4L80E TCC Unlocking defies logic

[Husker6.5]

Ok, '98 K2500 Suburban LT, 198K on odometer, tranny never rebuilt as far as I know, last fluid flush unknown. Fluid condition shows signs of overheating in past, no signs of "floaters" on dipstick, but doesn't mean there aren't any in the fluid. Hesitant to have a full system flush done at this time because I've seen more than one weak transmission decide that the time to fail is right after you flush all the "friction chunks" out of it and put fresh fluid in.

Ever since I bought it a year ago it has this totally annoying habit that once it's in OD and the TCC locks, all is fine while I'm driving up any sort of grade or the engine has a load on it and it's "pulling". When I come to a downgrade the TCC will unlock and RPM will jump up 150-200, then it will relock and the RPM will drop, then it will unlock and RPM will jump up, then it will relock and RPM will drop. It will cycle like this about every second or two, and is like riding on a bucking bronco (not Ford-kind!) all the way to the bottom of the grade until the cruise accelerates on the level or a slight upgrade and it locks back up, or I give it a little throttle on the downgrade to keep the motor "pulling", but there are times I hit the bottom of the hill at 90+MPH doing that! It does this unlock-relock cycle whether it's on cruise or if I'm using the throttle and my foot, however if I'm on the pedal, it's worse as the bucking makes my foot bounce up and down and I wind up getting swings of 300-400 RPM as it unlocks and locks, whee! (NOT!) This is definitely not coast clutch or override clutch disengagement, it's the TCC.

Logic says that as the load INCREASES on the torque converter and tranny going uphill, that's when the TCC would unlock to give the engine some more RPM (like it does on my '94 C2500 6.5 when I have a full load in the bed and am grossing 10K vehicle weight, but my '94 doesn't do the unlock-lock thing going downhill regardless of the vehicle load), NOT when the engine load lightens going downhill.

I took it to a very good and reputable local transmission shop that my mechanic uses. The owner (over 30 years experience) hooked up a SnapOn solus scanner to the ECM and did a scan for any codes in it or the TCM, there were none. With it hooked up, we took the 'Burb out for an hour test drive on the Interstate and local roads to replicate the unlock-lock (which it did). The scanner showed the solenoids operating for the upshifts and downshifts, line pressures were in normal ranges, everything seemed to check out ok, other than the damn unlock-lock thing on the downgrades. He concluded that it could very well be that the ECM was "telling" the TCC to cycle off and on for some reason, and that a reflash of the ECM might help.

I had my mechanic use his GM Tech 2 and go on-line to download the newest GM program for my ECM. The Tech 2 showed that the last update to the ECM was in mid '99, nothing since, and that there were two upgrades from GM since then, so the newest was downloaded and my ECM reflashed. Well, it didn't solve the unlock-lock problem of the TCC, but I did gain smoother idling, better throttle response and about 2.5-3 mpg better on the highway from it.

Now for the $64,000 question: Is it possible that there is a piece of debris in the TCC lockup solenoid circuit that causes the solenoid to release when the engine "backs off" that a good flush (tapping into the trans cooler line and flushing with the engine running while on the machine)would cure, or perhaps it's a case of the solenoid plunger bore being worn and it needs to be sleeved that's causing the unlock-lock sequence? While the shifts on this transmission are softer than those on my freshly (30K ago) rebuilt '94's 4L80E, and there is a slight hesitation in the 1-2 upshift occasionally (which hopefully would be cured with a flush and fresh fluid), my impression from the test ride with the transmission tech was that the overall health of the transmission is ok based off of line pressures and shifting via the scanner, and should get me by just fine for quite a while if we can just solve the TCC unlock-lock gremlin.

Any of you out there have a similar experience with your 4L80E, or have the technical experience to know what is causing this? Any help will be highly appreciated as this 'Burb is used primarily for long distance trips (like from Lincoln to Chicago, or to D.C., or to L.A/S.F. and back, or to Ann Arbor for a football game, etc.) and the downhill lurching gets old in a HURRY for my kids or friends after four or five hours of it.

Thanks.

 

[tanman_2006]

Mine did that going 45-55 iirc but quit after i got my 6 setting tune. Not sure what in the programming changed that. 45-55 is the typical lock unlock problem as far as i know.

 

[Husker6.5]

Mine did that going 45-55 iirc but quit after i got my 6 setting tune. Not sure what in the programming changed that. 45-55 is the typical lock unlock problem as far as i know.

