2006 Pontiac Grand Prix Transmission / Torque Converter Iss

It's nice to know you understand what a lockup torque converter is.

I'm not a transmission specialist, but it does sound like a torque converter issue. If the fluid was changed recently, I would first inquire if it's possible to use the wrong fluid. Some transmissions, Chryslers for example, will not be damaged by the wrong fluid but it will cause chattering and vibrations due to the clutch plates grabbing too hard. In that case it's just an irritation, but it's nice to know the cause.

GM had a lot of lockup torque converter trouble in the '80s, but you don't hear much about that anymore. Symptoms were much different too. I have to defer here to my experience with Chrysler products which are similar to GM's in many ways. If your converter uses an electric solenoid for the lockup function, a dropout in signal voltage from the throttle position sensor could trigger the computer to unlock the converter in preparation for coming to a stop. Driving with a hand-held computer connected will allow you to watch the lockup command. If the computer turns the lockup function off, it's in response to something it learned, (incorrectly) from a sensor. If the lockup stays commanded on, but you see engine speed increase, the lockup clutch is likely slipping. You can also watch sensor readings while driving. If there is indeed a dropout from the throttle position sensor, Chryslers will set a diagnostic fault code right away and turn on the Check Engine light. Often GM computers will sit and watch the sensor extra closely for a while, then set the code and turn on the light after the problem occurs often enough. The voltage dropout can also happen so quickly, the scanner won't pick it up and display it, but it's fast enough for the engine computer to react and unlock the converter.

If you don't think the wrong fluid is the cause, check the level, then visit a transmission shop for an opinion. Just about anything you can describe, they've seen before.

caradiodoc

The lock-up mode in these 4T65-E transmission is triggered with a Torque Converter Control (TCC) solenoid, which sounds like your problem. Lock-up usually engages somewhere between 35-45 mph, where the PCM will activate the TCC. Cardiodoc has good advice here about fluid and using a scan computer. My understanding with these transmissions is that they definitely have their issues, but they are not very elusive when it comes to diagnostics. Best of luck to you.