4L80E lockup issues

[jhellwig]

I am having some issues with my suburban not wanting to stay locked up. It has done it every now and then since I owned it but seams to be doing it more lately. When driving on relatively flat road around 55 it feels like it locks up when you let off the throttle but unlocks when you reapply throttle to maintain speed. The issue is eagerated even more at 75 it seams. The fluid way changed about 10k ago and the vehicle has 150k on it.

Where do I need to start looking on this? Being a 95 it is only obd1 so data is hard to collect. There doesn’t seam to be any other issues with it unless you manually shift it then it doesn’t want to shift on its own for a while or till you shut it off and turn it back on. I wonder if it is an electrical issue or sensor issue.

 

[Smoqueed]

My dad has a Chevy van with the 4l80e. He bought it with a rebuilt tranny/torque converter, but the torque converter was no good and they replaced it under warranty. He had similar issues once the tranny warmed up until they replaced the converter. The transmission temps would also skyrocket on the highway. I was reading that it could be the lockup solenoid going bad, or the torque converter itself dying. In his case, it was a bad rebuild (or maybe they never even replaced the TQ in the first place). Being obd1 idk if you can check the tranny temps on the highway, but you might be able to use an infrared thermometer and get an idea? The fact that the issue has been there for a while and is getting worse makes me think the converter is going out, but I'm by no means a professional.

 

[CourierDriver]

I use Lucas Transmission fix in my old 2005 Lasabre.Keeps mine running great

 

[bullwinkle]

TC trouble generally manifests as a "rumble" or shudder when gentle throttle is applied to accelerate or climb a small hill without uncoupling the converter clutch. On my '93 there is a PWM controlled solenoid that engages the clutch gradually-fortunately mine has only has had TPS issues on the mechanical injection pump that caused limp mode & codes, not TCC issues. This may help a little-
https://www.thoughtco.com/diagnosing-gm-converter-lock-up-problems-262653

 

[bullwinkle]

Also, at least in the 4L60 I had, failure of the TCC can cause fluid temps to skyrocket, to the point of burning fluid, if driven any interstate distances due to the TC slipping continuously & heating the ATF.

 

[jhellwig]

Originally Posted By: CourierDriver
I use Lucas Transmission fix in my old 2005 Lasabre.Keeps mine running great


That is not going to fix anything in an electronic transmission unless it is worn to the point that the higher viscosity would help.



I don’t know what the temp is running but it doesn’t seam to be getting extreamly hot. Today I drove the same amount and same roads with about the same outside temperature and it behaved just fine. Leads me to think it is an electrical issue.

 

[clinebarger]

Need more Input! Does it ever reapply after letting off & getting back-on the throttle??

The TCC releasing at 0% Throttle Position is completely normal for a stock 4L80E calibration (Any TP below 2% in most calibrations) & re-enables the TCC apply above 4% TP.
This is done because if your coming down a long steep hill with a heavy load....There is NOT enough line Pressure available to keep the clutch from slipping against the weight of the vehicle.

A reapply delay is generally also in the calibration to allow the PCM to do all it's TCC Enable self checks before reapplying the TCC.

No TCC apply/Falls out of lock-up HOT is quite common on high mileage 4L80E's.....
*Stock converter clutch & cover are trash!
*Leaking TCC regulated apply pressure from a worn TCC regulator valve bore.
*Leaking TCC PWM Solenoid....This solenoid gets worked to death by the PWM control. (Checkball seat erosion).
*Worn Stator Shaft Bushings.


Diagnostic Aids....
*Do you have a Graphing Volt Meter or a Lap Scope?..... Really the only way to view the PWM waveform to the TCC PWM Solenoid!

*Have you checked for codes stored in the PCM?

*Do you have a scan tool capable of reading live data, Believe it or not....Trans data is pretty good on your 1995 PCM!



Monitor the following....
*TCC PWM Duty Cycle, Read in percentage of ON time, Anything above 70% DC should fully lock the converter clutch.
*TCC SLIP, Read in RPM, Above 70% DC should drop TCC Slip value below 50 RPM.
*TCC Enable, Read Yes/No, Needs to display YES to allow TCC lock-up.

****Without using a GM Tech 1 or Tech 2, The TCC Duty Cycle CAN be displayed backwards (OFF Time)!!!! So 30% Off Time will equal 70% On Time,

 

[jhellwig]

Some times it reapplies after letting off the throttle and getting back on. It seams to go through fits of it when it does it. Sometimes it comes out of it after a few miles other times it does it till you slow or stop and start going again. There doesn’t appear to be a rhyme or reason to it yet.



I do have a portable scope but not graphing meter. I haven’t checked for stored codes yet. I wish I could find a obd1 scan tool(I haven looked for one that hard) but I get by with a jumper wire so far.


I am hoping that it is not rebuild time. Don’t realy need to deal with that at this time. When I changed the fluid and filter it did not appear like there was any cause for concern. Normal amount of metal on the magnet and the fluid was a normal appearance and didn’t smell burnt. Other than a couple stripped pan bolts under the cross member no issues. The previous owners covered up a lot of stuff so who knows.

 

[clinebarger]

Originally Posted By: jhellwig
I do have a portable scope but not graphing meter.


Here is a waveform I drew in paint, It's not to scale! But I would say this is right around 85% Duty Cycle, You will see higher frequency during actual testing.

The voltage spike in the second OFF pattern represents the Magnetic Field Collapse that happens with ALL solenoids, It won't be that pronounced....Again I was using a mouse to draw it & I'm not an artist!

