4l80e flakey rev and commanding 4th when in 3rd

[MattTheHam]

Good evening all. I am having some strange problems with the 4L80E in my 92 6.5L K2500, and I'm looking for some advice. The truck is new to me and has 180K on it, but the trans was rebuilt by a well known local trans shop at 152k. I've rebuilt blown manual transmissions before, but autos are new to me. Current codes are 68, 85, 87. Fluid was a nice red and smelled fine (as good as ATF can), small amounts of metallic grey stuff on the bottom of the pan. Gears 1-3 work great.

1st: Rev is flakey, it works great and will spin the tires in gravel when you first start the truck, but drive forwards for more than a minute and there is a 5% chance it will shift back into rev. If Rev is there, it is 'spin the tires' strong. If its not, it feels like neutral. No engagement or slipping, just nothing, you cant even tell it was shifted out of P/N. I do have compression braking and low at all times.

2nd: I have a snap-on brick and can see the TCU commanding 4th even when the PRNDL is in Drive (D). It is my understanding that it shouldn't command 4th unless the PRNDL is in OverDrive (OD). The gear ratio show 1.00ish so the trans is clearly still in 3rd. I have ohm-ed the harness from the TCU connector and couldn't find any shorts or out of spec solenoids. This one really has me stumped.

3rd: I have no OD or lockup (both 3rd and 4th lockup are missing) at any temp/speed/throttle level. This might be due to one of the above codes though, I defer to y'alls wisdom.

What has been done: All new speed sensors, AFL valve upgraded, TCC valve upgraded, new A-B solenoids, extended rev servo pin (set for 60-80 thou of travel, I think... I did that over the summer, can find the exact num if needed), new #7 rev/low check-ball seat, new separator plate check-balls.

Pressure Gauge @ idle (PSI):
Rev = 75 (will climb to 210 with light throttle)
N = 90
OD = 90
3rd = 90
2nd = 90
low = 110

1-2 and 2-3 shift time are under 0.4s

Im thinking it needs a full rebuild, but the weirdness of problem 2 makes me hesitant. I would hate to learn it was an electrical gremlin after I pull the trans and pay 1500 to get it rebuilt. Any ideas??
P.S. I'm a full time engineering student so I apologize for slow replies and bad grammar :')

 

[NickTransmissions]

Good evening all. I am having some strange problems with the 4L80E in my 92 6.5L K2500, and I'm looking for some advice. The truck is new to me and has 180K on it, but the trans was rebuilt by a well known local trans shop at 152k. I've rebuilt blown manual transmissions before, but autos are new to me. Current codes are 68, 85, 87. Fluid was a nice red and smelled fine (as good as ATF can), small amounts of metallic grey stuff on the bottom of the pan. Gears 1-3 work great.

1st: Rev is flakey, it works great and will spin the tires in gravel when you first start the truck, but drive forwards for more than a minute and there is a 5% chance it will shift back into rev. If Rev is there, it is 'spin the tires' strong. If its not, it feels like neutral. No engagement or slipping, just nothing, you cant even tell it was shifted out of P/N. I do have compression braking and low at all times.

2nd: I have a snap-on brick and can see the TCU commanding 4th even when the PRNDL is in Drive (D). It is my understanding that it shouldn't command 4th unless the PRNDL is in OverDrive (OD). The gear ratio show 1.00ish so the trans is clearly still in 3rd. I have ohm-ed the harness from the TCU connector and couldn't find any shorts or out of spec solenoids. This one really has me stumped.

3rd: I have no OD or lockup (both 3rd and 4th lockup are missing) at any temp/speed/throttle level. This might be due to one of the above codes though, I defer to y'alls wisdom.

What has been done: All new speed sensors, AFL valve upgraded, TCC valve upgraded, new A-B solenoids, extended rev servo pin (set for 60-80 thou of travel, I think... I did that over the summer, can find the exact num if needed), new #7 rev/low check-ball seat, new separator plate check-balls.

Pressure Gauge @ idle (PSI):
Rev = 75 (will climb to 210 with light throttle)
N = 90
OD = 90
3rd = 90
2nd = 90
low = 110

1-2 and 2-3 shift time are under 0.4s

Im thinking it needs a full rebuild, but the weirdness of problem 2 makes me hesitant. I would hate to learn it was an electrical gremlin after I pull the trans and pay 1500 to get it rebuilt. Any ideas??
P.S. I'm a full time engineering student so I apologize for slow replies and bad grammar :')

Two of the three codes you listed are gear ratio error/component slipping codes. They are sometimes caused by a faulty input speed sensor but most of the time it's slippage/burnt applied elements that sets and stores those codes (ie burnt overdrive clutch pack). The other one is related to shift solenoid B, which is installed behind the 2-3 shift valve but sometimes indicates worn spring(s) in the 1-2 and/or 2-3 shift valve bores.

