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Electrical and Vacuum Controls

The torque converter clutch should apply when the engine has reached near normal operating temperature in order to handle the slight extra load and when the vehicle speed is high enough to allow the operation of the clutch to be smooth and the vehicle to be free of engine pulses.

NOTE: When the converter clutch is coupled to the engine, the engine pulses can be felt through the vehicle in the same manner as if equipped with a clutch and standard transmission. Engine condition, engine load and engine speed determines the severity of the pulsation.

The converter clutch should release when torque multiplication is needed in the converter, when coming to a stop, or when the mechanical connection would affect exhaust emissions during a coasting condition.

The typical electrical control components consist of the brake release switch, the low vacuum switch and the governor switch. Some vehicle models have a thermal vacuum switch, a relay valve and a delay valve. Diesel engines use a high vacuum switch in addition to certain above listed components. These various components control the flow of current to the apply valve solenoid. By controlling the current flow, these components activate or deactivate the solenoid, which in turn engages or disengages the transmission converter clutch, depending upon the driving conditions as mentioned previously. The components have the two basic circuits, electrical and vacuum.

Fig. 1: Using electrical and vacuum controls to operate the torque converter clutch

Fig. 2: Typical diesel engine vacuum and electrical schematic for the torque converter clutch


All of the components in the electrical circuit must be closed or grounded before the solenoid can open the hydraulic circuit to engage the converter clutch. The circuit begins at the fuse panel and flows to the brake switch and as long as the brake pedal is not depressed, the current will flow to the low vacuum switch on the gasoline engines and to the high vacuum switch on the diesel engines. These two switches open or close the circuit path to the solenoid, dependent upon the engine or pump vacuum. If the low vacuum switch is closed (high vacuum switch on diesel engines), the current continues to flow to the transmission case connector, into the solenoid and to the governor pressure switch. When the vehicle speed is approximately 35-50 mph (56-80 kph) , the governor switch grounds to activate the solenoid. The solenoid, in turn, opens a hydraulic circuit to the converter clutch assembly, engaging the unit.

It should be noted that external vacuum controls include the thermal vacuum valve, the relay valve, the delay valve, the low vacuum switch and a high vacuum switch (used on diesel engines). Keep in mind that all of the electrical or vacuum components may not be used on all engines, at the same time.


The vacuum relay valve works with the thermal vacuum valve to keep engine vacuum from reaching the low vacuum valve switch at low engine temperatures. This action prevents the clutch from engaging while the engine is still warming up. The delay valve slows the response of the low vacuum switch to changes in engine vacuum. This action prevents the low vacuum switch from causing the converter clutch to engage and disengage too rapidly. The low vacuum switch deactivates the converter clutch when engine vacuum drops to a specific low level during moderate acceleration just before a part-throttle transmission downshift. The low vacuum switch also deactivates the clutch while the vehicle is coasting because it receives no vacuum from its ported vacuum source.

The high vacuum switch, when on diesel engines, deactivates the converter clutch while the vehicle is coasting. The low vacuum switch on the diesel models only deactivates the converter clutch only during moderate acceleration, just prior to a part-throttle downshift. Because the diesel engine's vacuum source is a rotary pump, rather than taken from a carburetor port, diesel models require bath the high and the low vacuum switch to achieve the same results as the low vacuum switch on the gasoline models.

Computer Controlled Converter Clutch

With the use of microcomputers governing the engine fuel and spark delivery, most manufacturers change the converter clutch electronic control to provide the grounding circuit for the solenoid valve through the microcomputer, rather than the governor pressure switch. Sensors are used in place of the formerly used switches and send signals back to the microcomputer to indicate if the engine is in its proper mode to accept the mechanical lock-up of the converter clutch.

Normally a coolant sensor, a throttle position sensor, an engine vacuum sensor and a vehicle speed sensor are used to signal the microcomputer when the converter clutch can be applied. Should a sensor indicate the need for the converter clutch to be deactivated, the grounding circuit to the transmission solenoid valve would be interrupted and the converter clutch would be released.

Fig. 3: Typical computer controlled clutch

Hydraulic Converter Clutch

Numerous automatic transmissions rely upon hydraulic pressures to sense, determine when and to apply the converter clutch assembly. This type of automatic transmission unit is considered to be a self-contained unit with only the shift linkage, throttle cable or modulator valve being external. Specific valves, located within the valve body or oil pump housing, are caused to be moved when a sequence of events occur within the unit. For example, to engage the converter clutch, most all automatic transmissions require the gear ratio to be in the top gear before the converter clutch control valves can be placed in operation. The governor and throttle pressures must maintain specific fluid pressures at various points within the hydraulic circuits to aid in the engagement or disengagement of the converter clutch. In addition, check valves must properly seal and move to exhaust pressured fluid at the correct time to avoid "shudders" or "chuckles" during the initial application and engagement of the converter clutch.

Centrifugal Torque Converter Clutch

A torque converter was used that locks up mechanically without the use of electronics or hydraulic pressure. At specific input shaft speeds, brake-like shoes move outward from the rim of the turbine assembly, to engage the converter housing, locking the converter unit mechanically together for a 1:1 ratio. Slight slippage can occur at the low end of the rpm scale, but the greater the rpm, the tighter the lock-up. Again, it must be mentioned, that when the converter has locked-up, the vehicle may respond in the same manner as driving with a clutch and standard transmission. This is considered normal and does not indicate converter clutch or transmission problems. Keep in mind if engines are in need of tune-ups or repairs, the lock-up "shudder" or "chuckle" feeling may be greater.

Fig. 4: Exploded view of the centrifugal lock-up converter

Mechanical Converter Lock-Up

Another type of converter lock-up is the Ford Motor Company's AOD Automatic Overdrive transmission, which uses a direct drive input shaft splined to the damper assembly of the torque converter cover to the direct clutch, bypassing the torque converter reduction components. A second shaft encloses the direct drive input shaft and is coupled between the converter turbine and the reverse clutch or forward clutch, depending upon their applied phase. With this type of unit, when in third gear, the input shaft torque is split, 30% hydraulic and 70% mechanical. When in the overdrive or fourth gear, the input torque is completely mechanical and the transmission is locked mechanically to the engine.

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