What does it mean?
OBD II fault code P0744 is a generic code that is defined as “Torque converter clutch (TCC) solenoid -circuit intermittent”, and is set when the PCM (Powertrain Control Module) detects an intermittent loss of continuity in the control circuit of the solenoid (or on some applications, the pair of solenoids) that controls the operation of the torque converter lockup clutch.
The purpose of the torque converter is to transmit the engines’ power to the transmission, an while torque converters that are not fitted with lockup clutches are reasonably efficient, the advent of torque converter lockup clutches has greatly enhanced torque converters’ power transmission ability.
In simple terms, a torque converter is a hollow drum that is attached to the engine, and encloses a turbine that is attached to the transmission, with no direct contact between the drum and the turbine. Both components are fitted with blades, or vanes; as the outer drum rotates, its vanes pick up transmission fluid (that is contained within the drum), that is then flung against the vanes of the turbine, causing the turbine to rotate, thereby transmitting the engine’s power to the transmission.
However, this method of power transmission necessarily results in an unavoidable loss of some of the engines’ power even in the most efficient torque converters , since it is the shear strength of the transmission fluid, and not a mechanical link, that determines how much of the engines’ power is transferred onto the transmission turbine.
Put in another way, this means that since some transmission fluid is forced through the small gap between the vanes on the drum and those on the turbine as the outer drum rotates, the fluid that is lost in this way cannot contribute to the transmission of power from the engine to the transmission.
This power loss is known as “slippage”, and while it can vary greatly between applications, it always means that not all of the engines’ power is transmitted to the transmission, which in turn, means that not all of the fuel being burnt in the engine is converted into energy that reaches the driving wheels. In short, this means that automatic applications without torque converter lockup clutches are less fuel-efficient than applications that do have torque converter lockup clutches.
To address all of these issues, car manufacturers have developed mechanical clutches that lock the two halves of the torque converter together; much like a clutch in a manual application that forms a mechanical link that locks the engine and transmission together.
The practical advantage of this is not only that slippage can be eliminated, which increases fuel efficiency and reduces emissions, but also that under certain conditions, slippage can be induced to both make gear shifts smoother, and to protect vulnerable transmission components against the effects of over torque conditions.
In terms of operation, a torque converter lockup clutch depends on pressurized fluid either to engage, or to disengage friction surfaces that loosely resemble clutch components found in manual applications. Pressurized fluid is supplied by the transmission pressure pump, and an electronically operated solenoid is used to direct/control/regulate the flow of pressurized fluid to and from the lockup clutch.
In a fully functional system, the lockup clutch is typically engaged when the vehicle is in motion, causing all of the engines’ power to be transmitted to the transmission since there is no clutch slippage present. When the PCM determines (based on input data from (among others) the engine speed sensor, vehicle speed sensor, and throttle position sensor(s) that a gear shift may be harsh, it may momentarily relieve some of the pressure keeping the lockup clutch engaged to induce some slippage.
This has the effect of “smoothing out” gearshifts, especially during hard acceleration, since some of the engine’s torque is now being bled off by the differential rotation of the two halves of the t