4L80E Slow 3-4 Shift

2004 Suburban 2500, 8.1 with the 4L85E. Transmission was rebuilt under prior ownership in 2015; I have no details on the rebuild.

Ever since I got it, if you got into the part of the trans map where the converter locked in 3rd and then the trans upshifted to 4th, it would shift, hold for a half a second to a second, unlock the converter, relock, and then be fine. EFILive datalogs show a bunch of weirdness, and the internet at large as been zero help. Hoping the hive can help with this one...

From the logs:

• The ECM thinks shift duration is taking about 2 seconds. All other shifts follow command, around 0.35 seconds.
• During the shift, reported gear ratio falls from 1:1 (3rd) to around 0.85:1, and then to 0.75:1 (4th) at the same time the shift registers as complete. I have not found any info on how the ECM calculates this ratio. for the other shifts, it ramps from one ratio to the next quickly.
• If the converter was locked in 3rd before the shift, the torque converter slip reports a negative slip during the shift. Once it unlocks and relocks, the slip goes to zero. Guessing this is a calculation quirk of the long shift time rather than the cause of the issue, but the converter was also replaced.
• If the converter was not locked in 3rd, the shift time still reports around 2 seconds with the odd ratio reporting, but you can't really feel it aside from the shift feeling soft
• Shift adaptation never activates for the 3-4 shift even though the shift times are set >0 (0.35 to 0.4 s)
• Torque management is active for the full ~2 second duration of the shift - I can see timing being pulled and can feel the power loss.
• ECM reported gear position is correct and quickly changes from 3rd to 4th, as do commanded shift solenoid positions

Other than this issue, the transmission works great. I've towed my enclosed trailer across the country through the I70 corridor west of Denver a few times no problem, it doesn't slip, doesn't get hot, nothing like that.

I'm assuming I'm looking at a hard parts issue because it does it with the stock tune, and upping the trans pressures and such doesn't seem to make much difference. That said, nothing is jumping out at me that would only cause a 3-4 shift issue without some other associated problem.

Any ideas? I'm fresh out of them and can't even find matching symptoms on any of the various GM forums except for one dude, who never posted a solution and hasn't been online in years.

Ensure the engine temperature the ecu is seeing is correct. If it reads too cold the ecu will delay the 3-4 shift.

In reply to bearmtnmartin (Forum Supporter) :

Thanks for the suggestion. Both the engine and transmission temperatures are reading properly. The 3-4 shift happens at the speed/RPM it is supposed to, it just isn't the snappy shift that it should be and the shift itself takes too long.

I found a hydraulic diagram for the 4L80E and it appears that the 3-4 shift works like this:

• AFL valve supplies a pressure to the 1-2 and 2-3 shift bores based on engine torque/PCM command
• during the 3-4 shift, the 1-2 solenoid switches on. 1-2 shift valve can't move because the 2-3 solenoid is still engaged and fluid pressure from that valve + spring pressure hold the 1-2 valve in the upshifted position. the 1-2 solenoid supplies pressure to the 3-4 shift valve
• 3-4 shift valve opens against spring pressure and allows 4th clutch apply pressure to go to the 4th accumulator and the 4th clutch
• 4th accumulator flow is also regulated by the accumulator valve.

So isolating things that can really only cause problems for the 3-4 shift, I think this is what I need to look at:

• 3-4 shift valve and spring
• 4th accumulator piston/bore/spring/check ball
• 1-2 solenoid

I guess at this point I just need to find some time to take it apart and see what I need to buy. Really hoping it is one of these things in the valve body so I can do it without removing the trans.

My trans builder did a lot of accumulator spring changes in both the 4L80E and the 4R100.  Since they all operate off of pressure each other (fighting pressure in some cases, hence the need for an accumulator), if one is off a bit it can change the way the other operates.  I also know that galling/scoring is a common problem in the 1-2 valve and can cause intermittent super hard shifts OR soft shifts.

Honestly I agree with the hard parts diagnosis.  I might try throwing a known good valve body on it and see if that fixes/changes things.