This is not speed dependent, it occurs once the TCC locks up in OD (haven't tried it in third, as I don't hiway cruise in direct, too many rpm's with a 3.73 diff, should try it in third some time) and I get in the situation of where the engine backs off of being under load or accelerating. It does it if I'm going 50 down hill, or if I'm going 80 down hill (it gets really wild at 85mph+ because the rpm difference approaches 400 or more, becomes like riding a mechanical bull!), the key is it's doing it when the vehicle is "coasting" and there is no real engine load on the torque converter. My '94 is rock solid, once I'm going fast enough for the TCC to lock in third it stays locked up in OD regardless of going uphill or downhill, the only time it unlocks going uphill is if I'm pulling some serious weight in the bed or trailering a heavy load (3.42 rear) and it has never unlocked going downhill.

It also was reflashed a couple of months ago with the latest GM factory stock tune for it, didn't do anything for the TCC problem, just changed the fueling and throttle characteristics so it idles smoother, more linear throttle response, and I picked up about 2.5-3 mpg on the highway (about 18 now running @ 70mph).

 

[WarWagon]

Look for pinched oil cooler lines or kinks in it. Possible debris plugging filter screen or oil cooler. If the transmission is on the way out a fluid change is a waste of money - just rebuild it. Look for a place that can do it for $1200.00 or so like they can here.

In 4th you should be 'freewheeling' as there is no compression braking in OD. I suspect the sprag is failing for 4th and staying locked up when it should be freewheeling. Mine blew up and went through the OD gearset adding $200 to the rebuild. Try 3rd below 55 MPH and see if it does the same. You could have an engine problem.

 

[Husker6.5]

I'm certain the tranny cooler lines are ok, lord knows I've been under the thing enough times, check and look everything over when it's up on the rack for an oil change. During the hour+ test drive with the scan tool hooked up, the line pressures were in normal range during driving and shifting at various speeds/loads. even when it was unlocking/locking the SnapOn Solus scanner said there was nothing wrong, all pressures and signals were normal and it threw no codes in either the ECM or the TCM. What does have me concerned is the burnt tranny fluid and the occasional "floater" I find on the dipstick (was this way when I bought the 'Burb last December) and that an orifice or passage may be partially plugged somewhere inside that is causing the TCC to unlock then relock in a regular cycle when going downhill and the drivetrain is "unloaded" ever since I bought it. I've put about 16,000 miles on it since I bought it, including towing my Camry on a two wheel toter behind it from Mooresville NC where I bought the 'Burb north through the Appalachians to Charleston WV then west back here. It pulled the Camry and toter just fine up the grades and the temp gauge never went above 200* on any of the grades and I didn't feel any transmission slippage. Trip to Chicago and back two weeks after that. 18 hours non-stop to DC first weekend of November, then 18 hours non-stop back two days later just last month. Innumerable trips back and forth between Lincoln and Omaha. Engine is tight and solid at 198K, I burn between a pint and a quart of oil between 3,000 mile changes, and about half of that is spotting on the driveway from the seeping 90* filter adapter O-rings. When I "get on it" on onramps, the shifting seems fine, not as firm as my 30K ago rebuilt 4L80E in my '94 (which was set up with extra clutches and frictions, beefed up sprag, shift kit, etc to deal with a loaded GVW of 9500lbs+ and occasionally a trailer, too), but decent none the less. It's definitely not the coast clutch, I can get that to disengage when I tap the brake pedal when it's on cruise and the rpms drop down to around 900-1000 at hiway speed. When the TCC unlocks, the engine rpm's go up a couple of hundred, then the engine accelerates as the cruise tries to bring the speed back up to the set speed while going down hill and the TCC locks back up. The rpms then drop 150-200 and the TCC unlocks again starting the whole cycle over again every couple of seconds. It's more pronounced the longer and steeper the down grade is. It also does it with the cruise off and trying to use "steady foot" when going down hill.

If the sprag for 4th was failing, it should have already failed in the last year and 16,000 miles you would have thought, especially trailering through the Appalachians. Also, if the sprag was failing, would it cycle repeatedly and rapidly on about a two second cycle rate? I will try it in third tomorrow on the way up to Omaha and see if it does the same thing. I'll let you know.

Although I want to eventually rebuild the transmission to be bullet-proof with a Hughes triple clutch converter, extra elements in the Koleen and Alto clutch packs, upgraded sprag, kevlar band, etc., unfortunately I am unemployed at the moment (well for the last 2 1/2 years of moments) and can't afford even a standard rebuild or parts. That's why I'm trying to brain pick to see if someone out there can match the symptoms to a cause. I'm hoping that it really is just a piece of junk caught in a passage that can be cleared with a good flush and fresh Dextron. $100 plus fluid I can swing, rebuilding a tranny I can't.

