4l80e rebuild gone wrong...

[HiCotton]

I'll put it back together tonight after I check that servo and with the new separator plate. The t-case stick is pulled back, so unless something changed in the t-case, it should be in high, but I will double check.

I have a twin stick NP205 and ORD shift rails. I believe breaking the output shaft is a thing when the t-case is in low and the other in high, but I was driving it for 3 years and almost never moved the sticks. I pulled off the highway and went to go at a stop sign and it went boom! It was making a rattling noise but I thought it was the loose shifter linkage. Could have been something else. I'll find out when I get out moving again.

I got ya. I’m just trying to think of everything. Don’t want us to miss anything.

 

[TubeTruck]

@NickTransmissions

Ok, Quick update. I'll probably be pulling the transmission out this weekend. I took it for a very short ride. It shifts from 1st to 2nd at about 2500rpm, normal, when it gets into second it bogs down, like I said like the brake is on. I rode it out for a second though into higher rpm and it freed up. I took my foot off the gas and it bogged again. What's happening is it's constantly trying to shift between 1st and 2nd gear and holding both. I'm assuming it has an internal leak?

 

[TubeTruck]

I got ya. I’m just trying to think of everything. Don’t want us to miss anything.

Np. Thanks. I could use all the help I can get at this point, lol. I'm thinking the first time I rebuilt the t-case I didn't get something right and it was barely in that lo/hi state. I didn't have a front driveshaft for the 3 years I was driving it so I wouldn't have noticed it. In the short ride I just took I didn't hear any of the rattle sounds it had before. So I'm hoping the t-case is good now.

 

[NickTransmissions]

@NickTransmissions

Ok, Quick update. I'll probably be pulling the transmission out this weekend. I took it for a very short ride. It shifts from 1st to 2nd at about 2500rpm, normal, when it gets into second it bogs down, like I said like the brake is on. I rode it out for a second though into higher rpm and it freed up. I took my foot off the gas and it bogged again. What's happening is it's constantly trying to shift between 1st and 2nd gear and holding both. I'm assuming it has an internal leak?

The forward clutch, which is responsible for giving you first gear in Drive, is applied and the low roller clutch assembly is holding...Upon the shift to 2nd gear, the intermediate clutch comes on, the intermediate sprag begins holding and the low roller clutch releases and begins to free-wheel. The forward clutch stays on at all times until you stop and shift into P, R or N.

So if it's binding and releasing intermittently, it sounds like something could be going wrong with the low roller clutch assembly (it's binding, debris/metal is getting into it, etc). It's also possible that there's a cross leak in the L/R circuit, partially applying the band as if it was in manual low when you're in second gear for some reason but that's a guess...

I'd recommend you replace all three one way clutch assemblies in the transmission; replace the low roller clutch assembly out of an abundance caution even if you replaced it initially as they are cheap and it's good insurance. Also pay close attn to the valve body to case gasket and valve body-spacer plate gasket for any restrictions, gasket blocking holes or tears in the gasket...

What model year is your 4L80E?

 

[TubeTruck]

The forward clutch, which is responsible for giving you first gear in Drive, is applied and the low roller clutch assembly is holding...Upon the shift to 2nd gear, the intermediate clutch comes on, the intermediate sprag begins holding and the low roller clutch releases and begins to free-wheel. The forward clutch stays on at all times until you stop and shift into P, R or N.

So if it's binding and releasing intermittently, it sounds like something could be going wrong with the low roller clutch assembly (it's binding, debris/metal is getting into it, etc). It's also possible that there's a cross leak in the L/R circuit, partially applying the band as if it was in manual low when you're in second gear for some reason but that's a guess...

I'd recommend you replace all three one way clutch assemblies in the transmission; replace the low roller clutch assembly out of an abundance caution even if you replaced it initially as they are cheap and it's good insurance. Also pay close attn to the valve body to case gasket and valve body-spacer plate gasket for any restrictions, gasket blocking holes or tears in the gasket...

What model year is your 4L80E?

It came out of a 1998 Suburban with a 454.

I took the transmission apart initially because of the output shaft, but it was ready to grenade. I cleaned it thoroughly and took apart/replaced everything to make sure all the bits were out. Here's what I found on my initial teardown.

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[Matt69olds]

Cracked reverse servo piston? Or defective seals on the accumulator piston inside the servo?