Try swapping the speed sensors, putting the rearward one in the front and vise versa then drive it and let us know what (if anything) happens.

Meanwhile, a couple questions for you:

> Do you have 4th gear at all (you note shift times for the 1-2 and 2-3 but nothing for the 3-4)?

> What happens when it's time to shift from 3rd to 4th?

You may want to verify your L/R servo travel and make sure it's not dragging on the reaction carrier's band-bearing surface... .080" should be fine but .060" may or may not be a bit tight (it may also be perfectly fine). You can check for drag by spinning your output shaft to see if there's any signs of the band binding/dragging on the carrier. I typically shoot for between .080" and .125"; other builders give numbers ranging from .075"-.175" so you have some wiggle room with the adjustment...The main thing is you want to avoid drag otherwise you will be wearing the band out prematurely.

Im thinking it will need to come out and be torn down, inspected and then rebuilt but you can try the above things first to see what happens.

 

[MattTheHam]

Two of the three codes you listed are gear ratio error/component slipping codes. They are sometimes caused by a faulty input speed sensor but most of the time it's slippage/burnt applied elements that sets and stores those codes (ie burnt overdrive clutch pack). The other one is related to shift solenoid B, which is installed behind the 2-3 shift valve but sometimes indicates worn spring(s) in the 1-2 and/or 2-3 shift valve bores.

Try swapping the speed sensors, putting the rearward one in the front and vise versa then drive it and let us know what (if anything) happens.

Meanwhile, a couple questions for you:

> Do you have 4th gear at all (you note shift times for the 1-2 and 2-3 but nothing for the 3-4)?

> What happens when it's time to shift from 3rd to 4th?

You may want to verify your L/R servo travel and make sure it's not dragging on the reaction carrier's band-bearing surface... .080" should be fine but .060" may or may not be a bit tight (it may also be perfectly fine). You can check for drag by spinning your output shaft to see if there's any signs of the band binding/dragging on the carrier. I typically shoot for between .080" and .125"; other builders give numbers ranging from .075"-.175" so you have some wiggle room with the adjustment...The main thing is you want to avoid drag otherwise you will be wearing the band out prematurely.

Im thinking it will need to come out and be torn down, inspected and then rebuilt but you can try the above things first to see what happens.

Thanks for the advice Nick!

I tried swapping the speed sensor, no change

There is no fourth at all. When in OD range I can see the computer command it, but it stays in 3rd and never shifts into 4th. After a few seconds it throws code68 and stops commanding 4th and puts it back into 3rd. Scanner is showing the 3-4 shift time as 0.02s.

Checked my notes on the L/R servo, it was set to 0.085".

 

[NickTransmissions]

Thanks for the advice Nick!

I tried swapping the speed sensor, no change

There is no fourth at all. I can see the computer command it, but it stays in 3rd and never shifts into 4th. After a few seconds it throws code68 and forces the trans back into 3rd. Scanner is showing the 3-4 shift time as 0.02s.

Checked my notes on the L/R servo, it was set to 0.085".

Gotcha, I retract my prev statement about it needing to come out in light of this new info (i thought you were neutralizing in 4th)...Problem seems to be sourced in the VB....I'd drop the valve body, remove/set aside the accumulator housing, spacer plate and then pull/inspect the O-ring on the 3/rev check ball capsule (replace it if need be if it's leaking fluid between those circuits, you'll have weird issues like you described), 3-4 shift valve as well as the springs behind the 1-2 and 2-3 shift valves (replace those with the Sonnax 1-2/2-3 shift valve kit...Ohm out all solenoids to ensure they are showing continuity. Inspect all valves for wear or other concerns....clean out the bores, ensure they are free of trans fluid then spray the bores and valves with WD40...install the valves and use a small flat blade screwdriver to confirm they move freely under spring tension and return to rest position when you remove the screwdriver.

Id also replace the pressure switch manifold assembly (oblong-looking thing that takes six 8mm bolts) and wiring harness with the Rostra harness for 1991-2003 model year units.

Here's Sonnax instructions with troubleshooting tips and a vb diagram...Its a fairly easy job that doesn't require any special tools; just a little patience, attention to detail and some nitrile gloves...