The ECM does a pretty simple calculation on ratio.  It takes RPM, turbine speed, and output speed and does the math.  Are the input and output sensors reporting correct data?  It's easy to just watch what the ECM reports the ratio is and trust it, but watch the reported ratio and compare it to what your ear hears.

Also, do a cold check on trans temps.  The temp sensor is a failure point and sometimes it reports what seems to be a logical number like 190 when it's hot, but it's hard to trust.  It could be actually way off.  The only way to know for sure is to go out on a chilly morning and check the trans temp. It should be within 5-10 degrees of ambient.  I've checked them before and the temp sensor had a dead short when cold and reported -247 degrees.  That's an outside guess because that usually causes shift anomalies across all shifts, but worth a check.

Another item to check is the pressure switch manifold, which is on the valve body and accessible with the pan off. The pressure switch pads get a build up of material/gunk on them and can cause strange shifts. On my 2003 8.1 Burb, it actually started throwing a code, and replacing it cured it. It may be possible to clean it but they are pretty cheap on RockAuto.  Pressure switch manifold

Thanks for the comments - I finally had some time to try some different parameters in the datalogs for more info. Being limited to 24 parameters is kind of frustrating; I'm used to logging hundreds at a time at work which can make it a lot easier to find unexpected relationships in the data.

Things I've learned:

• the torque converter slip ratio is correct as soon as the shift registers as complete, so that is a symptom and not a cause
• reported gear ratio is calculated from the difference between "turbine speed" and "output shaft speed". I can watch turbine speed doing the right thing through 1-2 and 2-3 shifts, but it is all wobbly on the 3-4 shift until it eventually locks into the gear.
• There is also a transmission input shaft speed parameter. I don't really understand what this one is as it seems to read very close to what the Turbine Speed parameter is reporting, but it doesn't register the 3-4 shift. When in 4th gear, it stays at a 1:1 ratio to output shaft speed.
• All the sensors seem to be reporting good, consistent, and correct data
• I still have no fault codes
• The 3-4 shift time is pretty consistently reported as 1.88 seconds. I've never seen it over 2 seconds, and I have seen it (inconsistently) down in the 0.35 second range where it should be if the shift occurs while I am rolling out of the throttle.
• I can't hear or see any issues with the 3-4 shift from in the truck, but it does feel a bit soft compared to the other shifts.
• Once the 3-4 shift is complete, there is no slipping in 4th based on looking at the turbine to output shaft speed ratio.

@Curtis73 - thanks for the thoughts. I have seen a lot of people deleting the 2-3 and 3-4 accumulators on the 4L80e and have seen similar things about bore wear. I'd have to buy another valve body to try since I don't have a known good one available, unfortunately.

The day generally starts a 75 degrees here right now so no cold days to check on, but I will do that if we have a colder morning. So far, all the data looks good with no hiccups.

@81cpcamaro thanks for the suggestion, I had not seen that one but it makes sense that this might have some gunk or wear on the 3-4 contact that is preventing it from registering the shift.. That is cheap enough that I will probably just throw one in as a precaution when the pan comes off.

Here is a great example showing all of the shifts, around 30% throttle with very little variation, and you can see converter lockup as well. Nice and clean except the 3-4. You can also see that I've got my shift pressures set on the high side since shift adaptation is pulling pressure on the 1-2 and 2-3 shifts.

Since this thread is now the #2 search result looking for slow 3-4 shift issues on a 4L80e, here's an update:

I did replace the valve body with a Sonnax unit, since I don't really have the time to throw parts at it and even if I have to ream one valve bore, the tools instantly put me over the cost of replacing the whole thing.

The fluid looked almost new and really had nothing in it. Even the magnet was fairly clean, and nothing meaningful in the filter.

