one of the transmission solenoids (they are located inside and on the valve body) as they determine which gear your vehicle is shifting in to.

You have just pointed out something that is very common, one problem that seems like another. For example, let’s say that when your car accelerates, there comes a point when it seems like the automatic transmission starts to slip. Indeed, it might even refuse to downshift at all when you put your foot into the accelerator to make it happen. It sees clear, at this point, there must be a transmission problem, right? It’s wrong, honestly.

As you pointed out, the slipping scenario can make you look in one direction under the hood, when you should be looking at the most obvious points of failure, especially if you pull the error codes and there’s nothing to indicate it could be a P0715-style error (0715 is a generic transmission failure code) then you have to look at mechanical failures like the plugs or perhaps the timing chain or even a sensor that is monitored by more than just the transmission.