Practical Transmission Tuning: Step 3-3: Downshifting

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Step 3-3: Downshifting

12.04

00:00	As we discussed prior, now we need to be mindful of how upshift schedules interact with downshift schedules.
00:07	Concerns include avoiding excessive shifting, which can feel like transmission confusion while driving, as well as avoiding requesting up and downshifts under the same set of conditions, which depending on the TCM and hardware, may cause damage.
00:23	Fortunately, as transmissions have become more advanced, with more gears, they generally received more safety systems to account for accidentally requesting two different behaviors at once.
00:35	Nevertheless, our goal should always be to avoid this with our calibration decisions.
00:40	I'll open the base downshift map here, and again, if we mouse over the button for the base table, we can see the in -application help states this is OSS -based.
00:50	The various up and downshift tables may not always be based on the same units, so it's best to play it safe by checking, and avoid unexpected behavior.
01:00	Some units are so drastically different that it's immediately apparent, such as one table having values in the thousands, and another having numbers from 1 to 10.
01:10	It's not always that obvious, though.
01:12	To our potential relief, this downshift table only has 9 columns.
01:17	It does not include the skipping of gears in the way the upshift table did.
01:22	In the upshift table, we were looking at the OSS, above which we exit the current gear, and upshift into a higher gear.
01:30	I know how downshifting on this particular TCM works because I've used it before, but I'm going to talk through my thought process the first time I saw it.
01:39	In some situations, we'll be working with super thorough and clear documentation, and sometimes we won't.
01:46	So, tuning becomes a combination of figuring out what each table is, and does, before editing the values.
01:54	Based on the way the upshift tables are laid out, perhaps the downshift table shows OSS values below which we exit the current gear by downshifting.
02:04	That seems it would make sense, right? But if that was the case, we'd expect the x-axis to have gears 2 through 10 in it instead.
02:13	So, where's 10th gear? If we never tell the TCM to downshift out of 10th, that wouldn't work properly.
02:21	Also, why is 1st gear here? There are no forward gears we can shift into below 1st, and we don't want the transmission to downshift from 1st to neutral as we slow down.
02:33	So, why is the table laid out this way? At this point, we have a pretty good idea that something else is going on here, and we need to figure out what this table really means.
02:43	Perhaps instead of the z-data being the output shaft speed at or below which we downshift out of the gear indicated in each column, perhaps this is the output shaft speed at or below which we downshift into the gear noted in the axis value.
02:59	For example, column 9 would show the max output shaft speed for a 10 to 9 downshift, not a 9 to 8 downshift.
03:09	If that's how it works, now the axis values being 1 through 9 instead of 2 through 10 make sense.
03:17	At this point, I have a pretty solid hunch I know what's going on here, and then I can test my theory by driving the vehicle, logging OSS, APP, current gear position, and seeing if my hunch was correct.
03:31	Of note here is this particular TCM has many downshift tables, and also has coast -down maps which come into play while coasting, so confirming when the base downshift table is active is likely best tested on flat ground with the accelerator pedal applied to avoid hill and grade modes and coast-down maps.
03:53	This is a situation where creating a spreadsheet to graph the behavior really comes in handy, but much like we adjusted the vehicle speeds for up and downshifting on the older Tahoe, we can make similar changes here.
04:06	If we want a sportier feel, we can increase up and downshift points to maintain a similar gap between them, and use both up and downshifting to keep the engine at a higher average speed while driving.
04:20	I'll get the base sport up and downshift tables open here so we can see what this looks like.
04:25	At 40% APP in 5th gear, sport mode upshifts to 6th gear at an OSS of 2310, versus the base mode upshifting at 2250.
04:38	As is typical, the sportier mode waits until a higher speed to upshift.
04:43	The downshift maps show at 40% APP, the sport map will downshift to 5th at or below 1000 RPM OSS, and the base file will downshift to 5th at or below 1025 RPM OSS.
05:01	So, the downshifting is more aggressive in base mode than sport mode here, and that's odd.
05:08	I've chosen this particular cell on purpose to illustrate a point.
05:13	When trying to figure out what a table is, or what it's doing, never only compare a single cell to other tables, because it may throw you off.
05:23	As we look up and down this column, we find every single other cell in this whole column is higher in the sport table than the base table.
05:33	So, while that one cell at 40% APP is an outlier, the table does generally fit the mold of using higher OSS values to make downshifts occur at higher speeds, which keeps the engine at higher speed as well.
05:48	This improves engine braking and requests for more aggressive downshifts when APP is applied to improve acceleration.
05:57	Along those lines, there's still a limit to how high we want to make the engine rev up when a downshift occurs.