You must be registered for see images attach

4L80E Valve body diagram (Stock) with vacuum testing instructions, if you have the Sonnax vac tester and an air pump:

You must be registered for see images attach

 

[MattTheHam]

Gotcha, I retract my prev statement about it needing to come out in light of this new info (i thought you were neutralizing in 4th)...Problem seems to be sourced in the VB....I'd drop the valve body, remove/set aside the accumulator housing, spacer plate and then pull/inspect the O-ring on the 3/rev check ball capsule (replace it if need be if it's leaking fluid between those circuits, you'll have weird issues like you described), 3-4 shift valve as well as the springs behind the 1-2 and 2-3 shift valves (replace those with the Sonnax 1-2/2-3 shift valve kit...Ohm out all solenoids to ensure they are showing continuity. Inspect all valves for wear or other concerns....clean out the bores, ensure they are free of trans fluid then spray the bores and valves with WD40...install the valves and use a small flat blade screwdriver to confirm they move freely under spring tension and return to rest position when you remove the screwdriver.

Id also replace the pressure switch manifold assembly (oblong-looking thing that takes six 8mm bolts) and wiring harness with the Rostra harness for 1991-2003 model year units.

Here's Sonnax instructions with troubleshooting tips and a vb diagram...Its a fairly easy job that doesn't require any special tools; just a little patience, attention to detail and some nitrile gloves...

You must be registered for see images attach

4L80E Valve body diagram (Stock) with vacuum testing instructions, if you have the Sonnax vac tester and an air pump:

You must be registered for see images attach

Thank you Nick for the expertise!

I'll pull the valve body and do as you have suggested. I don't have a vac tester, but I can see if a local shop has one and will test it. I'm about to go into finals week so it might be a few days before I can get you updated with results.

 

[NickTransmissions]

Thank you Nick for the expertise!

I'll pull the valve body and do as you have suggested. I don't have a vac tester, but I can see if a local shop has one and will test it. I'm about to go into finals week so it might be a few days before I can get you updated with results.

Yea man, take your time and focus on your exams...Trans can wait but you only have one shot at nailing your tests...

 

[MattTheHam]

Yea man, take your time and focus on your exams...Trans can wait but you only have one shot at nailing your tests...

Alright, finals are done and repairs completed, unfortunately not all is working. New oring on the 3/rev check ball shell, new harness, checked all solenoids, checked all shift valve movement, and replaced the 1-2 2-3 shift springs. Also replaced the tcc valve.

Fixed - TC Lockup is back and working great (in third)

Still broken -
Rev. When I first start the truck, it's there and strong, then nothing (same as neutral) after 2ish min of driving forwards.
4th. Computer commands the shift but the trans never leaves 3rd

Thanks!

 

[NickTransmissions]

Alright, finals are done and repairs completed, unfortunately not all is working. New oring on the 3/rev check ball shell, new harness, checked all solenoids, checked all shift valve movement, and replaced the 1-2 2-3 shift springs. Also replaced the tcc valve.

Fixed - TC Lockup is back and working great (in third)

Still broken -
Rev. When I first start the truck, it's there and strong, then nothing (same as neutral) after 2ish min of driving forwards.
4th. Computer commands the shift but the trans never leaves 3rd

Thanks!

What is the 'third and reverse check ball shell'?

Did you take any pictures of the work you did? Also did you replace the valve body and case gaskets? If so, did you use the correct gaskets (or did you purchase a paper and rubber kit)?

What does this mean: "Rev. When I first start the truck, it's there and strong, then nothing (same as neutral) after 2ish min of driving forwards" - Are you describing what is happening when you're in reverse or when you place it into 'Drive' and try to move forward?

 

[MattTheHam]

What is the 'third and reverse check ball shell'?

Did you take any pictures of the work you did? Also did you replace the valve body and case gaskets? If so, did you use the correct gaskets (or did you purchase a paper and rubber kit)?

What does this mean: "Rev. When I first start the truck, it's there and strong, then nothing (same as neutral) after 2ish min of driving forwards" - Are you describing what is happening when you're in reverse or when you place it into 'Drive' and try to move forward?

Sadly I didn't take any pictures

By check ball shell, I meant the 3rd/rev sleeve as shown in the diagram you sent me.

I did replace the gaskets, both of the gaskets in the valve body-plate-case stack are black gasket material.

4L80E Valve Body Separator Plate Gasket Set (1990-1996) Upper & Lower

This 4L80E valve body replacement upper and lower spacer plate kit covers model years 1990-1996. Fast shipping on this always in stock item!!

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When I first start the truck in the morning after it's been setting over night, rev works as it should. Truck goes backwards, everything feels strong. If I put the truck in drive and loop the parking lot (1/4 mile), stop and put it into reverse, it's still there and working. But if I drive across campus (1 mile?), stop and shift into reverse, rev is completely gone. It feels like I'm in neutral. Forward gears work great all the time, every time (except 4th which never works).

 

[Schurkey]

Post

I'm counting ten different threads where you leave a reply that says nothing but "Post".

WTF, dude?