4l80e hard shift from 2-3 gear.

[Mildphil]

Replaced the A and B solenoid and it didn't seem to help. I'm going to go on a long drive and see if it changes. Anybody have any other ideas?

 

[andy396]

Not sure if this is your problem or not, but definitely a good read:

https://www.sonnax.com/tech_resources/88-diagnosing-the-elusive-no-reverse-with-the-4l80-e

 

[Mildphil]

Not sure if this is your problem or not, but definitely a good read:

https://www.sonnax.com/tech_resources/88-diagnosing-the-elusive-no-reverse-with-the-4l80-e

Thanks. Lots of good information, but I'm still lost as to what may be causing it. The only idea I have is when I changed my fluid about a month ago when I was trying to solve my shuddering issue (ended up being cap+rotor). I'm worried the fluid change may have knocked loose some junk that may have been floating around. The old fluid wasn't too bad though.

 

[andy396]

Here's a complete guess, but if you look at what's happening on a 2-3 shift as well as R, which is where you describe the symptom's, it kind of points to an issue with the direct clutch. It's the only unique thing that is being applied in 2-3 and R. It's already applied going 3-4 which is perhaps why the 3-4 shift feels okay. I'm not familiar with how either a 4L80E direct clutch or something in the circuit could fail, but maybe it gives you something to at least start Googling on. I've heard a bad reverse boost valve can cause excessive pressure to bleed into the direct clutch (which would make for a hard shift) and break stuff. Might be worth while connecting to the pressure test port and getting some pressure readings.

Another guess is the intermediate sprag (like some how it doesn't want to release) but it doesn't appear to be doing anything with R and I'm not thinking that would give you a hard shift symptom.

You must be registered for see images attach

There's more info in Vol 4 of the '98 manual here:

https://redirect.viglink.com/?format=go&jsonp=vglnk_156475641208912&key=e4c2ffeaa62ac9a00fc17119badaef3c&libId=jyu7k9m601000blq000DA5rrmpfqi&loc=https://www.gmt400.com/threads/88-95-service-manuals.43575/#post-1029300&v=1&out=http://www.mediafire.com/download/clydtami61e4t2s/1998_GMT-98_CK-4_SERVICE_MANUAL-VOLUME_4_of_4.pdf&ref=https://www.gmt400.com/search/26155535/?q=repair+manuals&o=date&title=88-95 Service Manuals | GMT400 - The Ultimate 88-98 GM Truck Forum&txt=1998_GMT-98_CK-4_SERVICE_MANUAL-VOLUME_4_of_4

Sonnex has a fix for the reverse boost valve, but if that is it, it might be too late and a complete tear down may be in order.

https://www.sonnax.com/parts/1985-boost-valve-kit

 

[Mildphil]

Here's a complete guess, but if you look at what's happening on a 2-3 shift as well as R, which is where you describe the symptom's, it kind of points to an issue with the direct clutch. It's the only unique thing that is being applied in 2-3 and R. It's already applied going 3-4 which is perhaps why the 3-4 shift feels okay. I'm not familiar with how either a 4L80E direct clutch or something in the circuit could fail, but maybe it gives you something to at least start Googling on. I've heard a bad reverse boost valve can cause excessive pressure to bleed into the direct clutch (which would make for a hard shift) and break stuff. Might be worth while connecting to the pressure test port and getting some pressure readings.

Another guess is the intermediate sprag (like some how it doesn't want to release) but it doesn't appear to be doing anything with R and I'm not thinking that would give you a hard shift symptom.

You must be registered for see images attach

There's more info in Vol 4 of the '98 manual here:

https://redirect.viglink.com/?format=go&jsonp=vglnk_156475641208912&key=e4c2ffeaa62ac9a00fc17119badaef3c&libId=jyu7k9m601000blq000DA5rrmpfqi&loc=https://www.gmt400.com/threads/88-95-service-manuals.43575/#post-1029300&v=1&out=http://www.mediafire.com/download/clydtami61e4t2s/1998_GMT-98_CK-4_SERVICE_MANUAL-VOLUME_4_of_4.pdf&ref=https://www.gmt400.com/search/26155535/?q=repair+manuals&o=date&title=88-95 Service Manuals | GMT400 - The Ultimate 88-98 GM Truck Forum&txt=1998_GMT-98_CK-4_SERVICE_MANUAL-VOLUME_4_of_4

Sonnex has a fix for the reverse boost valve, but if that is it, it might be too late and a complete tear down may be in order.

https://www.sonnax.com/parts/1985-boost-valve-kit

Thanks for all the info, I'm going to start doing some research now. Driving it to work today it didn't really shift as hard in third. Very smooth in every other gear. The symptoms seem to be the worst when accelerating fast, when the outside temp is hot (95+ degrees) and when the coolant is hot (about 195 degrees, usually it's 188-192 in the cooler months).

 

[Supercharged111]

I have to wonder if it isn't in the valve body, accumulator or something? Those are moving parts that wear out over time and not the end of the world to replace.

 

[andy396]

The symptoms seem to be the worst when accelerating fast, when the outside temp is hot (95+ degrees) and when the coolant is hot (about 195 degrees, usually it's 188-192 in the cooler months).

Fast acceleration = high line pressure
High coolant temperature = low viscosity trans oil

Either one of those things would make a cross flow issue worse which supports the reverse boost going bad. Might be worth swapping in a Sonnax valve. You can get to it by just dropping the pan.

 

[MIHELA]

The reverse boost valve should really be replaced as a maintenance item if the pan is off because its easy to change but it's a trans killing issue. It would be a good idea to inspect the reverse servo piston while you are in there. That will require a gasket.

 

[Mildphil]

Fast acceleration = high line pressure
High coolant temperature = low viscosity trans oil

Either one of those things would make a cross flow issue worse which supports the reverse boost going bad. Might be worth swapping in a Sonnax valve. You can get to it by just dropping the pan.

Makes sense, especially considering if I accelerate very slow (below 1500 rpm) it shifts into 3rd gear fine. I'll order that valve and see what happens. Based on the information I've provided, do you think this will solve my issue? Should I replace anything else while I'm under there? Thank you.

 

[Mildphil]

The reverse boost valve should really be replaced as a maintenance item if the pan is off because its easy to change but it's a trans killing issue. It would be a good idea to inspect the reverse servo piston while you are in there. That will require a gasket.

What exactly is the reverse servo piston? I'm not very knowledgeable with transmissions besides the basics.