MegaShift 4L60E Control Strategies

The primary determinants of the target gear and line pressure are the vehicle speed and load. When in CAN enabled mode, this load is kPa of MAP (manifold absolute pressure in the engine's intake manifold). It is 'smoothed' for a few reasons, one of which is to prevent excessive hunting of the transmission. There is a user-settable factor ('LOAD filter factor' under 'General Settings→Shift Factors' in TunerStudioMS) that influences the degree of smoothing. The higher this number the more smoothing is done, and the slower the transmission responds (but it is more stable and less likely to hunt). The default value is 1000, user may have to experiment to find the best value for their configuration.

Line Pressure Control

The MegaShift hardware/software control the line pressure via the pressure control (PC) solenoid. This solenoid controls a bleed valve, and it is closed when no current is flowing. As more current is applied, the bleed vale is opened by the solenoid, and the line pressure is reduced. The current is controlled by applying 0% to 60% PWM on the PC solenoid.

It is important to note that this PWM% (called outpc.PC_duty in the code) is 60% at minimum pressure, and 0% at maximum pressure. However, TunerStudioMS subtracts this percentage from 100 when displaying the line pressure in TunerStudioMS. So maximum line pressure (=0% PC PWM) is 100% and minimum line pressure (=60% PC PWM) is 40%. This is done because it is much more natural for larger number to represent higher pressures.

Shifting

Shifting can be based on a 12x12 table (vehicle speed by load), or the trans can be shifted manual at any point.

All shift modes have a user settable 'Maximum Shift Line Pressure'. The 4L60E is much more durable if the shifts are not performed at full line pressure. If the line pressure set using the load is below the max. shift line pressure (typically at part throttle), then it is not altered. However, if the line pressure is high, then it is reduced to the set value before the shift is done to prevent extremely hard engagements.

There are two delay factors built into the shifts.

• The first ('Shift Pressure Adjustment Delay') is a user settable delay to allow the line pressure to adjust and the TCC to disengage where necessary before changing the shift solenoid states.
• The second delay is the 'Shift Completion Delay' that allows the shift to be completed before any new target gears are determined (and the LEDs/current gear display are updated). The completion delay has two factors: a user-specified fixed amount, that is added to a variable amount based on the line pressure PWM% (since shifts complete faster if line pressure is higher, and the line pressure is higher when the PWM% is lower).

Two MegaSquirt^® PWM channels are used for the pressure control solenoid and the 3/2 shift solenoid. All other controls are simple on/off switches.

The inputs and outputs are configurable in TunerStudioMS. You can set which should be on or off. The input pattern for the manual shift lever must unique identify the park/neutral, reverse, and forward gears. The outputs 1 and 2 must have a unique pattern in the forward (controlled) gears. The TCC lock up output and the output 3 (an PWM output) can be set freely. For an example, here are the patterns for the GM 4L60E:

• ¹ The Pressure Control solenoid controls main line pressure, pulsed at 0-60% duty cycles at 293Hz. There is no cleaning pulse (dither) every 10 seconds on this transmission (unlike the 4L80E).
• ² The torque converter clutch [TCC] is pulse width modulated (32 Hertz, with duty cycle varied) in OEM applications. Maximum current should not exceed 1.5 Amps.
• ³ The 3-2 Sol is pulsed at 50 Hertz, 0% duty in 1st, 90% duty in all other gears. During a 3-2 downshift, the duty cycle is decreased based on the vehicle speed; lower speed, lower duty.
• ⁴ The Lock-Up Feel solenoid was probably put in high end Cadillacs for seamless shifting. The Lock-Up Feel solenoid was added during 1995 Model Year. There is more information on the OEM control of the LUF solenoid here: LU Feel solenoid info.pdf (the LUF is called the "TCC PWM solenoid valve" in that document, while the TCC on/off control is called the "TCC solenoid valve").

In Auto mode:

• Gear is selected according to a 4 gear by 12x12 (mph x kPa) shift table which selects the gear based on the current vehicle speed and load,
• The torque converter clutch (TCC) will engage ("lock up") if:
  • MAP is below a threshold (< No TCC Above (kPa)),
  • transmission is in an appropriate gear gear (> Min Gear for TCC Lock),
  • transmission is at an appropriate speed (>Min. TCC Lock-Up Speed), and
  • transmission temperature is not cold (> Min. Temp for TCC Lock).
• Further TCC engagement is forced in 2nd and 3rd gear if transmission temp is high (>Lock TCC Above Temp),

In order for a downshift to occur after an upshift, the speed must drop below the original upshift speed minus the 'shift MPH hysteresis' (which is user settable). Setting this value to 3-5 mph or more will prevent the transmission hunting when near a boundary between gears. There is a similar setting for kPa hysteresis ('Shift kPa Hysteresis').

