Thread: GTP Consistent Weird Shift Into Overdrive (4th) Only When In "D"

Background info: I have a'99 Pontiac Grand Prix GTP with the 4T65E-HD transmission and 124,000 miles on it. My dad and I recently changed the pan gasket and drained the fluid as involved in that process. I noticed the issue about 6-8 months ago, and changing the gasket fixed a leak, but not this issue. The fluid was getting a little dark, but was about on schedule for a change. It does not smell burnt on the dipstick. There was a moderate amount of normal sludge on the magnet, but no metal shavings/hard pieces. It felt like grease. I also had a transmission shop look at it for a free inspection in August before driving 1000 miles to school, and they said things seemed fine, but my transmission computer wouldn't communicate with their computer, and they said the shift I'm concerned about was a little soft.

The Main Symptoms: The shifts are fairly smooth usually, as far as I can tell. (I haven't driven many different cars to compare because I'm only 21 and a girl.) When I have the car in "D" I can watch it shift through 1-2-3 with light throttle pressure by watching the rpm gauge(and listening to the loud exhaust). However, when it shifts again, into overdrive, the rpm gauge jumps up maybe 200-300 or so before it settles down to the 1700 rpms that it stays at while in overdrive with the TCC locked up (I always forget if the TCC locks or unlocks at this point, but whichever one it's supposed to, that's what it does here.) It sounds a little bit like when you forget to let off the gas all the way when pushing in the clutch on a standard. If I accelerate more quickly, which I don't do often, but my dad has tested it out, I'm not sure if it goes into third gear. It appears that it may just skip third and go from 2nd to overdrive in this case.

At first, one might think this is the common problem of the 3/4 clutch or the 4th shaft going out on this car, but I don't think that's the case. When I start out with the shift selector in "3" instead of "D," It will shift just fine 1-2-3. Then, if I wait until it's past 47 mph or so to slide it into D, it shifts normal, just like the other gears. I can also prevent the problem if I have it in drive and let off the gas at just the right moment as it shifts. And, one time, I started in third and slipped it into drive just a little too early, like at 44 or 45, and it seemed to shift down right away, but then I heard the rpms jump a small amount very briefly. That made me think that the TCC might be making it do that, but I'm not sure.

A Little More:
I went home to the Midwest (I go to school in the South) over Christmas and noticed more. . . When the engine is cold and I start it in "3," or "D," for the first five miles or so, it either doesn't shift into overdrive or the TCC doesn't lock up (unlock?). The rpms are lower in D than they are in 3 though, so I think that was just the TCC, which I've heard doesn't work until the engine heats up anyway. However, in this situation, I did notice that whenever the rmps went down the the normal 1700 after the car heated up (like 2200 until then in D, more in 3), they didn't jump like they would if the car had been warmed up.

Also, my shifter for PNRD321 seems to be a little loose. I think it might just be the year because before like 2002, they made it so you could easily go between 3 and D without pushing in the button (I think), but I thought I would include the information in case it might be affecting the issue.

Thanks for any ideas anyone might have!

@Fivefingerdeathpunch Would an OBD II system read the transmission info? I have one that should work with bluetooth on my phone, but haven't had a chance to see if the unit actually works ($30 one's from amazon are known to work about half the time).

@scottydoggs I know I get third because I hear it shift around 30-something with light throttle or if I let off the gas around 30 (higher speed with more throttle). If I have the shifter set to "3," it won't shift to overdrive until I put it in drive (D) around 47-52 mph (whenever I decide). If I start out in drive, it will shift as early as 43 mph with very light throttle, but that's when the issue happens. It just seems that if it shifts into overdrive at too low of a s

Also, I forgot to mention, the shifter is a little weird when I put it in reverse too. Sometimes I have to push it again to get it to actually go into reverse (from the direction of neutral, and yes the car is fully stopped at this point). That's why I think the shifter might be part of the problem.

I'm not sure what you mean by chugging. The best I can describe it is the sound a standard makes when you forget to let off the gas all the way when pushing in the clutch. It just jumps up once, then smoothly goes down to where it belongs at 1700rpm. I maybe wouldn't notice it if my car wasn't so loud lol. It has a custom exhaust that the previous owner put on it. The weird thing is that it only happens when I have it in drive when it hits the shifting speed. If I have it in third until then, it shifts normally as soon as I put it in drive (unless it's cold, then something is delayed, but not sure what).

I'll try the brake tap next time I'm in overdrive. It doesn't happen very often because the top speed limit most places I drive here at school is 40, but I could go down the road where it's 50. If I remember correctly though, I'm pretty sure it does that. It kinda scared me when I noticed cuz I thought something was wrong. Setting the cruise control also does that for me, but I never use it anymore because it caused weird rpm changes when I used it on the highway once.

I'm not really worried about the shifter unless it's contributing to this issue. My dad tried to look at where the cable links up and thought it looked normal, and it hasn't gotten any worse. I may look at it again though.

That sounds exactly like what's going on.

hmm. Maybe I should look at my muffler. I thought the noise was just the loud exhaust cuz none of the mechanics who have looked at it said anything.

When I had a the transmission shop look at it before my first long trip, they didn't seem to think it was a huge problem, but if it's a sign of failure, I should maybe sell the car before I lose money on it. I do have a service record from the previous owner of the fourth shaft or something like that being replaced around 90,000 miles. So I do know that the transmission has been opened up before. I've stuck a lot into the car (new fuel pressure regulator, spark plugs & wires, MAF sensor, front lower control arms, sway bar, camber adjusting bolt, front hub assembly, supercharger & serpentine belts, front axles & axle seals etc), but I paid a lot less for it than it's worth. If I end up replacing the transmission, I'm pretty sure it would negate any savings so far.

Yeah, my AC also doesn't work, so it's sometimes a choice between hearing and being cooled down, especially when I drove to Arkansas. My travel buddy hates that about my car. Sometimes I like it just a little bit though because I can hear how fast I'm going. When I drive my mom's GT, it doesn't have the HUD or loud exhaust and I have trouble keeping my speed.

And are you positive that it's not a sign of something going wrong? I just want to make sure I don't settle on an answer if I'm not positive about it. My mom has an '02 GT and it sometimes shifts hard around 2nd gear, but it doesn't do this if I remember correctly. It is a different transmission though (I think it's 4T65E instead of 4T65E-HD).

Yeah. I wouldn't consider 21 old though. hehe And I think it was the 4th shaft, not the hub, but I don't know the difference. Also, the supercharger is making noise too (thinking about trying to change the coupler myself), so that's another factor in the selling consideration. My dad told me I should sell it this summer because my transmission fluid was darker than my mom's (by his memory anyway), yet my magnet had less gunk on it he said.

And, I do keep it in 3rd in town. In fact, I rarely ever start in "D", unless I'm trying to diagnose the issue better. The guys at Aamco said they didn't know, but it couldn't hurt to just shift into drive myself every time. Also, it doesn't do it once I've been driving in overdrive for a while. For example, if it shifts down into third, unless I get below like 30, it shifts back into overdrive/4th just fine. If I get below 35, I usually shift back down to 3rd though and shift myself when I accelerate.

I really like my car. I even sanded, primed, and painted the rust on the fenders this summer. I still need to finish buffing it out though. . .

Thanks. Yeah, that's what I was thinking of doing with the supercharger. There's a chance that it's the rotors on the other side, in the blower though. I was told that the nose cone looks like it's already been replaced once. Though, that doesn't mean that the coupler was replaced or that it wouldn't go bad again. I have the fluid already. I just have to decide if I want to wait til I have time to do the coupler too, or just change the fluid for now.