To replace the torque converter requires removing the transmission but not disassembling it. It sits between the engine and transmission.
The torque converter is doing what it is supposed to do as evidenced by the fact it is locking up. If it had failed, the lockup would never occur or it would cause a shudder as the clutch plate slipped. The intermittent unlocking is usually due to a sensor problem. As one example, if there is a "dropout" in the signal voltage coming from the throttle position sensor, the engine computer might think you took your foot off the gas pedal. When the signal voltage returns to normal, the computer will energize the lockup system again within a few seconds. Where you might notice this under normal conditions would be after coasting down a hill with your foot completely off the gas pedal. You might feel the converter lock up a few seconds after you resume pressing the pedal. On some cars, Chryslers for example, if you watch the tachometer, you will see it drop in two steps, the first step is when transmission fluid starts to apply pressure to the lockup clutch, and another drop when the clutch is fully locked. Each step drops engine speed by 100 - 200 rpm.
The fact that you can press the brake pedal half an inch before the converter unlocks tells you the brake light switch is not misadjusted. It still could have arced or pitted contacts, but that would not be real common because there is just a tiny signal current going through them. That low current isn't very hard on switch contacts.
What might be the easiest way to narrow this down is to have your mechanic drive the car with a hand-held computer, called a scanner, to watch the sensor readings and the "inputs / outputs". Most scanners also have a record / playback function that can record about five seconds of data. Because that data is stored momentarily as it passes through the scanner, the recording actually starts a couple of seconds before the "record" button is pressed.
When the recorded data is played back frame by frame, the mechanic can watch for incorrect sensor values and the inputs that affect lockup. Lockup should not occur if engine temperature is too low, vehicle speed is too low, or the transmission hasn't shifted into 3rd gear yet. The "output" will be displayed as "TCC" for torque converter clutch, "yes" or "no", or "on" or "off". If the display says the computer is requesting the lockup clutch to be energized when in fact it is not, the sensors and switches can be ruled out. That would indicate an electrical problem between the computer and the transmission. The most probable suspect would be a corroded or loose pin in an electrical connector. The solenoid could also cause the same problem, but on most cars, a break anywhere in that circuit will be detected by the computer which will memorize a fault code. Any fault that could adversely affect tail pipe emissions must turn on the Check Engine light. Since fuel mileage would drop if the converter wasn't locked up, emissions would go up. If that light is not on, there still could be other fault codes in the computer, but if there are no codes related to the torque converter, that would suggest the intermittent unlocking is due to the computer requesting it to unlock in response to the sensors or switches.
Thank you for the donation, but you don't have to keep adding to it. I'm happy to help people who are appreciative. The dollars go toward constantly repairing three laptops I use. I still have an old tank I built about ten years ago for a backup, but that one will never need repairs. It's too bulky to accidentally tie to the trailer hitch of my minivan!
Wednesday, March 31st, 2010 AT 2:14 AM