All multi-speed transmissions have some form of hydraulic control for shift quality optimization. One would think that the emergence of 10-speed transmissions would result in extremely complicated valve bodies and hydraulic control, but in a way, the opposite has happened. As a quick comparison, the once popular General Motors 440T4 (4T60) 4-speed transmission was hydraulically operated and contained over 25 valves to control shift timing and shift quality of those three upshifts, plus the application of the torque converter clutch. By contrast, the modern General Motors 10L80 transmission has only 17 valves controlling the timing and quality of the nine potential upshifts, and TCC apply. How is this possible? In a nutshell, the complicated 4-speed valve body was replaced by electronics, actuators, and software, which made for a simpler valve body and hydraulic control. The sophistication transitioned from the old-school features of the hydraulic system, such as accumulators, bias valves, and calibrated springs and orifices, to the high-tech and adaptable electronic control, processors, and software calibrations.
HOW COMPLICATED IS AN UPSHIFT?
Have you ever noticed the data PIDs for “shift time” on a scan tool? It’s common for shift duration to last between 0.125 and 0.5 seconds. This duration is pretty standard whether the shift is at light throttle or wide open throttle. For example, at light throttle and low vehicle speeds, the engine RPM change during a shift is relatively small. As you can see in Figure 1, the light throttle 2-3 shift on this 2022 10L80 equipped truck happens at 17 mph and only changes the engine speed from 1790 rpm to around 1420 rpm – a 370 RPM drop. Meanwhile, that same shift at WOT occurs at 53 mph, and the engine speed drops from 5570 RPM down to 4350 RPM, a 1220 RPM drop as shown in Figure 2. The light throttle shift that lasted 0.300 seconds is smooth, and the WOT shift that took 0.375 seconds is firm because of the difference in engine RPM change that occurs during that fraction of a second. Also, keep in mind that this example is a 10-speed transmission. A four or five-speed transmission would have even greater engine RPM drops during each shift.
Controlling clutch application and release during the shift by modifying hydraulic pressure to a piston ultimately determines the quality of the shift. The manufacturer controls the pressure in a specific manner to achieve desirable, repeatable shifts. Not surprisingly, there are patents related to this shift control strategy. Searching patents. google.com for US 2006/0135316a1; US 7,912,617 B2; US 7,314,128 B2; US 8,364,361 B2 from GM and Ford will return information on the shift control that’s likely used in their modern transmissions.
I first read these patents when experimenting with HPTuners on the GM 6L80 transmission. I was intrigued enough to outfit a 6L80-equipped truck with pressure transducers that directly measured the clutch pressures. I used a scope to outline the actual clutch pressures while operating the vehicle on a chassis dyno under various conditions and calibrations. In this article, I’m primarily focusing on the segments that make up a shift. But the experiment provided great insight into what HPTuners actually change (or doesn’t change) in transmission operation when modifying calibrations and tables. The results from the scope were equally as eye-opening as a learning tool and made me further appreciate what goes into controlling a shift.
PHASES OF A SHIFT
A modern electronically controlled shift will include these four stages or phases during the shifting process. These terms are generic, and manufacturers might use different terms, but they all relate to the same function in the shift process, as shown in Figure 3.
Fill stage (A): The shift sequence starts with a fill stage where the computer commands a solenoid to supply pressure to an applying clutch while simultaneously commanding another solenoid to reduce pressure at the releasing clutch. The applying pressure is typically enough to stroke the clutch’s hydraulic piston to the point where the piston takes up all the clutch pack clearance but doesn’t start to apply the clutch. In Figure 3, which shows a 2-3 upshift on a 6L80, the fill stage is shown in segment A. The green trace (3/5/R clutch) builds pressure to about 38 psi and lasts about 275 ms. That length of time is pretty standard across all clutches and all driving conditions and doesn’t relate to the actual shift time. This is a calibrated pressure and time that an engineer determined this clutch requires to fill and take up clutch pack clearance. Expect extra tight or loose clutch pack clearances to cause issues during this portion of the shift process.
