2-stage overdrive???

Something's been on my mind for a while...it feels like my car has a 2-stage overdrive. It's a '95 S/C don't forget. So here's what happens:

As I drive from a stop to 50, it shifts into 2nd, 3rd, then as I approach 45, it'll go down into overdrive at about 1,500rpm. As I keep accelerating, or if I hold that speed, the car will shift into 3rd, and then into overdrive again, but this time at a much much lower 1,200rpm. Are there 4 gears and then overdrive? I always thought the 4 gears included overdrive, like 1-2-3-OD(-OD1-OD2 in this case!), not 1-2-3-4-OD.

Anybody else know what I'm talking about? The car's always done this...I'm wondering, I don't think they make 2-stage overdrive units...

Its called Torque converter lockup.. The torque converter is programmed to lock up at cruising speeds to allow a low RPM for cruising , keep trans temps lower , and increase fuel economy.

So THAT'S what lockup feels like...good idea, bravo GM!

Haha thanks for the answer!

Your observation is accurate. We have 4 gears + overdrive. The way our PCMs are tuned stock, you almost never get to use 4th gear by itself, so effectively, you have 1-2-3-OD (TCC lock-up).

With a PCM tuner, I've been able to space out my TCC lock-up so that it only kicks in when throttle is less than say 12%, or during cruising in 4th gear. Also, TCC will disengage easily when you add throttle. This lets you enjoy 4th gear whenever you need to. Interestingly, my fuel economy isn't negatively impacted. I think GM set the car to run in OD most of the time so the driver wouldn't detect the shifts. I like frequent shifts, because it tells me what the car is doing.

Using the above idea, my transmission effectively is a 5-speed. I've been told that it technically is a 4-speed + TCC lock-up, but the result really is the same as a 5-speed, especially when you tune TCC lock-up to work as a 5th gear. On a graph, the RPMs are just like a 5 speed - 1-2-3-4-OD(TCC lock-up). That's an extra gear in there. Best of all, when you use cruise control, you keep the old OEM economy settings, so you can really feel a big difference here!

If you ever get another Riv and it's a '97-up, get a tuner and you'll swear you're driving a totally different car. Mine revs like a Mazda through each gear all the way up, but once cruising, it uses torque to hold in gear. Remember pushing the throttle 1/2 way to escape low RPM on the freeway? Remember tapping the brake pedal to "fool" the transmission into releasing TCC lock-up before you need to accelerate? I haven't experienced those things in years, and also haven't experienced the shudder that comes with TCC lock-up as you try to release it while increasing speed. Also, you've probably heard some say to throw it in 3rd when towing a trailer. That's to avoid TCC lock-up. But with a tuned PCM, this happens automatically.

What you are feeling, Ilan, is the TCC lock-up engaging in 3rd gear, and then again in 4th, so it kind of is a 2-stage overdrive. It's pretty good that you are familiar enough with how your car operates to see what's happening. I don't think many drivers would even notice. To say it's a good idea by GM though, I wouldn't go that far. It's not so good for the performance driver, imo. It's probably the #1 reason domestics are perceived as heavy and slow. It's not that the power isn't there, it's how that power gets delivered.

Have a read for more: https://rivperformance.editboard.com/series-ii-scans-tuning-pcm-f5/aa-s-dhp-powrtuner-log-t41.htm

Yay I is smarts!

I think you are feeling the TCC lockup in 3rd gear, then the shift to 4th, then the TCC lockup in 4th.

deekster_caddy wrote:

I think you are feeling the TCC lockup in 3rd gear, then the shift to 4th, then the TCC lockup in 4th.

Mine does it too

So technically it's a 6 speed auto!!! Haha, now it goes 1-2-3-3TCC(4)-4(5)-4TCC(6).

Haha... almost, but not quite. The only problem with that is, going from 3TCC to 4 make the engine rev higher in RPM!

But you are right, there are 6 "effective" gear ratio possibilities. And remember, you almost never get to use 4th without TCC lock-up on a stock car.

AA wrote:

Your observation is accurate. We have 4 gears + overdrive. The way our PCMs are tuned stock, you almost never get to use 4th gear by itself, so effectively, you have 1-2-3-OD (TCC lock-up).

I disagree, unless you're counting reverse. Overdrive only refers to the fact that with 4th gear having a ratio of 0.7, the output speed is greater than the input speed.

IBx1 wrote:

So technically it's a 6 speed auto!!! Haha, now it goes 1-2-3-3TCC(4)-4(5)-4TCC(6).

no, the jump in RPM you describe is, as others have mentionned, the torque converter locking up, which physically links the pump and turbine (in the torque converter), thereby turning the converter into a purely mechanical coupling (vs fluid coupling before lockup).

Quote :

AA wrote:
Your observation is accurate. We have 4 gears + overdrive. The way our PCMs are tuned stock, you almost never get to use 4th gear by itself, so effectively, you have 1-2-3-OD (TCC lock-up).


I disagree, unless you're counting reverse. Overdrive only refers to the fact that with 4th gear having a ratio of 0.7, the output speed is greater than the input speed.

Jason is technically correct. When I say 4 gears + overdrive, I mean 4 gears + TCC lock-up. I use the term OD to mean TCC lock-up, but the two are different things. The reason I think this way is there are many cars with autos that have an "OD" button that, when pressed, disables the TCC lock-up feature. Because of this, I've always made this connection.