I don’t have a hydraulic chart handy, if 2nd clutch oil was leaking into the band apply it might stroke the piston just enough for the band to drag, causing the bind up sensation.

Might be worth dropping the valve body and servo first for inspection. If you find nothing, it’s one less thing to remove during the autopsy!!

 

[NickTransmissions]

Cracked reverse servo piston? Or defective seals on the accumulator piston inside the servo?

If the L/R piston itself is cracked, l would think he'd have no reverse (depending on location and severity of the crack). I also suspect the 1-2 acc circuit could be x-leaking into the L/R circuit (or low roller issue) but we wont know until the post-mortim report...

It came out of a 1998 Suburban with a 454.

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Ouch.

 

[TubeTruck]

If the L/R piston itself is cracked, l would think he'd have no reverse (depending on location and severity of the crack). I also suspect the 1-2 acc circuit could be x-leaking into the L/R circuit (or low roller issue) but we wont know until the post-mortim report...


Ouch.

Pulled the trans apart. Couldn't see anything obvious other than my assumption. Something was staying engaged. Found that the direct clutches are smoked. What would keep them engaged?

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There's no nicks in the piston. Everything still felt good and spun fine when I was taking it apart. No sign of a band dragging. No signs of heavy wear inside the direct clutch where the rings seal. I can't catch my finger on anything and it doesn't feel wavy. Both sprags only spun ccw.

I carefully pulled the valve body off and the gaskets were centered perfectly.

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But since my tranny had 250k+ miles on it I ordered a good used direct drum and center support from Global Transmissions. A full set of clutches and steels, and a bushing kit just in case.

Just to give you an idea of my skill level. I'm an Industrial Technician by trade so doing this type of thing is in my wheelhouse, but this is the first time attempting a transmission.

What else to check?

 

[NickTransmissions]

Pulled the trans apart. Couldn't see anything obvious other than my assumption. Something was staying engaged. Found that the direct clutches are smoked. What would keep them engaged?


What else to check?

Damn, lol - smoked is right!

Those look like were dragging real bad, if that was the source of your issue.

What was your clearance when you rebuilt the trans for the first time?

Rip all parts from that stupid HD2 kit from the transmission except the direct clutch high rate return springs (assuming you installed them). If you had that ******* sandwich style HD2 plate installed, that is my chief suspect (major cross leaks).

 

[TubeTruck]

Just getting back to be able to work on this thing, and I figured out what I did wrong in 2 seconds.
I ordered a new forward drum and center support and laid them on the table next to my old ones. Today I come out and just for fun, spun the sprag on the bottom of the forward drum.. yeah, the new one went CW, mine goes CCW.

Hopefully I'll be able to get this thing back together and in by this weekend.

@NickTransmissions when you say rip out the HD2 kit, do you mean everything? I've already drilled the pump, so no going back unless I buy a new one. I have a stock plate to replace the sandwich style HD2 plate, and I've patched the relief hole with a set screw. Can I leave everything else alone? Or is there something else I should remove?

 

[NickTransmissions]

@NickTransmissions when you say rip out the HD2 kit, do you mean everything? I've already drilled the pump, so no going back unless I buy a new one. I have a stock plate to replace the sandwich style HD2 plate, and I've patched the relief hole with a set screw. Can I leave everything else alone? Or is there something else I should remove?

You are good...Drilling the pump is fine, i do that on all my builds (same with all my lube mods to those units). You can also leave in the high rate return springs in the direct drum. Sounds like you patched the hole in the valve body and replaced the spacer plate with a stock plate so everything else should be fine. Dont feel bad about the sprag, anyone can make that mistake. It's on the direct drum, which nests in from of the center support...

 

[Matt69olds]

Installing the sprag backwards is a mistake you only make ONCE.

One of the first 400s I built I put the sprag on backwards. No 2nd gear, locked up in 3rd.

 

[TubeTruck]

Got it back together and in. It's drivable. Something still doesn't feel quite right, but I'll live with it. If I get on the gas at any speed it hesitates a bit and occasionally feels like it's shifting up and down rapidly before it holds a gear. Some of it might be the engine from sitting for 4 months, and should probably be professionally tuned. Time will tell. I'll run a tank and a can of Seafoam and hopefully it will clear the engine out.

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