The 2-6 clutch is the releasing clutch during the 2-3 upshift, as shown by the gold trace. Notice how, during the fill stage, the releasing clutch pressure dropped to about 70 psi to prepare for the disengagement of the clutch.
Torque stage (B): After the hydraulic piston has taken up the clutch clearance during the fill stage, the control system will command the applying shift solenoid to increase hydraulic pressure at the applying clutch and strategically command the releasing solenoid to reduce pressure at the releasing clutch. On synchronous shifts (a clutch is released as a clutch is applied), the releasing clutch will drop the pressure to fully release the disengaging clutch. During the torque stage, there is no ratio change, but the applying and releasing clutch continues to transfer torque between their respective hub and the drum. Some literature identifies a “torque hole” during the end of this phase where transmission output torque drops as a result of gear ratio tie-up. In Figure 3, the torque phase is identified in segment B. The gold trace shows the pressure of the releasing 2-6 clutch dropping, and the green trace shows 3/5/R clutch pressure raising. Once the torque stage is complete, the releasing clutch has handed off torque transfer to the applying clutch. As mentioned, during this phase, there’s no ratio change; it is just a transfer of torque from the releasing to the applying clutch. Similar to the fill stage, the torque stage doesn’t relate to the actual shift time because, at this point, there hasn’t been a ratio change or an engine RPM drop.
Inertia stage (C): The inertia stage is where the ratio change occurs, and the engine RPM drops during an upshift. If there is a releasing clutch, it’s completely released during this stage. The inertia stage dictates the shift timing as measured by the engine speed drop. In Figure 3, you can see that the complete shift pressure control from start to finish lasts about 1.2 seconds, but the inertia stage identified in segment C is only about 0.3 seconds. Does that sound familiar? That’s the actual shift time where the ratio changed!
Final stage (D): The final stage occurs after the shift and ratio change is complete. The torque and inertia pressures are typically much lower than line pressure, but after the shift, it’s common for pressure to increase to ensure the clutches do not slip. In Figure 3, the pressure jumps to 180 psi during segment D. Notice how pressure is much higher after the shift than during the shift. I find it interesting that this shift, at WOT, uses only 65 psi (or so) to complete the ratio change and then jumps to almost 3x that pressure to ensure it holds. Keep this in mind because it directly relates to the torque management system covered later in this article.
SCAN DATA
Most transmissions do not give you the ability to monitor actual clutch pressures. The scan data might have solenoid pressure PIDs, but those are calculated and not verified. I have the privilege of using donated vehicles for these experiments, so I can drill holes in the valve body or a transmission case to monitor actual pressures and hope for the best. So, you might ask how the actual pressure results align with the vehicle’s scan data. Incredibly well!
Take the scope shot from Figure 3 and compare it to the HPTuners scan data recording in Figure 4. For these experiments, I used the HPTuners scanner because it records the PIDs at a very fast refresh rate and can record for hours if needed. It’s a powerful scan tool for supported powertrains. For clarity, I purposely turned off the traces for most of the solenoids except for the two solenoids responsible for the 2-3 upshift.
In purple, PCS 2 ramps the 3/5/R clutch pressure up through the fill, torque, inertia, and final stages. In white, PCS 4 ramps the 2-6 clutch pressure down to release the clutch. Notice the engine RPM (red trace in the top graph) drop during the inertia stage. Right after the RPM drop, the PCS 2 pressure jumps to the max to allow line pressure to hold the 3/5/R clutch during the final stage. Also, notice how the releasing clutch is completely released before the engine RPM drops, indicating the division between the torque and inertia stages.
