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4L80/85E Transmissions. 6L90 Transmission Discuss the 4L80/85E transmissions found in the 6.2/6.5L equipped vehicles and Duramax equipped vans. 6L90 discussion.

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Old 05-29-2019, 02:51 AM  
Beastwerks
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4th Gear Shifting Problems When Hot

Hi Y'all!

Longtime lurker, occasional question asker. I usually am able to figure out the issues on this truck myself but this one has me stumped and I really need some guidance because some of the FAQs and other threads I've lurked have assumed maybe a level of background that either I don't have or my addled brain isn't up to at the moment.

This is what's going on. 1994 2500 6.5 w/ a 4L80E, limited modifications (mostly engine stuff homemade turbomaster, exhaust upgrade, that kind of thing). The transmission has been uppity lately in a strange way. When it's cool it'll do its thing no problem, I can hop on the freeway and hit 60 if I want. It's shifting up and down through stop and go just fine too. After the transmission gets warm though, it's stuck in what I assume is limp mode, not really wanting to get past 25mph or so.

Okay, so I start checking. Fluid first. I've had a little leak on this thing develop sometime last year that I try to stay on top of, but that looks like it's in the right range. Bring out the computer and try my scan tool (GMTD Scantech, whatever the version is that lets you do the tdc offset) checking to see if there's codes and whatnot. DTC 85. Take it for a little spin and see what gears and ratios are happening when and what looks like what's happening is that the computer is commanding 4th and it's still in 3rd (I think at least, not sure if I'm reading that right).

Okay, so there's electrical tests asked for, hop under to take the passthrough off and do some probing. I find some corrosion on the wires and I know that's not helping so I change out the connectors that lead to the trans. Still got the problem. I find a kit that's got the internal electronics so I pull the trigger on that since I know I at least will need the wiring harness since I'm getting leaking there. Change out the VSSes first since they're quick and... well that helps. Now the shifting problem doesn't arise until after the transmission has been warm for a little while it seems, so it's taking longer?

Okay, get around to dropping the pan and changing out the internal electrical bits. The harness definitely needed swapping, new one is much tighter. The old gasket was crap too so that's nice now too. The Pressure Control Solenoid (the big one mounted next to the little on the engine side of the trans) I didn't switch out because the damn screw stripped out on me and I didn't have the tools to deal with that today. I didn't have time to take it on a long drive after I got fluid back into it, but it seems like my issue is back to square one

So that's where I am. Trans gets warm, no 4th (or maybe no OD? I'm not sure what the distinction is). What do I start testing? I'm reading about the TCC PWM and how that may be a culprit with wear over time, but lots of the photos on the faq about it are missing it seems. What are some good next steps/tests and what info can I give you guys? What can I do with the GMTD that would be useful here?

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Old 05-29-2019, 08:59 AM  
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Here is a thread from the stickys above on the " No 4th condition"
https://www.dieselplace.com/forum/57...-how-deal.html

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Old 05-29-2019, 01:37 PM  
johnH123
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If its commanding 4th and not going, it sounds like classic AFL valve wear. Drop that valve body and put a sleeve and new valve in the bore.

https://www.sonnax.com/parts/1990-ac...imit-valve-kit

As far as getting that screw out, take a dremel or die grinder with a cutoff wheel and make a cut across the head of the screw. Then use a big flathead screwdriver to get the screw out (the flathead needs to be wider than the screw head) I had that same thing happen so I replaced all my screws with allen screws so I didn't have to worry about getting out the torx bits.
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Old 05-29-2019, 01:54 PM  
Beastwerks
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Quote:
Originally Posted by johnH123 View Post
If its commanding 4th and not going, it sounds like classic AFL valve wear. Drop that valve body and put a sleeve and new valve in the bore.

https://www.sonnax.com/parts/1990-ac...imit-valve-kit

As far as getting that screw out, take a dremel or die grinder with a cutoff wheel and make a cut across the head of the screw. Then use a big flathead screwdriver to get the screw out (the flathead needs to be wider than the screw head) I had that same thing happen so I replaced all my screws with allen screws so I didn't have to worry about getting out the torx bits.
Yeah, that's the root of the question I'm trying to get to. If the electronics are checking out good what's the next failure set. If I do have to change little inner valve components how do I test which ones are the right ones? Do I have to drop the... valve body? That's the tunnel network that sits directly above the pan, yes? Do I have to pull the transmission to work on that? I've seen some videos of people rebuilding these and it looks like I can't work on the valve body while it's in position, is that correct?

I'm pretty sure I'm using the wrong keywords when trying to look things up too, which I'm sure isn't helping.

Thanks for the advice on the bolt, that's pretty much what I was hoping would work. I didn't have the tooling for it yesterday but I'll get around to that. Do you know what that bolt size is for when I replace it? And why the AFL valve and not the TCC valve? That seems to be one of the big ones on the FAQ, but I have no idea how to test them...

