What is my 4L60E doing?

[redhat]

This has been something I've noticed since I have been driving my Tahoe (Feb 2022). It happens when you are going over 55... or I'll put it this way, when you're in 4th gear and I assume lockup. You let off the gas and coast going down a grade maintaining or picking up speed.

You feel a grab then release, grab, then release feeling. Like a slow pulse.

Absolutely nothing under acceleration. Since putting on a cat-back exhaust, you can hear the engine change tone while this happens as well. Fluid looks good. Relatively unknown service history on this trans besides knowing it is not original and was probably changed around 150k in 2014. Trans. in question has probably 150k on it.

It has done it so long that I'm not sitting by its bedside concerned, but genuinely curious on what it is.

Something with the torque converter?

 

[Bailes1992]

I get this on both my Wife and I's Dacias. On our cars at least it's the 'smart' alternator increasing the voltage to help charge the battery on overrun.

 

[DriveHard]

It could also be your A/C engaging and disengaging. The first brand new car I bought that was current tech had an A/C compressor that cycled. I could feel it pulse on and off, and ended up taking it back to the dealer because I had never experienced that before. That was 2002 ;-)

 

[ekrampitzjr]

DriveHard is probably correct. You might not have the AC on to cool the interior, but if the defroster is on to clear the windshield, most recent vehicles run the AC with that for drying the condensation. On plenty of vehicles the AC cycling is noticeable at certain times.

You don't notice it on hard acceleration because the AC compressor cuts out when the accelerator is floored.

 

[redhat]

DriveHard is probably correct. You might not have the AC on to cool the interior, but if the defroster is on to clear the windshield, most recent vehicles run the AC with that for drying the condensation. On plenty of vehicles the AC cycling is noticeable at certain times.

You don't notice it on hard acceleration because the AC compressor cuts out when the accelerator is floored.

I would agree with that as well, and I thought that could've been it over summer -- but... when I did a motor swap months back, I put in I drained the system, put a new compressor on and re-evacuated the system. Presently I do not have a belt on the A/C or the clutch connected.

Was going to put the belt on and re-charge it come closer to the summer months.

 

[clinebarger]

Try this, Set all the cells in the Minimum TCC Duty Cycle table to 90, This will cure TCC bobble if the TCC Regulator Valve Bore is a little worn.....Upon deceleration, The TCC Duty Cycle drops pretty low in the stock tune which can lead to reversion slip if the bore is worn/leaking.
This mod reduces the constant oscillation of the valve which is what wears the bore in the first place, ALSO effectively eliminates the PWM ramp apply & release of the TCC which can cause slight bumps when applying & releasing.

 

[clinebarger]

You can sometimes see the bobble in the TCC Slip RPM pid, Really depends on the scan tool software.....My Snap-on Solus Ultra is pretty good if I isolate TCC Slip RPM pid & graph it.....Reversion slip will read in the negative.

A good healthy unit with the above Duty Cycle mod will maintain Zero slip RPM (+/- 4 RPM) anytime the TCC is enabled.

 

[redhat]

@clinebarger Thank you! I am going to try this today or tomorrow and will report back. Appreciate your input as always!

 

[redhat]

So I made those changes to the Min TCC duty cycle tables. I have driven a few times and think it's still there. I am taking a longer drive this evening where I will be traveling down a hill for a few miles with relatively low throttle input (cruise set to 65 mph). So, I will confirm it then. For those of you in the WNY area, getting on the 219 going south, and then turning around at the Genesee exit then heading back north. From Rice Rd. to Boston is a perfect case for this to happen.

Also worth noting, another thing this thing does is when letting off the brake and about to accelerate, about every 1 out of 8 times, you feel a very positive grab/shudder. It accelerates normal. No slip. Maybe TC on its way out?

Maybe I should start planning a rebuild.

 

[redhat]

So I connected the MVPI2 and ran VCM scanner and datalogged a complete drive including up and down this stretch of road where it happens.

Could feel it still and could see some interesting things in the scanner. What I noticed is it is coming out of lockup, goes to coast, then to locked, then to coast, etc. when this happens. Also while coasting, I am seeing a few misfires. I also have some misfires when idling. No misfires at all when driving or in the throttle. I have been told that even with my baby cam, some misfires at idle are the nature of it being cammed so I am assuming thats what I am seeing there during coast.

