A great retired instructor, Dr. David Gilbert, who taught alongside me here at SIU, frequently said, “If you want to understand how to diagnose a system, you first need to understand how that system works and what normal looks like.” I often reflect back on that phrase when I come across new technology, and especially when I have questions. As an instructor, I have the luxury of spending time (and a few resources) digging into those answers, and I’ve learned quite a bit throughout that process. There’s only so much information you can gather from service information, and sometimes, perplexing questions are left unanswered. I’m grateful to ATRA and Gears Magazine for allowing me to share some of the findings of my exploratory research while I attempt to answer those questions.
In this article, I’m going to share some findings on how the shift is controlled in the GM 10L transmission. Hopefully, this September, you are planning to attend the ATRA Powertrain Expo in San Antonio, where this topic will be discussed in greater detail. I’m going to dive deep into how these modern transmissions control shift feel, and how, through the use of tuning tools, some of those controls are adjustable. This topic isn’t new. Maybe you read my article published in the Oct/Nov 2024 issue of Gears, where I did similar experiments with the GM 6L transmission. This article builds on that research by providing insight into the GM 10-speed, and you’ll find some distinct similarities and a few differences between the 6-speed and the 10-speed.
EXPERIMENT
Like the experiments performed on the 6L80 in the 2024 Shift Anatomy article, I outfitted a 10L80 transmission (Figure 1) with eight 500 psi pressure transducers in order to monitor actual clutch pressures. From the factory, the only pressure tap that’s available is line pressure, so plumbing pressure sensors at the clutch passages provide insight into actual clutch pressures. This setup also allows me to monitor the rate of pressure change and clutch overlap timing with an 8-channel scope.
By doing these measurements, we can gain a better understanding of how the transmission is electronically and hydraulically controlled. Also, if you dabble in transmission tuning, these pressure measurements allow us to see exactly what is changing when modifying tables and pressures with tuning software. Spoiler alert, many modifications had absolutely no effect on the transmission either electronically or hydraulically. I will delve into these experiments in depth at the ATRA Powertrain Expo in September.
SHIFT STAGES
In general, a clutch-to-clutch ratio change takes place in about 125 to 500 milliseconds. There are four stages to the shift, and in total, the complete shift sequence takes about 1 to 2 seconds. The 2024 article explains a bit more in-depth about the four stages of the shift, but for a quick refresher, here is what happens during a normal upshift where the TCM is controlling the release of a clutch and the application of a clutch (synchronous shift). Refer to Figure 2, which shows an upshift from 8th to 9th gear at 30% throttle. The left side of the Figure shows scan data from the HPTuners scanner, and the right side shows pressure measured at the clutches. The labeling depicts the four stages to the shift: (A) = Fill Stage, (B) = Torque Stage, (C) = Inertia Stage, and (D) = the Final Stage.
Stage One (A) – the Fill Stage: As its name implies, the fill stage delivers fluid pressure to the clutch housing and strokes the apply piston enough to compress the release spring. This brings the clutch to its “kiss point,” where the clutch clearance is taken up, but there’s no torque transfer. At the same time, the releasing clutch pressure drops, preparing for the next stage.
Stage Two (B) – the Torque Stage: The torque stage is where the releasing and applying clutches “hand off” their roles. There is no engine RPM drop yet; therefore, there is no ratio change during this portion of the shift. The purpose of this stage is to transfer responsibility from the releasing clutch to the applying clutch.
Stage Three (C) – Inertia Stage: During the upshift, this is where the intended ratio change and engine RPM drop occur, and the amount of time this RPM drops is reported on the scan tool as “shift time.” During the inertia stage, the releasing clutch has been released, and the applying clutch is pulling the engine speed down. The TCM works with the ECM to reduce engine torque to prevent excessive clutch slip. You can see from the scan data, that the engine RPM drops during this phase. If you observe the shift speed and gear ratio PIDS, you will see that they are “live” monitors of the process and that the shift speed reports a final result after the gear ratio PID settles in the upshifted gear ratio.
Stage Four (D) – The Final Stage: Once the shift is complete, the TCM commands the solenoid to deliver line pressure to the clutch to prevent it from slipping under full torque. Full engine torque is restored after the ratio change has taken place.
Many manufacturers use these four stages when controlling shifts. It’s not widely discussed because we, as technicians, don’t have access to clutch pressures on most transmissions, and most technicians don’t have pressure transducers and oscilloscopes to monitor multiple pressures and graph them out. So I’m hoping these articles shed a little light on what’s going on in the background when the 10-speed transmission completes nine upshifts when accelerating from a stop to highway speeds. One item of note is that with fast scan data and a normally operating transmission, by monitoring solenoid amperages or pressure commands and overlaying them with engine speed and gear ratio, you can gain a pretty good representation of what’s happening hydraulically in the transmission. 
The pressure won’t be verified, but the pressure control sequence is almost a perfect match when comparing the measured pressures to the displayed solenoid command. Check out the solenoid command in Figure 2, and you’ll see a pretty similar reaction when comparing it to the Pico capture. But please realize the limitation – the scan tool is displaying a “command,” and that command is not verified through hydraulic measurement. If the transmission has issues, this command might not be a fair representation of the hydraulic control.