 

[great white]

4th OD doesn't over run, it compression brakes.

3rd OD overruns.

3rd manual doesn't over run, it compression brakes.

Right from the from powertrain manual for the 4l80e.

Just putting the info out there.

 

[Husker6.5]

Thanks for the info, GW. Now, can anyone tell me why the TCC keeps unlocking-locking when on a downhill?

 

[WarWagon]

Thanks GW, I must have burned something up in 4th for compression braking as well as 2nd... :rolleyes5: Not sure the gears that require overrun clutches for compression braking, but, the ones that do will shift up fine and run well with power while having trouble holding a vehicle back because it is a separate powerflow/clutch set to do so.

So watching the scanner when the truck 'bucks' what is the commanded gear and TCC lock/unlock? Do they stay the same? This eliminates 'other things' like a bouncing brake switch etc. causing the TCC to be commanded to drop out. Watching input and output RPM's could give you an idea if something is 'unlocking' in the transmission.

The computer will not code if compression braking 'fails' and the RPM's are out of whack. Mine would drop to idle in manual 2nd gear due to an overrun clutch failure and not code. Before the TCC clutch failed I would get some pretty good 'hopping' in 3rd with the TCC override switch on. This was a clutch loosing it's static friction and slipping, and then grabbing for a second before slipping again. Last time I used the TCC override on it before rebuild... I rebuilt it with ratio errors/limp mode before the fluid went bad. GM is sometimes good about giving you warning before you walk from a failure.

The 4L80E is known for valve body wear in the TCC lockup valve boar. This allows slipping as fluid bypasses the valve instead of holding the clutch. This wear typically codes and goes into limp mode preventing OD and TCC lockup. Thus the reason I think it is somewhere is the overrun clutch power flow side of things. They really need fluid changes every 25K to reduce grit wearing this valve.

You can drop the rock guard/ torque converter cover and look for bluing of the outside of the converter where the TCC rides in the front. This would indicate the TCC has burnt up.

Check the main electrical connector for fluid leaks and clean it up if it is wet.

You may be able to get away with a fluid change as the filter may be plugged or o-ring filter seal leaking air. Next would be torque converter rebuild and TCC valve body service although you should rebuild it if you are going this far and I am not sure the TCC is the problem.

Other things to think about are u-joints binding when 'unloaded' and check the rear end out in case it is eating something.

 

[great white]

In simple terms; The 4l80e will compression brake in 4th with the TCC locked up in OD mode because of the way it selects the components in the case. There is no overrun clutch in 4th OD. (need to look that up again to be 100% positive, I'm 99% positive thats right....I read too damn much tech stuff!! )

In 3rd OD it employs an overrun clutch even if you have the TCC engaged. The intent is for it to "freewheel", although I find with my TCC engaged in 3rd OD there does feel lik there's compression braking. BUt reduced. I haven't figured that one out yet. Might just be fluid drag inside the freewheel/sprag clutch itself.

2nd OD is the same a 3rd OD. It employs a freewheel/sprag.

When you move the lever down to 3rd "manual", the freewheel is not employed and you get compression braking. Same with 2nd manual.

It all has to do with power transmission in the transmission hard parts themselves and the valve selections.

Now this is all for "coasting" without power applied. "Power on" is a different story.....

TCC activation is a function of vehicle speed and programming. for example:

That's a stock .cal file for my 1998 L65 (the one I'm running in my truck is.....*ahem*......"slightly" modified)

there are a few other modifiers in there such as temperatures, but that governs whether or not the PCM will even try the TCC lockup.

This one shows why the TCC unlocks when you come off the throttle:

Also a stock .cal file.

If you let off the throttle quickly, it disengages, if you drop to idle (APP setting, not the engine) is disengages.

Abnormal action of the TCC outside these can be a (as WW mentions) a worn valve body (common in 4l80E's with the aluminum body vice the TH400 steel.iron one), a bad solenoid just or another system causing the PCM to kick it off.

Something as seemly unrelated as a PMD going south can give you TCC codes and faults.

Unfortunately, without seeing/scanning your truck I can only give you a feeling for what it should be doing and where you might look.

One thing that's always mentioned on these forums is grounds. In this case, it's easy and they're worth checking/cleaning. Noisy signals can play havoc with the PCM.

Good luck.

 

[great white]

Just a thought; if you remove your foot off the throttle all together, does the TCC stay unlocked?

The programming indicates it should.