Interesting, I have some more research to do and look through my data log and tune more.

FWIW, slip rpm while locked is +/- 4 rpm.

 

[clinebarger]

GMT800 4L60E's don't normally come out of Lock-up at closed throttle/coast per the calibration unless,
A. The brakes are applied.
B. The minimum TCC Release Speed is met.

On the other hand.....GMT800 4L80E calibrations are set to release the TCC at anything below 4% TPS.

 

[clinebarger]

Did you monitor the brake pedal position PID? Different circuit than the brake lights FWIW.

 

[redhat]

Did you monitor the brake pedal position PID? Different circuit than the brake lights FWIW.

Finally getting back to this, the only PID I can find in VCM scanner close to that is TCC brake off. That didn’t look like it was doing anything out of the ordinary. Unless there’s another way you would recommend monitoring that specific PID. I fired up Tech2Win and I didn’t really see any sort of reading for that, unless I’m not looking at the right thing…

I monitored this some more under VCM scanner, and took quite a few data logs. It looks as if I’m getting this feeling going down a slight grade with the cruise control on, and I am switching between locked and coast rapidly. Steeper downhill grades, it seems to go into coast and stay in coast. I also tried reducing the high-speed TPS threshold percentages both to 0%. That seemed to keep it locked downhill versus switching to coast. I would still get some of these slight “pulses/shudders” when it would switch between modes, but nothing like the locked/coast switching.

I also maxed out my idle (moving and not moving) and 1,400RPM medium RPM misfire tables thinking maybe it’s bringing it out of lock up because it’s seeing misfires. That really didn’t change much but I think it’s unrelated because it’s not really coming out of lock up, just switching between coast and locked quickly.

Wonder if something is worn in the drivetrain itself that is causing this, where a normal/healthier 4L60 would just do this work and you would never notice it. I should put the VCM scanner on the Suburban and see what it does.

 

[redhat]

I think I finally figured this out completely, and it was a few things.

The issue that I was feeling was when descending down a grade, usually with cruise control on, or with a very steady accelerator pedal, I'd experience a jerk/shudder/surge (it was a surge). I had data logged using HPTuners VCM Scanner and would notice this situation when the torque converter would be locked up... it'd then unlock (go to Coast mode), then lock again, then coast, etc. This would oscillate between modes to what I though was rather unusual.

Since I am cammed, the PCM was still at times detecting misfires. Which I have come to learn is normal for my situation. With that said, GM logic will take a torque converter out of lockup if the PCM sees the engine misfiring. So I disabled misfire detection throughout all of the misfire tables in my calibration. This now helped keep the torque converter locked when it should be locked and coast when it should.

However, I still felt this jerk/shudder/surge situation. I will claim that it was better as it was happening much less frequently as the torque converter was staying locked when it should. However, during times when GM logic would call for the TC to unlock and coast, then lockup again, you'd feel this shudder.

On a Gen III P59 PCM (and P01), your usual driving and accelerating (above TPS threshold... usually around 2%) spark is controlled by an interpolation of your high and low octane spark tables, also known as your main spark tables. When you are idling, your spark is controlled by your Idle Spark Advance (In-Gear and Park/Neutral) tables. With that said, when your torque converter is Locked, you're usually above the TPS threshold either maintaining speed or potentially even accelerating and working from your main spark tables. When your TC unlocks and you are in "Coast" mode, your TPS is below the threshold and you're idling, even if you are rolling. So these situations will pull from the Idle Spark advance tables.

The picture below shows what the tables were in my calibration for main spark and idle spark advance.

So my TC would unlock and I'd be around 25-27 degrees of timing, then since I'm below the TPS threshold, it would switch to the idle spark advance table and at that similar cylinder airmass and RPM drop to 16 degrees of timing. THIS is where this surge was coming from. Timing would drop 10 degrees, then the PCM would try to compensate for the drop then add more timing and it would create this viscous cycle of surging and in doing so it lock the TC back up... what a mess!

From reading around online, I copied my high octane spark tables to both idle spark advance tables. Took an hour long drive and the truck drove great. No more surging... lockup and coast TCC modes apply and happen flawlessly.

I wanted to take the time to document this as I have been working at this situation for months now. I am sure part of the length of time it took to nail down is in part due to me not explain it correctly... but it has seemed like a few different things over the course of time.

