4L80E lost fourth and lockup

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As I left my parents house today in my suburban pulling a lightly loaded trailer when it went to shift into fourth the Rpms dropped for a split second like it shifted then revved up like against nothing. I have 1st 2nd 3rd and reverse like normal but no lockup in third when the shifter is in third. No engine braking either. No clunk or bang when it happened and I wasn't hard in the throttle. 170k miles on it. Not sure if it has been into before or not. I got it with 140k on it. Fluid looks like ok and has 30k on it. It was shifting in and out of fourth on the way down like it should due to head winds.

Just looking for an opinion on where I need to start to look from these symptoms.

A quick internet search looks like it might be a valve body issue but I need to do some more looking.
 

jhellwig

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That sucks.
B.T.W. I tow in 3 in my Avalanche
I never have needed to since it has 4.10 gears so it normally stays in fourth even when towing. Keeping it in third wouldn't have been a problem if the lockup worked. 40mph was the most I could get with out just revving the wee out of it.
 
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Have you scanned for codes? I have a '92 with the same problem. It was throwing some code, I don't remember what, but the diag led to a bad TCM. Replaced it (pretty cheap) and although the code went away, the issue still remained.
 
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Agree with Snagglefoot. Even if you can repair the valve body, with this kit, you probably have clutches and drum bearings which are worn.
 
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Needs to be scanned for DTC's.....Or if need be.....Flashed out. But it will need to be test driven with the symptom present before extracting DTC's.
 

jhellwig

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It got driven for a hour and a half from my parents house to mine. It never set the service engine soon light when it happened and I have another unrelated issue that has been setting the light on and off for a while now so the light does work. I will see what codes it has if I can find my chilton manual.

I assume the TCM has its own pins to jump on OBD1 to get codes.
 
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There's no standalone TCM....Trans control functions are built into the PCM on a '95 gas truck, Trans DTC's get flashed out the same as Engine DTC's.

The issue with not have a scan tool-Live Data is you can't see what state the Solenoids are in.
 

jhellwig

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Got the codes. 12 44 and the one I’m looking for, 87. Drove it again today and the issue is gone.

The biggest thing I dread about fixing this is the checkballs especially upside down in my driveway.
 
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Does this unit have a good strong Reverse? I usually use my Tech2 to command 3rd gear at a standing start to rule out any Direct Clutch issues.

If the Direct Clutch can physically hold.....You likely have a VB/control issue, I would replace both shift solenoids, AFL Filter at the end of the Manual Valve Bore.
*Ream AFL Valve Bore & replace AFL Valve & Spring using TransGo 48-ACT-TL
*Use Sonnax part# 34994-02K to replace the Shift Valve Springs.
*Install Sonnax TCC Regulator Valve part# 34994-01K.
*The Separartor Plate is likely beat to crap, Replace with TransGo 48-PLT-01.

You can use TransJel Red to stick the checkballs up in the case while you install the VB.
 
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its only 760 miles to fort worth...:whistle:
actually thought it was alot closer
1150 from where I'm at.

That would surpass my most epic roadtrip to Grand Lake(orient) Maine.
 

jhellwig

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Does this unit have a good strong Reverse? I usually use my Tech2 to command 3rd gear at a standing start to rule out any Direct Clutch issues.

If the Direct Clutch can physically hold.....You likely have a VB/control issue, I would replace both shift solenoids, AFL Filter at the end of the Manual Valve Bore.
*Ream AFL Valve Bore & replace AFL Valve & Spring using TransGo 48-ACT-TL
*Use Sonnax part# 34994-02K to replace the Shift Valve Springs.
*Install Sonnax TCC Regulator Valve part# 34994-01K.
*The Separartor Plate is likely beat to crap, Replace with TransGo 48-PLT-01.

You can use TransJel Red to stick the checkballs up in the case while you install the VB.
I will give that a try. Reverse is good. I know you suggested the trans go afl thing before due to delayed lockup issues. I just haven’t done it yet due to not driving it hardly any to driving it a lot back to hardly any.

Depending on parts costs it might be cheaper to go with a sonnax rebuilt valve but I still might give it a try if it is a wash.

Speaking of cost, it is now worth fixing the frame on this thing where it is rotting. Also need to replace control arm bushings as those are gone. I looked for a replacement suburban and wanted to cry.
 

jhellwig

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Couple of questions.

Do the check balls need replaced? Some of Precision Transmissions videos on youtube mention replacing the check balls due to them wearing on the separator plate.

I can't find a good source for that Lubejel Red. Looks like there are other brands. I am considering the Lube-guard green stuff.

Probably just going to order a whole gasket/seal kit in case I ruin anything. It also looks like I should replace the internal harness.
 
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The checkballs have a hardened chrome surface & rarely ever have wear, The do beat the snot out of the Separator Plate though.

Plastic checkballs are an option, But the Plate HAS to be prepped for their use! I have a punch that I machined the end of to @ a 30° point, I cover the end of the punch with some Emory Cloth, Then dress the holes that the checkball seat against.....You just want to knock the sharp edge off.
This will prevent the plastic checkballs from wearing prematurely.

Lubegard "assemblee goo" works fine.
 
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