2006 Pontiac Grand Prix Transmission / Torque Converter Iss

VENGEBEAR

MEMBER
2006 PONTIAC GRAND PRIX
V8
FWD
AUTOMATIC
65,000 MILES

I have recently noticed a couple of issues with my 2006 Grand Prix GXP.
Problem 1 - While stopped at a stop light (car still in drive and it appears to idle fine), when I slowly accelerate when the light turns green, my car feels sluggish (~2-4 MPH) then I feel something lock (like a small slam that my car just went into gear) and then my car accelerates.

Problem 2 - If I am cruising at roughly 69-73 MPH, I see my RPMs bounce up and down a little bit. Then, my torque converter will lock (at about 2100 RPMs). If I start to slightly go uphill, car starts to shake a little bit and then my torque converter unlocks (goes up to about 2600 RPM) and the shaking stops.

Are these symptoms torque converter or tramissions problems or both?

2 Replies

CARADIODOC

MECHANIC
33,752 POSTS

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