4L60E Operation

The 4L60E is an electronically shifted General Motors RWD 4-speed automatic transmission. MegaShift is an electronic controller for this transmission that is based on the MegaSquirt^® GPIO hardware.

The GM 4L60E 4-speed Automatic RWD Transmission

The basis for 4L60E rear wheel drive transmission was introduced in Chevrolet cars and trucks in 1982 as the 700R4 overdrive-equipped replacement for the TH350. Originally, only the torque converter was computer controlled.

Just for a bit of background, the 700R4 was one of the first overdrive transmissions, and it coincided with two other developments: lock-up converters, and EFI.

EFI was necessary because carbs typically won't meter well at the low cruise rpms and relatively high manifold pressure (low vacuum) that an overdrive transmission allows (around 1500 rpm at legal highways speeds in 4th (OD)). EFI allows the fuel to be precisely metered at any speed/load - if tuned properly, allowing lower rpms and higher manifold kPa. This meant less frictional losses (lower speed) *and* less pumping losses (higher MAP) *and* better combustion efficiency (higher combustion pressures because the higher MAP allows better cylinder filling so this is a bit like a compression rise).

The lock-up torque converter (sometimes called the TCC for torque converter clutch) was necessary with overdrive because if a non-lock-up torque converter is operated below it's 'stall speed' for long periods of time, the slippage will heat the transmission fluid, possibly excessively. But lowering the stall speed to less than 1500 rpm would make the car a 'dog' off the line. So by having a lock-up converter, you can have a decent stall, but still lock it up while cruising to prevent excessive heating.

The common reasoning is that the lock-up improves mileage because it reduces the slippage, but this is a minor effect. Instead it's primarily there to allow sub-stall rpms without overheating the transmission fluid.

These three things (EFI, overdrive transmission, and lock up converters) combined with the EGO sensor to make for a new era in engine developments that has resulted in the return of performance while achieving very good mileage and low emissions (remember when everyone thought the muscle cars of the 1960 and early 1970s were going to be the high watermark for horsepower forever?).

There were frequent improvements to the 700R4 through it's life. Early versions had weak 27 spline input shafts, but these were upgraded to a larger 30 spline shaft in 1984. Many other minor improvements were made, almost every year, and generally the later the transmission the better it is.

Later editions of the 700R4 were renamed the 4L60, and the fully electronically controlled version (the 4L60E) became available in 1992. When the Corvette went to a rear transmission location in 1997, the case of the 4L60E was redesigned with a removable bell housing, and this was offered in all subsequent uses of the transmission. The internal components remained interchangeable, though.

The 4L60E's principal feature is a 30% overdrive. It also sports a low first gear ratio of 3.06:1 for improved acceleration 'off the line'. These transmissions are quite strong and reliable. Earlier models can be improved by the stock upgrades, and can be greatly improved with many aftermarket parts and building techniques.

For those upgrading from a 700R4 to a 4L60E, the differences are primarily:

• The pans are slightly different (though the same bolt pattern and general appearance). If some versions of the 700R4 pan are used on the 4L60E, the filter/pickup will hit the pan as it is tightened down. This will push the filter enough that the rubber seal on the pick-up no longer seals, and air will be drawn into the pump along with fluid. The result is that the transmission will not work because it has no line pressure. So be sure to use a 4L60E pan (or one that is approved for both 700R4 and 4L60E usage).
• The bolt pattern for the converter cover is different on one bolt (so it pays to drill that hole before putting the trans in the vehicle).
• The output shaft on Corvettes are about 1.0" shorter than those on Astro-Vans, etc. The output shaft is easier to shorten with a cut-off wheel - duplicate the existing configuration with a grinder.
• The VSS sensor for the 40 tooth wheel on the 4L60E's output shaft may interfere with the housing designed for for the 700r4'S speedometer gear driven VSS sensor. You may need to use a die grinder to create enough clearance in the tailshaft housing for the 4L60E's sensor if reusing the tail shaft housing (such as with a Corvette, where a specific housing is needed to attach the C-channel drive line beam). You may need to reposition the 40-tooth wheel somewhat. You can move it easier with a gear puller.

4L60E Shift Logic

The following table illustrates the hardware logic required to shift gears in the 4L60E. Note that the transmission can be electronically shifted between the 4 forward gears, neutral, park, and reverse must be selected with the shift lever (though the shift lever position can be detected via the switch manifold feedback). Once in Drive, the transmission's forward gears can be controlled with MegaShift.