@clinebarger - question for you sir, what would you do? Would you keep idle spark on a Gen III PCM same as high octane? Maybe pull a 2-3 degrees across the board? Or copy low octane?

I looked at a stock LQ4 2500HD calibration and it looks like these idle spark advance tables are bone stock.

 

[350Rocket]

I think I finally figured this out completely, and it was a few things.

The issue that I was feeling was when descending down a grade, usually with cruise control on, or with a very steady accelerator pedal, I'd experience a jerk/shudder/surge (it was a surge). I had data logged using HPTuners VCM Scanner and would notice this situation when the torque converter would be locked up... it'd then unlock (go to Coast mode), then lock again, then coast, etc. This would oscillate between modes to what I though was rather unusual.

Since I am cammed, the PCM was still at times detecting misfires. Which I have come to learn is normal for my situation. With that said, GM logic will take a torque converter out of lockup if the PCM sees the engine misfiring. So I disabled misfire detection throughout all of the misfire tables in my calibration. This now helped keep the torque converter locked when it should be locked and coast when it should.

However, I still felt this jerk/shudder/surge situation. I will claim that it was better as it was happening much less frequently as the torque converter was staying locked when it should. However, during times when GM logic would call for the TC to unlock and coast, then lockup again, you'd feel this shudder.

On a Gen III P59 PCM (and P01), your usual driving and accelerating (above TPS threshold... usually around 2%) spark is controlled by an interpolation of your high and low octane spark tables, also known as your main spark tables. When you are idling, your spark is controlled by your Idle Spark Advance (In-Gear and Park/Neutral) tables. With that said, when your torque converter is Locked, you're usually above the TPS threshold either maintaining speed or potentially even accelerating and working from your main spark tables. When your TC unlocks and you are in "Coast" mode, your TPS is below the threshold and you're idling, even if you are rolling. So these situations will pull from the Idle Spark advance tables.

The picture below shows what the tables were in my calibration for main spark and idle spark advance.
View attachment 184803

So my TC would unlock and I'd be around 25-27 degrees of timing, then since I'm below the TPS threshold, it would switch to the idle spark advance table and at that similar cylinder airmass and RPM drop to 16 degrees of timing. THIS is where this surge was coming from. Timing would drop 10 degrees, then the PCM would try to compensate for the drop then add more timing and it would create this viscous cycle of surging and in doing so it lock the TC back up... what a mess!

From reading around online, I copied my high octane spark tables to both idle spark advance tables. Took an hour long drive and the truck drove great. No more surging... lockup and coast TCC modes apply and happen flawlessly.

I wanted to take the time to document this as I have been working at this situation for months now. I am sure part of the length of time it took to nail down is in part due to me not explain it correctly... but it has seemed like a few different things over the course of time.

@clinebarger - question for you sir, what would you do? Would you keep idle spark on a Gen III PCM same as high octane? Maybe pull a 2-3 degrees across the board? Or copy low octane?

I looked at a stock LQ4 2500HD calibration and it looks like these idle spark advance tables are bone stock.

I was thinking this was similar to something that happens in my truck except it's completely stock...when I'm coasting downhill with cruise control on in third gear (I usually keep od locked out as the truck is heavily loaded and will downshift on any little grade to maintain speed), it will jerk like it's letting off the throttle completely on and off again.... initially I would turn the cruise off and take over but then I figured out if I just add a small amount of throttle it doesn't do it. Pretty sure in my case it's staying locked up at all times during coasting though.

 

[clinebarger]

@clinebarger - question for you sir, what would you do? Would you keep idle spark on a Gen III PCM same as high octane? Maybe pull a 2-3 degrees across the board? Or copy low octane?

Long as it doesn't cause weird high idle issues during coasting events....Nothing wrong with running a little more advance on the Base Spark table.

 

[OILJUNKIE]

Try this, Set all the cells in the Minimum TCC Duty Cycle table to 90, This will cure TCC bobble if the TCC Regulator Valve Bore is a little worn.....Upon deceleration, The TCC Duty Cycle drops pretty low in the stock tune which can lead to reversion slip if the bore is worn/leaking.
This mod reduces the constant oscillation of the valve which is what wears the bore in the first place, ALSO effectively eliminates the PWM ramp apply & release of the TCC which can cause slight bumps when applying & releasing.

Thanks for that info. I'm trying brain surgery next week.