ELECTRONIC AND HYDRAULIC CONTROL
These transmissions are busy shifters. Take a look at the range reference chart in Figure 3, and see how busy the C, D, and E clutches are. That’s a lot of shifting! Unlike the Ford 10-speed, the GM transmission doesn’t make frequent use of “skip shifts”. The ratio change is very minimal; notice how the ratio spread is very little, especially between the 9th and 10th gears. 
The ratio spread indicates how much RPM change will occur between gears. The lower the number, the less change.
With ten speeds, the transmission always seems to be shifting, and at the heart of all those shifts is the valve body. This valve body is relatively simple, with a PWM linear solenoid controlling valves that direct pressure to clutches. The layout is simplistic, but complexity exists through the software and electronic control. A linear solenoid physically actuates a valve and does not use hydraulic control to manipulate valve position. The linear solenoid has a very sensitive electronic control strategy. It still uses PWM to control the position of the solenoid pintel, which only moves about 0.098” (Figure 4), but it takes very little PWM change to adjust pressure. The TCM pulses the solenoids at 3140 Hz and changes the duty cycle to achieve the desired pintel position and ultimately clutch pressure. 
Refer to Figure 5, which shows a zoomed capture of the D and E solenoid control during a 4-5 upshift. During this portion, the D solenoid (gold) is starting to reduce pressure, and the E solenoid (purple) is beginning the “fill” stage of the E clutch. This capture is only 30 milliseconds long, so it’s only a brief glimpse of what’s going on during the very beginning of the shift. When the voltage is high, the solenoid is deenergized, because the voltage measurement is taken on the ground side of the solenoid, which the TCM controls. It might seem backward, but the more the pattern shows low voltage, the more the solenoid is energized.
Referring to Figure 6, during this Fill stage, the duty cycle was about 36% energized. The frequency never changed, but the duty cycle increased to about 60% energized at the end of the Final stage.
To show how incredibly responsive these solenoids are, the scope pattern on the right side of Figure 6 shows short bursts of pulses from the TCM controlling clutch E (purple/bottom trace), and you can actually see the ripples from those pulses in the pressure measurements on the left scope pattern (purple applying clutch). The vertical time cursors on both scope Figures show the pulses are separated by about 25 milliseconds, proving that the electronic control is causing the ripple in the pressure measurement.
There are no pressure sensors in the 10L transmission, so the TCM relies on the programmed solenoid characteristics and the feedback information from the speed sensors for proper shifting. The electronic control is very advanced, and it’s apparent how much this transmission is a software-dependent unit. 
As you could imagine, proper solenoid operation is critical, and because of that, these solenoids have an etched code (Figure 7) that indicates how they are calibrated. Please keep the solenoids in their correct position when disassembling. Ford discusses this in their service info, but GM doesn’t mention it. But then again, GM doesn’t have its technicians disassemble the VB. If you refer to the GM service info, you’ll see that the repair information only includes removing and installing the valve body, and it doesn’t have any disassembly instructions or diagrams. It’s at this point that we must thank all the aftermarket suppliers, diagnostic support, and training providers for picking up where the OE left off. There’s plenty of great aftermarket service info regarding the multiple generations of the GM 10L valve body.
10L TORQUE MANAGEMENT
Torque management is vital! If you remember from the 2024 Shift Anatomy article, the 6L transmission reduced torque so much during the shift that there was a complete torque reversal, meaning that the calculated engine torque went negative, and the driveline was driving the engine during the shift. This is similar to a manual transmission, where the driver fully releases the throttle when shifting to remove all engine torque from the driveline. On the 10L, torque reduction is active, but not quite to the level of the 6L. The 6L, at times, reduced the throttle during the shift, but it always reduced ignition timing. During my experiments, the 10L never reduced throttle, and it only reduced timing. 
In fact, the throttle is very active during the shifts, even when the accelerator is held steady. Refer to Figure 8, which shows that the ECM actually increases the throttle a few degrees during many shifts, and it increased the throttle from 50 to 100% just before the 1-2 and 2-3 shifts while steadily accelerating with 50% throttle! This is all in the name of shift feel and clutch durability. The TCM uses input from its four speed sensors to speed match the clutch housings to achieve a quality shift. The speed sensors not only measure shaft and housing speed, but they can also determine rotational direction!
DYNAMIC FUEL MANAGEMENT
Here’s one final observation to close this article that you might find interesting – on deceleration, after a 20-40% acceleration, the ECM commands ALL of the engine valves OFF to limit engine braking. This allows the vehicle to coast for a long time in an effort to improve fuel economy. On the dyno, this was a bit annoying because I typically try to avoid using the brakes as much as possible while coasting down, so it took forever for the driveline to slow down to 30 mph, which is where I gingerly apply the brakes to get the dyno rollers to stop. 
We all have our opinions on GM’s Active (or in this case, Dynamic) Fuel Management System, but now you know that this system isn’t only activated during cruise. Look at the scan data in Figure 9, where you can see during this deceleration event that the throttle is closed, there is no intake manifold vacuum, and there is no airflow entering the engine. Right at about 15 MPH, the valves reactivate, and you can hear the sound in the engine (it sounds like a slight spark knock) when everything kicks back into operation. Please try to attend my seminar on Friday, September 5th, at 10:30 am, where I’ll go deep into the results of my HP Tuning experiments and cover more interesting features of a modern shift. The Powertrain Expo is always a great time, and I think this year is going to be one to remember!