If you/the cruise are/is trying to "trail throttle" down the hill, it's possible you're "confusing" the PCM and leading to the lock/unlock scenario.

If this is happening, you may have a component breaking down (temperature, App, etc) as my stock .cal file will allow me to accelerate/trail throttle down hill (or the cruise) without the violent "jerking" behavior you describe.

This type of problem also tends to steer me even more towards a grounding issue and a "dirty" signal. Although it probably would do it on acceleration also if it was grounds. Still, cheap (IE: free) and easy to check.

One other quick check you can make it to pull the electrical harness pass though plug on the side of the trans. The seal often weeps and allows fluid into the plug, causing all manner of strange faults. It's unlikely with only jerking on coast down, but again it's cheap and easy...

Hopes for it being a dirty valve or solenoid are slim. Your transmission guy confirmed this when you did a live/drive scan and saw normal operation and pressures.

There's something else going on.

Since you're OBDII, a manual TCC lock up switch is not an option either. The OBDII system will throw a fit if you wire one in.

Lockup behaviour can only be changed with a reflash and a modified .cal file on an OBDII truck.....well, I think it was BDpower had an add on box but it's something foolish like 600 bucks and has mixed reviews....

 

[Husker6.5]

Thanks so far guys. WW, it's not driveline bind, it is definitely the TCC unlocking and locking (RPM climb and drop on the tach). I have considered the bore for the lock up clutch being worn as the culprit, especially as when we did the live drive the scanner showed all systems normal (electrical/hydraulic) even while it was doing the unlock-lock cycle. Is replacing the solenoid and resleeving the bore something that can be done in the driveway without dropping the tranny and/or the valvebody?

BTW, replacing the brakelight switch on the pedal was the first thing that was done.

GW: ECM was just recently reflashed with the newest GM program for my VIN. If I pull my foot off it completely (with cruise off) I'm not sure, will be out on the Interstate later, will try and see. I think the coast clutch lets the engine drop down to 900-1000rpm till I apply the brake then it drops to idle. I'll check and make sure. I am also beginning to come to the conclusion that the tranny is just getting tired and is in need of a complete rebuilt front to back (rear seal between output shaft and transfer case is leaking a little). I'm one of these people that if the tranny is going to be opened up for something, EVERYTHING gets gone through. Do it right the first time, don't have to do it a second. Now that being said, I am aware of the TCC solenoid bore wear issue, and if the Sonnus kit is an easy driveway install that will cure the problem, then I'm all for it at this time vs. a complete rebuild because I'm on a very limited budget (haven't worked for nearly 3 years due to a severe injury at old job, dealing with the work comp insurance company has been a real PIA, more painful than the two reconstructive surgeries on my shoulder!) and other than the TCC issue the tranny shifts and holds fine. I get NO slippage of the TCC when going up hill pulling a load (see Camry towing in Appalachians comment above).

On a side note, I bought one of the GM service manual DVD's off Ebay for the 'Burb. It doesn't seem to be as detailed as I hoped, kind of lacking in really good detailed diagrams-exploded views and trouble shooting procedures. Doesn't seem to be as detailed as the GM DVD my mechanic has. I went to it's transmission troubleshooting section, and it didn't even have anything on the TCC unlock/lock cycling on deceleration. Are there different versions of this DVD, and is there a GM part # for the best one? This DVD did have GM logos and a PN on it, but it could be a fake. One site advertises that their DVD had 7.2G of info. The one I loaded onto my hard drive was "only" 4.2G of data from another vendor.

It's the old Maintenance Officer in me - always give me the manual so I can do it by the book. Hoo-ah! Scouts out!

 

[Burning oil]

Check you brake light switch. If its shorted it will cause TCC to unlock, or if pedal at a wierd ajustment and barly bumping the switch.
Worth looking at anyway.

 

[dka99burb]

I had the exact same issue as you with my 99 burb. I checked everything too. Even installed a tranny electronics kit that included the harness and solonoids. Of course I replaced the filter and fresh fluid. It didn't help. After coming to a similar conclusion: that it needed the PCM reflashed I noticed the throttle getting touchy. So, having read all the threads on the subject I cleaned my coolant temp sensor, checked all grounds/resistance, tired another APP, etc.:mad2: All the while the throttle pedal kept getting more sensitive. I finally concluded, with help here, that it needed an IP. If you havn't jumped ahead of me by time I am done I will let you know if it worked. Please post if you find an alternate solution.