(Also, thanks for the responses guys, I really appreciate the help with this beast. Automatic transmissions are... well they're not my favorite.)
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Old 05-29-2019, 01:57 PM  
Beastwerks
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Also, I'm not sure if it's no 4th or no Overdrive. I was under the impression those were the same thing, which now seems wrong of me. What can I read to tell me that when I'm looking at the scan tool? Or is there some other clever way?
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Old 05-29-2019, 06:38 PM  
johnH123
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4th = overdrive.

yes the valve body is the 'tunnel network' that is above the pan. You can drop the valve body while the trans is in the truck. Just be mindful of the checkballs- don't lose them because they will roll away never to be seen again in the blink of an eye. The bright side is that if you do lose them, you can get new ones at the hardware store- they are just ball bearings.

The only thing you can do to the VB while its in place on the trans is the solenoids. Any internal repairs will need the VB removed. If you remove it, break the bolts loose in a cross pattern, like you would torque an engine head, then remove to keep from warping it. Also remember you'll need and INCH POUNDS torque wrench to reinstall (not foot-pounds).

The TCC and AFL are very, very different. TCC= torque converter clutch, which is a clutch inside the torque converter that locks it from 'slipping' so you get better mileage. The TCC valve is controlled by the TCC solenoid, which is the one next to the big fat pressure control solenoid. it only engages in 3rd or 4th at speeds above 40 mph.

The AFL valve is integral to every GM transmission- it controls line pressure, (in laymans terms, the pressure supplied by the pump to the clutches and components) and the computer adjusts line pressure based on the load and engine speed and various other factors. It is operated by the PCS (pressure control solenoid- the big fat round one)

The issue with them is they wear the bore and get sloppy, so the fix is to drill it out and put a sleeve in to bring it back to spec.


Not sure what size bolt you'll need, but just get it out and take it to the hardware store and they should be able to hook you up. I'm pretty sure it is a metric thread though. (I think it might be m6-1.0x12 or something. but I really don't know)
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Gone but not forgotten: 2001 silverado 2500hd CC/SB, 4x4, 6.0 vortec, 229k miles when I traded it.
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Old 05-30-2019, 12:02 AM  
Beastwerks
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Going back to the electrical I pulled the external plug off to test the PCS prongs ohms to make sure the old one in there is still functioning. Put it back together and drove home with my laptop hooked up to double check what gears I was in. I double checked the DTCs I have and now I'm getting...

28
81
82
83
87

... Which is definitely not what I was getting earlier, except for one time when I forgot to attach the harness. That makes me second guess my external harness, or maybe something further up the food chain. I thought it tested good when I installed it, so what would I look at next in the electrical system? I know how much the electrical system matters on this persnickety thing, so I freshened up the grounding awhile ago for all the ones I could find in the engine bay. How does the transmission ground itself?
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Old 05-30-2019, 09:33 PM  
Glagulator
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Tranny is grounded mechanically. The only ground wire is for the temp sensor.
You might check the 12v. ign. supply (PNK/BLK) to the main conn.
If the grounds or voltage at the PCM were bad the engine would not run.
I have seen some harnesses really bad at the tranny with corrosion.
TF makes the insulation swell then water gets in.
Clear those codes and see if they come back.
The Sonnax AFL repair requires a very expensive tool kit.
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Old 05-31-2019, 08:54 PM  
Beastwerks
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Quote:
Originally Posted by Glagulator View Post
Tranny is grounded mechanically. The only ground wire is for the temp sensor.
You might check the 12v. ign. supply (PNK/BLK) to the main conn.
If the grounds or voltage at the PCM were bad the engine would not run.
I have seen some harnesses really bad at the tranny with corrosion.
TF makes the insulation swell then water gets in.
Clear those codes and see if they come back.
The Sonnax AFL repair requires a very expensive tool kit.
The 12v. power line (Pin E if I remember right) to frame ground is reading 12.4ish volts with the key on. I had replaced both the external and internal harnesses during this because of that exact reason, there was some kind of leak from the internal harness that was doing some kind of evil and the external had some wires poking through the insulation. They're both new now, and much cleaner. I question whether I did the external harness correctly, but I'm getting the same symptoms as I was before I replaced it, just different codes.

Cleared the codes, double checked them. Says I have none currently and 28, 81, 82, 83, 87 in the history. Clear them and it says the same thing. I'm using the GMTD scantech to clear them/read them. Whatever is happening seems to be throwing codes pretty much immediately .

I do have a fairly nice amount of tools at my disposal, and I've got friends with ones I don't so I'm not totally against a process that might need them (we run an independent fabrication shop). I looked into a sonnax reman valve body too in case it would be a big time savings, the guy at the transmission supply near my house was asking almost $500 for one, which is definitely an amount of money where I want to make sure the thing I'm doing will fix the problem for a good length of time (hopefully).
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Old 05-31-2019, 11:03 PM  
johnH123
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when I rebuilt my 4l80e, after about 4 days I had a shifting issue and p0756, shift solenoid error. After months of diagnosis, I pulled the trans and ripped it apart to find that the 2nd clutch lip seal had a big chunk missing, causing the computer to 'think' there was a shift solenoid issue. The moral of the story is that sometimes the computer is tricked into spitting out a code, but the thing it says is faulty is not the real cause of the issue.

I did my AFL in my trans by having a local trans shop do the tooling. I just bought the kit (no tools) and had him do the drilling and I did the install.

Look up Global Transmission parts for all your trans parts needs. They have a VB for less than 300 shipped. great guys to work with.

https://globaltransmissionparts.com/...2003-24204267/

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