4L60E 2-4 servo

[Leeztruk]

This is just a point of interest to anyone considering a Corvette servo for their transmission. For my last swap, I used the Corvette 2-4 servo and found the 1-2 shift quite firm (at least for my liking). The swap I'm doing now ('89 Cheyenne) I was just going to use the stock servo, with some minor Sonnax upgrades. I got a couple of ATSG manuals to guide me through the changes, and came across an interesting service notice about the harshness of the Corvette servo. Their recommendation was to use the servo from the 4L70E trans. All well and good, but that trans was only used for a couple of years before the 6 speed and 8 speed automatics were introduced. So I tried to find the 4L70e servo. Turned out to be almost as illusive as the dial speedometer for the '90-'91 trucks. To make a long story short (too late?), I found one. Actually found the speedometer too, but that's a story for another time. Here's some pics to show the difference (left to right, stock, 4l70E, Corvette)

 

[tayto]

you can change the 1-2 shift feel by: the 1-2 accumulator stack up, office size in the separator plate, accumulator valve, etc

 

[Leeztruk]

Tayto, please explain "stack-up". Not being a transmission guy, I only know from what I read. I know that various spring rates will soften or firm up the action of the accumulator, which in turn, affects the servo and its' operation. I'm just trying to learn here. Thanks

 

[tayto]

you can run the spring against the separator plate, piston in the 1-2 accumulator housing. with this setup the firmer the spring the FIRMER the shift. (700-R4's)

you can run the piston against the separator plate and spring in the accumulator housing. with this setup the firmer the spring the SOFTER the shift. (Most 4L60E's)

you can also run an "over/under" spring arrangement like transgo does. this shortens the piston travel

The FASTER an accumulator strokes, the softer the shift will be
The SLOWER an accumulator strokes, the firmer the shift will be
LARGER accumulator oil volume=softer shift
SMALLER accumulator oil volume=firmer shift

You also have different code accumulator valves. The most popular are A, B, L, M. There are well over half a dozen types going back from the early 700R4's to last 4L60E's. Different valves have different pressure curves, so you can globally make shifts firmer or softer. I tend to leave this valve alone, but lots of shift kits will change this spring for firmer shifts.

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Lastly orifice hole size. On all of the units I build I run a corvette servo, i usually leave the 1-2 shift hole in the .050-.060" range and it produces a slightly than firmer stock shift. If the unit is for a relatively stock unit or where the owner doesn't care about "performance" feeling shifts I usually leave the 1-2 accumulator stackup & accumulator valve stock. this seems to produce a nice quick shift with added benefit of the increased clamping of the bigger servo.

 

[Leeztruk]

Thanks for the info. Not looking to transform the transmission into a performance unit, just a nice daily driver. I do plan on using the Sonnax pinless pistons in the 1-2 accumulator and valvebody, but don't intend on modifying the seperator plate. The only other mod to the trans. will be a torque convertor with 1800-2200 stall

 

[tayto]

I would not run the sonnax pinless on the 1-2 accumulator. get a new GM pin and transgo piston. run the stock springs in the same orientation (spring in accumulator, piston against separator plate)

 

[Leeztruk]

Thanks again for your input. I really appreciate your advice