 

[Husker6.5]

Well, was driving it over the weekend to Omaha and back and here's what I noted: It is far worse in Drive (3rd) than in OD (4th) whether the cruise is on or off when going downhill due to the greater RPM's the engine is turning and hence the greater swing between TCC locked and unlocked. When pulling a hill the TCC stays locked with no slippage whatsoever, even if I accelerate at just below "kickdown" to passing gear - no slippage whatsoever. As I crest the hill and the throttle backed out to maintain the cruise-set speed, the TCC will unlock and RPM will go up 150-200 RPM (depending on road speed). Now, if it was a long, gentle downslope, the TCC would stay unlocked until reaching the bottom of the hill and then would lock back up as the engine RPM started to climb as the cruise tried to maintain the set speed and stay locked up the grade.

Now, if the downgrade was steeper, like most of them were, then the unlock-lock cycle would occur, with more frequency and lurching correlating to the steeper/longer the grade was.

These situations occurred whether I was using the throttle pedal or the cruise, and was always worse using the throttle as my foot is nowhere near as precise as the PCM/ECM at giving input signals to the IP.

I think dka99burb is on to something. If the IP is unable to meter the injection pulse as precisely as the PCM/ECM is commanding (like when being "told" to feather back slightly on fuel as it goes to a light load when going downhill), it may cut out and the TCM then interprets that as "Closed throttle coasting" and unlocks the TCC. That sudden decrease in road speed is sensed by the Cruise Control Logic as "Give 'er more fuel to maintain speed" so the IP is commanded to increase its pulse, the engine begins to pull, the TCM then relocks the TCC, the Cruise then sees the VSS signal as the vehicle is now traveling faster than the set speed, so the PCM/ECM tells the IP to back off a wee bit and the whole cycle starts all over again.

It is very possible that at 199K miles on it, the IP is wearing out. Especially without knowing the vehicle's past history of usage and maintenance (or lack thereof), like if a good diesel fuel supplement like Power Services was used to compensate for the much lower lubricity of the IP by the introduction of ULSD.

It would be just the same when off the Cruise Control, only it would be my brain and right foot substituting for (and far less precisely than) the PCM/ECM and Cruise Control Logic Circuit, with of course much more radical results.

Now let me restate this for those of you just now coming into this thread: The ECM HAS been recently reflashed with the latest program for my VIN from GM. The brakelight switch was replaced and set properly. Transmission fluid level is full (but the fluid does show some signs of past owner overheating and internal slippage). There is not any slipping involved, just the unlocking/locking of the TCC when driving downgrade.

 

[NVW]

It is normal for them to unlock on a down slope.

 

[Husker6.5]

No, it isn't normal for the TCC to unlock and then relock in a rapid cycle when driving downhill on the Interstate, highway, or street. I fact, in my '94 C2500 HD with a freshly rebuilt 4L80E and new ECM, once I'm above 45MPH in either 3rd or OD the TCC stays locked up at all times, except when the road speed drops back down below about 35MPH. The only exception to that is if I'm pulling a good grade under load (9,500-10,000lbs GVW, + or - trailer) in OD, the TCC will unlock for a little more "grunt" up the hill, then downshift to 3rd if need be, before shifting back up to OD and relocking the TCC on the way down the hill. What I'm trying to find is the cause of the unlock-lock cycling of the TCC when going downhill in my '98 K2500 Burb.

 

[Husker6.5]

So, does any one have the answer for my TCC unlock-lock cycling under light load when I'm running downhill?

 

[dka99burb]

Sorry but not yet. Sadly, very sadly, I have not had time to work on mine lately. At the rate that I get to find time in will probably be a few more weeks. If the IP fixes it I will definitely be posting! Come to think of it; if it doesn't I will be posting too....:willynilly:

 

[Husker6.5]

Well, I'll be waiting to hear if your replacing a worn IP solves your problem of the TCC unlock-locking cycle problem when under light load coasting downhill. Unfortunately, I drove half way across the country to buy this ebay special from a used car dealer in Mooresville, NC. While clean looking, it was nowhere near in the mechanical shape he claimed it was after I got it home and had the chance to really go through it. The TCC problems manifested itself going northbound out of Mooresville with the Camry in tow on a toter once I started hitting the grades towards Virginia. The only good things were the motor's solid, new generic rubber on the rims, new carpeting and the leather wasn't torn. Tranny issues since day 1, the factory AM/FM/cass/CD comes on once in a blue moon for a few seconds then goes back off, it had a previous accident and had GMC doors put on the rear that don't line up right, and I have to put new pins and bushings in both front door ASAP when it warms up, needs brakes all around, right rear axle seal seeping, need idler and pitman arms....

 

[Husker6.5]

Any other brains out there wanting to be picked on this subject?:mad2: