Deciphering the alphabet soup of shift feel

Deciphering the alphabet soup of shift feel

The GM 4L60 and 4L60-E transmissions have been around since 1982, so virtually everybody in the industry is familiar with their designs and how they work. However, one thing that still generates numerous technical calls and confusion is the alphabet soup of different letter codes that are stamped on the accumulator valve. What do they mean, and how do they affect the transmission? A short review of hydraulic theory and some simple graphs should help bring clarity to these questions and provide guidance on which letter code would be best in various situations. 

A condensed oil circuit that highlights the 2-4 servo, accumulator valve, 1-2 and 3-4 accumulators is shown in Figure 1. The 2-4 servo is used to apply the 2-4 band in second gear using second clutch (line) pressure on the second apply piston, and in fourth gear using second clutch and fourth (line) pressures on the second and fourth apply pistons. There are various sizes of OE and aftermarket servo pistons available, which will allow for greater holding power and reduction of 3-4 clutch and 2-4 band failures (Figure 1, above).

The 1-2 and 3-4 accumulator pistons are responsible for the 1-2 and 3-4 shift feel by providing the hydraulic cushioning of second clutch and fourth fluid pressure and altering the band apply rate. Both pistons use accumulator pressure fed from the accumulator valve to aid the springs in cushioning the band apply rate for a smoother shift. This accumulator pressure is regulated and varies in relation to the engine torque. In heavier throttle conditions, a greater apply pressure is required and a quicker apply rate results. Light throttle conditions demand lower pressure, and a slower apply rate results.

The other influencer on shift feel comes from the differences in the accumulator valve spool sizes and the associated spring, which is what the letter codes on the accumulator sleeve indicate. Therefore, knowing how each of these codes influence accumulator pressure will help in determining if they will make the 1-2 and 3-4 shifts lighter or firmer. The graph in Figure 2 shows the accumulator pressure curves related to EPC/Throttle pressure in 700-R4 or very early 4L60-E applications. Because this accumulator pressure helps to resist the stroking or cushioning effect of the accumulator piston, the pressure curves that are lower (K, N, A) on the graph will allow for a lighter apply rate and shift. The pressure curves that are higher (B, M, L) on the graph will provide a firmer shift. A common practice is to alter the accumulator valve spring during a rebuild, so putting in a spring that’s stronger than the OE will push the curve higher and provide firmer shifts (Figure 2, below).

The graph in Figure 3 shows the accumulator pressure curve related to EPC pressure for early (C) and later (YZ, CX, DX) 4L60-E applications. By comparing the firmer 700-R4 codes (B, M, L) with the 4L60-E codes, it is noted that at lighter throttle the overall pressure and shift firmness is relatively equal. However, in heavier throttle conditions, the 4L60-E accumulator valve designs provide a noticeably firmer shift. (Figure 3, below).

Oct-Tasc-Force-Figure-3-EPC-Acc-Pressure-2
In heavier throttle conditions, the 4L60-E accumulator valve designs provide a noticeably firmer shift.

Choosing the correct accumulator valve code and associated spring for your customer can be as simple as A, B, C when you understand the overall hydraulic theory and relative effect of each code.

Maura Stafford is Sonnax vice president of transmission products. She is a member of the Sonnax TASC Force (Technical Automotive Specialties Committee), a group of recognized industry technical specialists, transmission rebuilders and Sonnax technicians.

You May Also Like

Solving electronic control problems in today’s automatic transmissions

Special tools are often needed when diagnosing transmission problems and overhauling units.

The modern automatic transmission and attached drivetrain system has many “mechatronic” parts that control and protect the traditional clutches and planetary gears. These devices shift the gears, lock the clutches and regulate the fluid pressures. Most of these items can be diagnosed and replaced while the transmission is still in the vehicle.

How to fix GM 8L45/8L90 no-move and no-pressure conditions

By now you may have heard of, or even experienced, a GM 8L45 or 8L90 transmission with a no-move or no-pressure condition. Conditions of no movement or no pressure after repair have plagued this unit since its inception. Here’s what you can do about them. Related Articles – Are you ready to rock in 2023?

Tasc-Force-December-Figure-1-Pressure-Regulator-Valve-Assembly-Correct-1400
Searching for the source of an unexpected transmission fluid leak

As the installer was pumping fluid into the transmission from the right side of the vehicle, he was having a difficult time getting it full.

techspeakfeature-1400
Transmission slipping in fifth gear: An elusive cause of a common problem

Sometimes, finding the cause of a complaint isn’t as straightforward as one would expect; sometimes, these causes can be very elusive.

December-Shift-Pointers-Figure-1-1400
Stop/start accumulators: What you don’t know can hurt you

It’s no secret that the auto industry is making big moves these days, but while a lot is new, one thing that never changes is that the men and women servicing vehicles need to be learning almost constantly in order to do their jobs safely and effectively. Everybody knows not to touch hot exhaust parts,

Figure-1---Stop-Start-Accumulators-1400

Other Posts

How to get started with OE ECU reprogramming

The conversation goes like this: I have been paying mobile programmers or remote access programmers (RAPs) far too long. I believe I can save money and time by doing this myself. What type of equipment is needed and what is the cost to get up and running? Related Articles – FCA 62TE missing caged needle

FCA 62TE missing caged needle bearing

The complaint When the Low Clutch Housing is removed from the Underdrive Center Shaft of a 62TE transmission, one notices that the caged needle bearing located on the shaft between the sealing rings was missing (Figure 1, above).  Related Articles – A 3D printing solution for 948TE offset pump access – 6F35: A classic bolt

Tech-Talk-Q4-2022-Figure-8-1400
FCA 62TE slipping in second and fifth

The complaint Immediately after overhaul, a 2013 Chrysler Town and Country with a 3.6L engine and a 62TE transmission faults to limp mode. The scan tool reveals trouble codes P0732 “Second Gear Ratio Error,” P0735 “Fifth Gear Ratio Error,” P0792 “Compounder Speed Error,” and P1790 “Fault After Shift” are set. During the overhaul procedure, the

Tech-Talk-Q4-2022-Figure-9
Comparing the pumps on Ford 6F35 generation 1 and 2

The change For the 2013 model year, the Ford 6F35 transmission received a high-volume pump which increased the intake channel in the stator shaft support from 0.432” to 0.520”. The pump body intake channel was also increased to the same matching dimensions. Related Articles – Japanese CVT no-move conditions: Failsafe or defective TCM? – Having