The table gives the state of transmission's inputs and outputs for each gear, as well as what needs to change during upshifts and downshifts.

Manual Shift Lever

Note that Switch A, Switch B, and Switch C indicate the position of the manual valve (the shift lever, in essence). This tells MegaShift when the transmission shift lever is at park/neutral, reverse, D1, D2, D3, or D4. The values for the switch manifold shown above indicate manual valve position, NOT the currently selected gear. The switch manifold information is used to limit upshifts, where desired. Note that engine braking (via the overrun clutch in the transmission) is only available when the manual valve is in position D3, D2, D1.

Shifting Strategies

So the basic shift strategy looks like this:

Steady State Monitoring Configuration

Reverse

• Sol A = on
  
• Sol B = on
  
• 3/2 Sol = off
  
• PC = Load based
  
• LED 1 = off
• LED 2 = off
• LED 3 = off
• LED 4 = off
• notes LED display flashes

Neutral/Park

• Sol A = on
  
• Sol B = on
  
• 3/2 Sol = off
  
• PC = Load based
  
• LED 1 = off
• LED 2 = off
• LED 3 = off
• LED 4 = off

First

• Sol A = on
  
• Sol B = on
  
• 3/2 Sol = off
  
• PC = Load based
  
• LED 1 = on
• LED 2 = off
• LED 3 = off
• LED 4 = off

Second

• Sol A = off
  
• Sol B = on
  
• 3/2 Sol = 90% PWM
  
• PC = Load based
  
• LED 1 = off
• LED 2 = on
• LED 3 = off
• LED 4 = off

Third

• Sol A = off
  
• Sol B = off
  
• 3/2 Sol = 90% PWM
  
• PC = Load based
  
• LED 1 = off
• LED 2 = off
• LED 3 = on
• LED 4 = off

Fourth

• Sol A = on
  
• Sol B = off
  
• 3/2 Sol = 90% PWM
  
• PC = Load based
  
• LED 1 = off
• LED 2 = off
• LED 3 = off
• LED 4 = on

Torque Converter Clutch

• If in P, N, R, or 1st then GOTO TCC_RELEASE
• If KPA > TCC_MAP GOTO TCC_RELEASE
• If Current_Gear=2 && min_TCC_gear >= 2 || trans temp (TTS) < 200°F Then TTC_APPLY
• If Current_Gear=3 && min_TCC_gear >= 3 || trans temp (TTS) < 180°F Then TTC_APPLY
• If Current_Gear=4 Then TTC_APPLY
• GOTO TCC_RELEASE
• TCC_APPLY (set PTD2 = 1, set timer?)
• TCC_RELEASE (set PTD2 = 0, set timer?)

Shift 'Mechanism'

This is the basic operation of the shift in sequential mode. (The code also has the capability to go direct to a commanded gear, skipping intervening gears.)

* Upshifts *

First-Second

• 3/2 Sol PWM from 0% to 90%
  
• switch off Sol A
  
• switch LED1 off
  
• switch LED2 on
  

Second-Third

• switch off Sol B
  
• switch LED2 off
  
• switch LED3 on
  

Third-Fourth

• switch on Sol A
  
• switch LED3 off
  
• switch LED4 on
  

* Downshifts *

Fourth-Third

• switch off Sol A
  
• switch LED1 on
  
• switch LED3 on
  
• switch LED4 off

Third-Second

• switch on Sol B
  
• 3/2 PWM reduced from 90% based on speed and load
  
• switch LED2 on
  
• switch LED3 off
  

Second-First

• switch on Sol A
  
• 3/2 Sol PWM from 90% to 0%
• switch LED1 on
  
• switch LED2 off
  

Note that shifting the transmission using the shift solenoids versus shifting it using the manual lever has 2 effects:

1. There is no engine braking in any gear if the transmission shift lever is in 4th, regardless of the shift solenoid operation. To get engine braking, the lever must be in 3rd or lower. If engine braking is important to you, you will have to leave the lever in 3rd and either forgo the overdrive or shift manually into 4th as required.

2. The transmission is physically stronger in first gear if it is shifted there using the lever than it is if it is put in 1st using the shift solenoids. This has to do with how the hydraulic circuits engage the rotating elements in the transmission, and cannot be changed with electrical changes. If ultimate first gear strength is important in your application (high horsepower drag racing on slicks, for example) you should select first manually using the gear lever.