Odd 4l60e behavior

[emmett442]

I've seen a few trans experts in here...

2008 Tahoe

1-2 shifs hard at part throttle. Little throttle and full throttle good. Other shifts feel good. MAYBE 2-3 slightly soft part throttle, but not noticable.

Symptoms improve as temps increase. Cold is worst. Hot is very good.

Replaced 1-2 accumulator. No change.
Replaced valve body, no change.

While in there, replaced TCC valve with updated sonnax parts.

Replaced 2-4 servo with the infamous "Corvette" apply area. 1-2 worse, 2-3 ok. Yeah, getting to be parts cannon time.

Here's the odd part: if I reset TAP data, it shifts perfectly for a few days, then as TAPS data builds, the symptoms return.

Getting even more odd: TAP data on the 1-2, which shifts hard, show boosted pressure. The 2-3 shift that's maybe slightly soft, show pressure reduced. It's like a self-inflicted problem.

To my uneducated self, the perfect shifting after data reset means hard parts and hydraulics are likely ok. I'm curious if this could be an EPC problem?

Please advise!

 

[emmett442]

For those who care, ended up figuring this out:

The parts cannon attempt in my story above foiled me.

The Corvette ratio servo I bought came as an assembly. I simply popped it in and didn't think twice.

Eventually figured out the pin was way, way too short. 190 thou short.

Ground down an extra long pin to about 0.080". Shifts are SEAMLESS.

Interesting to me how it influenced the TAP data.

Case closed, though in the back of my mind, I wonder if band clearance was always the issue, and makes me wonder how much band wear there is to need a longer pin...

 

[clinebarger]

The TCM noticed the 1-2 Shift was taking too long & increased line pressure to shorten shift time.

4L60E Band Clearance is always loose even brand new, Even though GM makes different length Servo Apply Pins, They're ALL built with the 2-Groove Pin.

You're suppose to run a extra Servo Cushion Spring with 'Vette 2nd Pistons.

 

[emmett442]

The TCM noticed the 1-2 Shift was taking too long & increased line pressure to shorten shift time.

4L60E Band Clearance is always loose even brand new, Even though GM makes different length Servo Apply Pins, They're ALL built with the 2-Groove Pin.

You're suppose to run a extra Servo Cushion Spring with 'Vette 2nd Pistons.

Not sure what the original was, but the pin that came with the new servo was a 2-ring pin.

I saw that extra spring in a set of transgo instructions. I also saw that Sonnax calls it optional.

Unfortunately, I couldn't find the extra spring in stock anywhere. That is when using the GM part number anyway. Therefore, I went without it.

Regarding the pin likely being on the short side new: I'm concerned that needing a longer pin means I'm taking up slack from a worn out band. You're thinking the pin was always too short from new, and minor band wear threw it out of spec enough to change shift quality? I hope so.

 

[MSCH]

I

Here's the odd part: if I reset TAP data, it shifts perfectly for a few days, then as TAPS data builds, the symptoms return.

Getting even more odd: TAP data on the 1-2, which shifts hard, show boosted pressure. The 2-3 shift that's maybe slightly soft, show pressure reduced. It's like a self-inflicted problem.

To my uneducated self, the perfect shifting after data reset means hard parts and hydraulics are likely ok. I'm curious if this could be an EPC problem?

Please advise!

I dont have any advice except to say the 4L603 in my 93 GMC was the same way.. on that model it would relearn if you disconnected the battery ... I learned how to use my right foot to minimize the issue..