06:02	So, we'll notice the 8th and 9th gear columns at higher accelerator pedal positions actually have the same values in both tables.
06:11	This TCM does have downshift protection.
06:14	Here the info states this is the maximum engine RPM that's allowed after the downshift.
06:20	If the change in gear ratio at a given output shaft speed would send transmission input speed over this value, and potentially overspeed the engine, the shift won't be executed until conditions are safe.
06:34	Much like some TCMs have additional tables for settings for upshifting, there may be others for downshifting as well.
06:41	We shouldn't feel like we need to adjust all of them.
06:44	That's almost never necessary, and adjusting unnecessary tables can cause confusion or trouble, so at this point I suggest keeping it simple.
06:54	We should test drive the vehicle in all the basic modes we can achieve, and continue adjusting until we're satisfied with both up and downshifting while coasting or at partial throttle.
07:07	After you're happy with the primary drive mode related tables, you can repeat for the additional drive modes.
07:13	You can refer back to the information in the transmission tuning section about drive modes if a refresher would help.
07:20	This step of the tuning process has been broken down into a set of modules, so let's take some time and run back through the main points of them before finishing up.
07:30	We started with an older, more basic 4 -speed TCM, and went all the way to one of the latest, greatest, and most complex, but the principles remain the same.
07:41	If you can tune a 4-speed, you can tune a 10-speed.
07:45	While I've used a GM and a Ford for illustration because they're quite popular, what we've gone over in this module are universal principles.
07:53	Our worked examples will dive deeper into the intricacies of each system, but don't wait for a worked example for your exact application to dive in.
08:03	Shift scheduling will generally be based on vehicle speed or output shaft speed, and I've taught you how to calculate either one from the other so you can work back and forth between them.
08:15	Then the primary axis is likely either accelerator pedal or throttle position, and most of the TCMs using throttle position are cable throttle, so throttle position is the same as APP anyway.
08:28	With this in mind, as different as TCM software may seem at first, hopefully with this knowledge in hand, the little differences don't feel like such a big deal.
08:38	I talked about starting out by looking over the tables and weeding out the ones that aren't used, or we just don't need to adjust.
08:47	That could be safety systems for overheating, extreme ABS braking, traction control, or drive modes we plan to never use.
08:56	We also discussed we can make simple and safe changes temporarily to help identify which table is active at a given time.
09:06	Make the change, test drive, data log the monitors associated with the X, Y, and Z data in the shift schedule table, review the data log, and correlate the shifts in the log with the settings in each table to figure out when each table is active.
09:24	If we set the vehicle in sport mode, test drive and the up and down shift points happen at vehicle speeds or output shaft speeds that match up with an upshift table called upshift table number 57, and the data we recorded doesn't match any other table, we probably just identified the sport upshift table.
09:45	If, on the other hand, we're in a table labeled sport mode, but the vehicle behavior doesn't match the sport upshift table data, a few things may have happened.
09:54	It's possible we've found an error in the software, and that's not really the sport mode table.
09:59	Alternatively, it may be correct, but there's another system overriding the default sport shift schedule.
10:07	This could be an inferred mode like the hill descent or inferred grade modes the 10 or 80 has.
10:12	It could be something as simple as a coast down mode that takes over while APP is zero, or a wide open throttle shift mode that takes over during wide open throttle acceleration.
10:24	When working with a TCM that's new to you, read the available documentation, then review the data, and use your own judgment if some aspect isn't clear to you.
10:35	Much like my process to figure out how the 10 or 80 downshift table worked, we can apply a similar thought process to other tables as well.
10:44	The axis labels, values, and the Z data can all be clues to help us figure it out.
10:51	I also went over avoiding overlapping shift requests by leaving gaps between up and down shifts to avoid the transmission behavior feeling confused or overly busy and annoying.
11:04	I showed you how to convert transmission input speed to output speed simply by dividing the input ratio by the gear ratio, and converting the other way is just as easy.
11:15	Multiply OSS by gear ratio to get transmission input speed.
11:19	Remember, transmission input speed may resemble engine speed, but they can differ, so always data log both.
11:28	When calculating final output speed of the drivetrain, remember to factor the transfer gear ratio in if one is present and it's not 1 to 1.
11:37	To avoid an upshift, set the OSS or vehicle speed to a value higher than the vehicle can achieve.
11:45	If you don't want to use first gear, set the 1 to 2 upshift speed to 0.
11:50	Now, that we've settled on our partial throttle, coast, and drive mode shift scheduling, let's move on to adjusting shift feel, shift speed, and torque converter clutch lockup.