I would start with a 20 volt DC scale, And a 1 second time base. Hopefully you can record the data??

*If the waveform disappears And just rides the 12VDC.....The PCM is most likely commanding TCC release.

*If the Field Collapse waveform starts changing over time....The solenoid has a Pintle movement issue or a Coil issue.

 

[XL1200RFan]

While not specific to the original post, I thought I'd mention (for those who may not be aware) that Sonnax.com is a great resource if you want to learn what the most common problems are for a given transmission. Sonnax makes well engineered upgrades for most automatics, many of which can be added to the valve body without transmission removal. And while valve body work requires skill & patience, it is not rocket science. If a trans is not too far gone, incorporating the upgrades (from TransGo as well) can add much life to an automatic.

 

[jhellwig]

Originally Posted By: clinebarger
Originally Posted By: jhellwig
I do have a portable scope but not graphing meter.


Here is a waveform I drew in paint, It's not to scale! But I would say this is right around 85% Duty Cycle, You will see higher frequency during actual testing.

The voltage spike in the second OFF pattern represents the Magnetic Field Collapse that happens with ALL solenoids, It won't be that pronounced....Again I was using a mouse to draw it & I'm not an artist!

I would start with a 20 volt DC scale, And a 1 second time base. Hopefully you can record the data??

*If the waveform disappears And just rides the 12VDC.....The PCM is most likely commanding TCC release.

*If the Field Collapse waveform starts changing over time....The solenoid has a Pintle movement issue or a Coil issue.

I will give it a try when I get a chance. Thanks.

 

[jhellwig]

Here is some pics of the waveforms and the speed and rpm. The higher duty cycle one is going uphill and the other is either downhill or relatively flat ground.

One thing I did notice is that after lockup is commanded it takes about a second or two before it applies. When I let off completely on the throttle lockup disengages completely. Some times if you were heavy enough into the throttle with it commanding almost full lockup and slip off the throttle it disengages with a jerk. I can never get it to go below 50-60% duty cycle before it disengages. Duty cycle varies with the amount the throttle is pressed. More throttle gives me higher duty cycle. It also disengages when you give it enough throttle to downshift as it should.

I will see if I can get the actual waveforms off the scope later tonight.

It is a relatively nice day today. Lower 70s with a breeze. The operation of the trans today was what it typically does in this weather.

 

[clinebarger]

Thanks for the waveforms....Looks very healthy (electrically)! I would really like to see what happens when it loses lock-up altogether.



"One thing I did notice is that after lockup is commanded it takes about a second or two before it applies."
This could very well be leakage at the TCC Regulator Valve, Can also cause loss of lock-up hot.

Sonnax 34994-01K is a drop-in TCC Regulator Valve that uses a PTFE seal to shore up TCC apply pressure leaks. Requires removal of the valve body but not a very difficult repair.
Replace the TCC PWM Solenoid at the same time!

 

[jhellwig]

I apparently don't have the right software on my computer to talk to the scope so here are better pictures of the waveforms.

If I need to take the valve body off for that one part is there any other wear item I should look for or replace while I am in there?

 

[jhellwig]

And apparently I was holding my iPad upside down when I took the pictures.

System voltage was about 13.9 when I was doing this for reference. I like that the scope give the rms values also.

 

[jhellwig]

I am woundering if I need to do the manual valve kit also for my issues of sticking in second gear after i have manually shifted down to first.

 

[clinebarger]

Originally Posted by jhellwig
If I need to take the valve body off for that one part is there any other wear item I should look for or replace while I am in there?


AFL Valve bore is known to wear from valve side-loading. This valve regulates pressure to all the Solenoids.

TransGo part# 48-ACT-TL is the most cost effective solution, Comes with the Reamer & Guide plus 6 oversized AFL valves though you only need one. You can sell the kit after your done.
Summit Racing has it for $90.

I recommend replacing all 3 thimble filters. One in the case (TCC Solenoid Feed), One under the separator plate in the valve body (PCS-AFL Feed), And one at the end of Manual Valve (Shift Solenoid AFL Feed).

Replacing the rest of the electronics.....Assuming they're original is a good idea based on age alone.

 

[jhellwig]

I will give all that a try eventually. I have a good couple of months of work I need to do before I can get to this.

 

[clinebarger]

Originally Posted by jhellwig
I am woundering if I need to do the manual valve kit also for my issues of sticking in second gear after i have manually shifted down to first.


??? The only Manual Valve issues I've seen on these is when it gets bent from human error. Now I have seen symptoms like this with low AFL pressure caused by a worn AFL Valve bore. As both shift solenoids are "Off" in 2nd gear.....So if you don't have enough AFL pressure, That will be the default ratio!

But, I would like some shift solenoid scan data while it's acting up!

 

[jhellwig]

Originally Posted by clinebarger
AFL Valve bore is known to wear from valve side-loading. This valve regulates pressure to all the Solenoids.

TransGo part# 48-ACT-TL is the most cost effective solution, Comes with the Reamer & Guide plus 6 oversized AFL valves though you only need one. You can sell the kit after your done.
Summit Racing has it for $90.

Is there any advantage to the transgo kit verses the sonnax 34200-16K kit other than the cost? I realise the tool kit would be needed for the sonnax kit which is about $300 bucks for the tools but the valves look very different. It looks like the sleeve part of the sonnax would be a better long term solution.