I've only put maybe 50 miles on it so far. I was not able to find a way to reset transmission adaptation parameters (TAP adjusts the pressure within a range defined in the cal to try to hit the target shift speed). I tried flashing a cal in EFILive, using the EFILive DVT tool, and an overnight sit with the battery disconnected with no success - I could see in the logs that adapt shift was pulling pressure, basically as much as the limits would allow, on the 1-2 and 2-3 shifts, because for some reason the original valve body was causing harsh shifts. So for now I'm driving carefully and watching as the adapt shift pressures work their ways back towards 0 psi where they should be. The 1-2 shift is now in the 2 psi range, with max authority of +16 psi. Previous was -6 psi, which is the maximum authority allowed.

Adapt shift does not apply to "performance mode" (tow/haul) shifts. Previously, every shift felt like you were getting rear ended in tow/haul, except the 3-4.

Adapt shift, even though it appears enabled in the calibration (shift time targets are nonzero and max authority is +16 psi for all engine speeds), does not seem to apply to the 3-4 shift. In datalogs, every time a 3-4 shift occurs, the "adapt shift in progress" parameter goes from "yes" to "no". It switches back to "yes" whenever an upshift to another range occurs.

So, with all that background, here's the impacts:

• Tow/Haul is actually usable now. I didn't know it wasn't supposed to be that way because I didn't have anything to compare to, but this is much more believable in terms of "how it should be".
• As the trans adapts work their way back to normal, the 1-2 shift occasionally triggers the "long shift" flag (0.7+ second duration) on light throttle shifts. It's fine with more load on it, and it's barely noticeable driving because the converter slip masks it.
• 2-3 shift is great and continues to firm up as shift adapt does its thing.
• 3-4 shift is now the way it is supposed to be up to around 200-250 reported ft lbs, which is pretty typical shift torque for moderately aggressive acceleration like on a highway onramp.
• 3-4 shift is still a little long (0.8s+) at higher load (~340 ft lbs), but I haven't had a trailer on it on the highway to test more. Without a trailer to add load, it's a little hard to test because the rate of acceleration + the shift delay timer means you're moving along pretty well by the time it shifts. Regardless, 0.8 seconds is dramatically better than 2+ seconds I was seeing before. I only did two 3-4 shifts with the converter locked and while TCC still shuttles in and out, it happens very quickly now. My hope is that I can get trans adapt to work and that will fix this, but if not, maybe I can bump up the shift pressures to compensate. Regardless, still feels like something is off nominal, but now it is good enough that I'll probably let it be.

This experience continues to erode my trust in automatics.... so much easier to troubleshoot a manual! Making progress, though.

Still scratching my head on this one.

I've towed plenty with it, when it does a slow 3-4 shift I just let off the throttle until the converter locks again and then it's fine. Very annoying though, especially in the mountains. Doesn't slip once it is in gear, doesn't get hot, no other issues. Manageable, but I know it is wrong so I want to fix it.

This is shifter in 3rd, converter locked, steady throttle. I bumped the shifter up into 4th, converter stayed locked (as it should). Steady throttle. It can't be felt from the driver's seat. The engine speed is correct for 4th gear converter locked the whole time and it feels like a nice snappy shift, but somehow the turbine speed is "hanging". It hangs for the exact same time duration every time it does it, which is basically every 3-4 shift, it's just not noticeable if the converter wasn't locked before the shift. The fact that it is ALWAYS 1.9 seconds is so strange to me, and I can't wrap my head around how the engine speed can be lower than the turbine speed with the TCC locked. If something was slipping, I'd expect the engine speed to be higher than turbine speed. 

The 4l80 in my 03 Sierra Denalli would require you to fill out forms in triplicate and submit them requesting a gear change.  At over 400K miles on that trans, it was a bit slow.  

In reply to dean1484 :

This transmission shifts great... from the driver's seat even this 3-4 shift feels fine. It's just obnoxious to have it shift and then have to lift off the gas to wait while it unlocks and relocks the converter. If I don't lift, it sometimes takes a very long time to command lockup again, and my 4L60e experience makes me very leery of putting any real power down in 4th gear (towing) with the converter unlocked. Logs show the unlock occurrs due to long shift error - there is a status message for TCC state. 

This is now in the top 5 search results for everything I try to google about my problem, so I'm just going to keep documenting things here because there's at least 2 other people who have posted about this but never posted a resolution.