IMPORTANCE OF TORQUE MANAGEMENT
Most electronically controlled transmissions utilize torque management. The white trace in the top graph in Figure 4 shows throttle reduction, which occurs on the 1-2 shift but not on the 2-3 shift (in this example). The yellow trace shows a timing reduction, which happens on all shifts. Torque reduction might pull power during a shift, but it’s necessary to ensure clutch durability. This feature can be easily modified using a tuning device, so it would be a good practice to ensure that torque reduction is intact on any of your performance builds. Earlier in the article, I mentioned that the transition between the inertia stage and the final stage resulted in a 3x pressure increase in Figure 3. That prompted me to review the torque PID on the scan data to see how much the calculated torque was reduced during the shift.
Review the calculated engine torque (white trace) in Figure 4 in the bottom graph. At WOT, just before the 2-3 shift, the calculated torque hovered around 330 lb-ft; during the shift, it dropped to -100 lb-ft. So, during this shift, the engine wasn’t producing any output; there was a torque reversal! This explains how the lower shift pressures are adequate to bridge the engine RPM gap during a shift and why the pressure needs to jump substantially after the inertia stage. This also reinforces the need to ensure torque management is in place on your builds!
SO, HOW NEW IS THIS CONCEPT?
Electronic shift control strategy is anything but new, but the methods are a bit different based on the manufacturer. I went back to analyze pressure graphs I collected in the past with this new arsenal of knowledge.
Figure 5 shows a 2011 Dodge Charger with a 42RLE. Like many Pentastar transmissions, this transmission is based on the long-lived A604/41TE. If you are old enough, you might remember way back in 1988, the transmission world was upturned by the Chrysler 41TE. This transmission used completely computer-controlled synchronous shifts. As mentioned earlier, synchronous shifts precisely time the releasing clutch with the applying clutch, so near-perfect transitions are required, or they risk binding, flaring, bumping, and a host of other shift complaints. And if you were working on transmissions back in those days, all of those shift issues were on the menu. The 41TE transmission problems were notorious, and there were various classaction lawsuits (unlike GM’s 8L90 today). That was 36 years ago and that basic transmission is still used today in the 62TE and the 41TE, which just ended production with the Dodge Journey in 2020. What an impressive run! After various improvements through software and hardware, the 41TE became a pretty reliable unit, just like this 42RLE in the Dodge Charger.
The scope trace in Figure 5 shows the 1-2 upshift pressures. The fill, torque, inertia, and final stages are pretty clear in this scope Figure. They might not define these portions of the shift as such, but the general concept seems to apply. The scope Figure even shows the pulses from the 2/4 solenoid ratcheting during the torque and inertia stage. Have you ever wondered what caused those weird wear marks in the accumulator bores on these units? It would be interesting to compare this 2011 shift pressure pattern to an early 41TE to see how much the pressure control has evolved. The control module’s processing power and software enhancements no doubt influenced the transmission’s durability.
Figure 6 shows the 4-5 shift on a 2004 Acura MDX transaxle. The Honda/Acura shift control was pretty elaborate back in the 2000s with a set of pulse width modulating (PWM) clutch pressure control (CPC) solenoids controlling CPC valves, which controlled clutch pressure for the releasing and applying clutches. On/Off shift solenoids controlled shift valves that either directed line pressure or CPC pressure to a set of clutches. The scope Figure shows the fourth clutch releasing way before the 5th clutch starts to apply.
The fifth clutch appears to go through various stages of fill, torque, and inertia before the shift solenoids shuttle and direct line pressure to the clutch. If you’ve worked on many Honda/Acura units, you know how important the pressure switches are for shift quality. Those pressure switches provide the controller with valuable information on how the CPC solenoids react during the shift sequence.
Currently, at the shop, I’m outfitting a GM 10L80 (Figure 7) with pressure sensors to run experiments on the dyno with HPTuners. I have an unlocked TCM on the way and hopefully, I’ll have some great content to share with you regarding the latest transmission shift control strategies. At this year’s Expo in Vegas, I’ll be sharing the results of these experiments and much more. I hope to see you all there!
