What would cause a torque converter to stay locked on a 700R4 causing the engine to stall? I just replaced the torque converter after replacing the pump in the transmission. I also replaced the TV cable and reset the position. I had perfect operation of the transmission for a couple hundred miles then suddenly started having an occasional hard shift into second — as if the torque converter lock was engaging. The converter is still locked as I limped home. Is this a mechanical or an electrical issue? Thank you. — L.N.
This is actually a fairly common problem with the 700R4/4L60E overdrive transmissions but luckily the solution is also relatively easy.
You didn’t mention what vehicle this is in, but the solution is still the same. It sounds like the torque converter clutch solenoid has stuck or failed in the open position.
In these transmissions, when the ECU signals the transmission to engage the torque converter clutch, this electrical signal is sent to what is called a torque converter clutch solenoid in the transmission. When engaged, the solenoid opens a passage in the valve body that applies hydraulic fluid pressure to the back side of the converter clutch assembly.
This fluid is applied to a large disc inside the converter with a clutch material on the leading side of the plate that locks the input shaft to the outside of the converter housing much like a manual transmission clutch. If the solenoid fails in the open position, the clutch will not release when vehicle speed reduces to the point where the ECU should cut the signal to the converter lockup solenoid.
If this is in an earlier car without an ECU, the lockup converter is controlled by a pressure switch inside the transmission that is triggered when the transmission shifts into fourth gear. It could be that this switch has failed and is feeding a constant signal to the converter to lock up. So this would need to be checked before merely replacing the torque converter clutch solenoid. If there was a loss of voltage feed to the transmission, the solenoid would not be triggered.
It is possible, but not likely, that it is a problem with the brake switch, as the fourth gear switch should disable the circuit when the transmission downshifts out of the overdrive.
In talking with transmission shops they say you may experience problems with the converter even after repairing the solenoid issue. They say that the converter will often fail after experiencing this lockup where the vehicle stalls, so you at least should be aware of it. In these cases, they suggest replacing the converter along with the torque converter clutch solenoid.
This is probably not what you want to hear, but if the converter has been damaged, it could lead to other transmission problems down the road if the converter fails.