The operating system in my truck is "rare" - seems to be unique to 2004 8.1 4L85E trucks, and even some of those are different. I finally found another version of that OS that was identical to mine - except for the transmission control segment. So I did a segment swap and went for a drive. My reported shift times are now almost normal at lighter throttle, but who knows what other adaptations I erased in the process.

The TISS still shows turbine speed hanging high, even though engine speed is correct. The "shape" of the shift isn't a nice clean saw tooth that I like to see. Instead, the engine speed quickly drops and then kind of settles to target, like the barrel of a wave.

My last ditch thing to try is take another look at the 3-4 accumulator in the valve body. These things are known to partially fail and bind. I checked it when I replaced the valve body and I remember the piston being very tight, so for about $40+oil and a few hours of my time I think I'm going to install a pinless accumulator piston. If that doesn't fix it, I will have to assume it is something in the trans itself, and I'll deal with that if/when I have a bigger failure.

In reply to gearheadE30 :

One weird thing I have learned about GM PCMs from the era is that some adaptives cannot be cleared, and the only way to reset them is to reflash the computer.  Maybe burning A new OS did the trick, and not specifically THAT OS?

I kinda wonder if someone put a modded valve body in to solve whatever problem.  

I would also like to know where you got your SI.  Auto transmissions are still a little voodoo to me and I would like some deep info on how certain ones work, in the "this does this, that does that" sense.

In reply to Pete. (l33t FS) :

Yeah, I have noticed that. I'm sure the dealer can clear things, but taking modified vehicles to the dealer always seems like an unnecessary risk to me...

The shift signature still looks a little funny on the 3-4 shift. It feels and looks exactly the same with the new trans segment, but for whatever reason it doesn't trigger the long shift logic as frequently. I do think I'm about at the end of my misguided hope that it's a software issue though.

When I replaced the valve body, it had some non-OEM gaskets in there. The trans was definitely rebuilt in 2015, but I'm not sure what they did to the VB if anything. The new Sonnax VB fixed everything except this 3-4 shift time. I went back and checked, the new VB included accumulators, manifold switch, and solenoids which pretty well rules out all the external stuff. Guess I'll just drive it until it has a failure worth pulling the whole trans out for. 

If you want some light reading, I have a 156 page .pdf manual for the 4L80E that actually does a pretty excellent job of showing what's going on in there. Looks like I can't attach it to a post but if that's useful to you, let me know and I can try to email it or something.

I'd personally would program to have 3rd unlocked all the time and Lu only occur in od. Gm is famous for this and ford too for locking the converter way before it needs to for that fraction of a sec in mpg and emissions improvement.

I would also check the wiring. I wonder if there is a high resistance causing the voltage to drop at the solenoid. It still works but it's going to take longer for it to latch.

In reply to Ranger50 :

stock, this is one of those that will lock in 2nd if you're really on the gas. I've changed some of that stuff around to make it never lock before it shifts into 4th in normal mode, but in tow mode it makes a huge difference to have the converter lock in 3rd like it does stock. Yeah, there's some mileage improvement, but the real important part is it keeps the trans temperatures down. Towing the toy hauler especially in the mountains out west, it spends a lot of time in 3rd. Right now I manage it using the shift lever and not using cruise control and its fine, but any time someone else is driving they end up with a flubbed 3-4 shift, TCC unlocks, and then they're just cruising with a trailer in 4th with the converter unlocked which is a really good way to kill a transmission.

Good thought on wiring. That's an easy one to check.

Someone mentioned that I should have replaced the separator plate when I did the valve body, which of course I didn't do. It had some minor wear, but it's been suggested to me that even minor wear can cause some issues. So it sounds like a separator plate might be in my future to rule that one out.

I did bump up shift pressures and added torque reduction to the 3-4 shift (stock has no torque reduction) just to see what would happen. It actually kind of looks like it is tying up the shift a bit, turbine speed drops too far now and I actually get a ratio undershoot. But shift times are generally decent with these changes. So strange. But hey, I guess I'm getting to learn a lot about transmissions.