The MegaShift controller is a dedicated controller for the electronically shifted automatic transmissions.
It has the following features:
In Auto mode:
In Manual mode:
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.
Note that MegaShift-4L60E only controls the forward gears. You still require a mechanical shifter to place the transmission into Park, Reverse, or Neutral.
The internal transmission temperature is sensed by a standard sensor and used in place of Megasquirt's CLT sensor.
The 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.
LIMP 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.