Auto mode is switched to manual mode if either shift buttons are used, or if both buttons are held fort a second or so.

In Manual mode:

• Full manual control, unless rpm exceeds user-specified redline or drops below user-specified minimum,
• TCC engagement only if transmission temperature is high and MAP is low.

The torque converter clutch always unlocks during downshifts. It unlocks under certain conditions during upshifts, depending on the vehicle speed and engine MAP. It is safe to upshift under light load while the TCC is locked. Most OEM systems keep the torque converter clutch applied during light throttle upshifts.

Manual mode is active until:

• The speed drops below 8 mph (or 8 kph) from more than 20 without selecting first,
• the engine minimum or maximum rpms are exceeded,
• both the upshift button and downshift buttons are held simultaneously for a second or so.

The Manual Valve Position

Note that MegaShift-4L60E only controls the forward gears. You still require a mechanical shifter to place the transmission into Park, Reverse, or Neutral. MegaShift knows which position the gear lever is in by reading the state of the pressure switch manifold, and it can determine all of the gear positions (except it cannot distinguish between Park and Neutral). Furthermore, the shift lever position limits the top gear selected in D3, D2, or D1 (and also provides overrun engine braking in those positions). So in D3, we have engine overrun braking, and MegaShift will not select 4th gear (in if 4th when the lever is moved to D3, MegaShift will downshift the transmission). So in D2, we have engine overrun braking, and MegaShift will not select 3rd or 4th gear, etc..

Pressure Switch Manifold

The pressure switch manifold (PSM) is a multiple switch assembly consisting of 3 normally open (NO) pressure switches and 2 normally closed (N/C) pressure switches. These provide outputs to 3 pins: Pin N, Pin R, and Pin P (aka. swA, SwB, SwC, respectively).

Fluid from various hydraulic control circuits is fed to this the pressure switch manifold which allows the ECU to determine which position the transmission manual valve (i.e., the shift lever) is currently in. The switch contacts are normally open and close when fluid pressure causes them to. Depending upon the circuit, the switch may provide a ground path when closed. The table below shows a pin that is grounded by the pressure switch manifold (PSM) as a "0", while an open circuit shows a nominal 5 (volts).

Limp Home Mode

"Limp Mode" is enabled when the vehicle computer recognizes a problem based on input sensors becoming out of range or when they conflict. When a sensor signal value is received by MegaShift and it is not within the computer's programmed specifications, "limp mode" is activated by the computer. These are intended to protect the transmission from the damage that an improper sensor signal might inflict.

The computer is always expecting certain signal values from certain sensors i.e. the temperature sensor, the speed sensor, the MAP, the pressure switch manifold, etc. If the computer receives one or more signals from the sensors that are out of the normal range expected from this sensor, it will go to "emergency" or "secondary" measures.

These emergency measures vary depending on the severity of the defective signal. All this is preprogrammed into the computer's logic by the manufacturer. The manufacturer has decided that as long as a certain parameter of a particular signal is sent from a sensor to the computer, all is well. The manufacturer decided that if this signal is higher than their highest parameter or lower than their lowest parameter, something is wrong with that sensor and the computer should make someone aware of the situation and take action to try to "save" the vehicle systems or power train.

MegaShift will cause the internal transmission fluid line pressure go high to protect clutches and bands. The transmission also turns off the shift solenoids to cause the unit to default to a single gear, usually second or third. All normal instructions to control line pressure are overridden so a "slipping condition" cannot occur easily. If the bands and clutches were allowed to slip excessively, the transmission could wear out very quickly.

All this happens so that the vehicle's driver can get the damaged vehicle to the nearest available spot for repairs. When the transmission goes into this state (high line pressure, one gear, etc.), it is said to be in "Limp Mode". You limp to the next town using third gear only, at full line pressure so the transmission clutches won't slip, minimizing potential damage.

Note that if the MegaShift cable harness to your transmission was to become detached, severed or damaged, your transmission would also go to "limp mode". The electrical and fluid circuits of the 4L60E were designed this way to avoid leaving people stuck at the side of the road.

Fluid Temperature Sensor

The transmission temperature is sensed by the 4L60E's internal 'GM standard spec' sensor. This is used to prevent TCC lock-up at low temperatures for faster warm-ups, and force lock-up at high temperatures in 2nd, 3rd, and 4th gears to cool the transmission.