Updates?...


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^Well, I've been working until dark every weekday, and I've been out of town the last two weekends.
Hopefully I'll get to it this weekend if it will ever stop raining.

 

Apologies in advance for bumping a thread from December, but I too have been trying to figure out a few things as to how the tranny in my 06 X operates.

Primarily, when coasting (completely off the throttle) at 5-20mph, the RPMs drop to idle and the tranny essentially goes into a "Neutral" mode. Lightly dabbing the throttle simply revs the motor up with no change in speed, exactly as if you're in neutral.

In order to accelerate you have to stab the gas more forcefully, causing the RPM's to jump up to 2k (for example) at which point the tranny engages with a JERK and you begin to accelerate. This can be replicated each and every time when traveling 5-20mph, such as when rolling through a stop sign. Why??

As far as fuel cutoff question, we know that all manual cars employ a fuel cutoff when you release the gas with the tranny in gear and the clutch out. This is called engine braking. It essentially turning the engine into an air compressor, which helps deceleration.

Most cars with a torque converter, to my knowledge, do not cut fuel because, unlike a clutch, the torque converter is a fluid coupling which prevents the forward-spinning wheels from turning the engine. The engine will stall if fuel is completely cut. I'm sure many of us have experience with the engine stalling out while the car is in motion (due to malfunction) in a car with an automatic transmission. You pull over, put the car in Park, and try to restart the engine.

However, if you lock the torque converter then you are essentially simulating a manual transmission with a clutch. Based on my experience, my G35X does cut fuel but only when the torque converter is locked up while you're decelerating. This happens when you are simply coasting with your foot off the gas and brake, and usually at lower speeds (under 45mph) is where this is noticeable..

You can hear the exhaust & motor noise change and run a bit rougher when it happens, because now the engine is acting as a compressor. You can also feel the deceleration even though you're not applying the brakes. And you can see the RPM stay up and not drop to idle.

Somewhere around 10 or 15mph (I don't remember the exact number), the torque converter unlocks, the RPM drop to idle, fuel kicks in, the engine vibration smoothens out, the engine note gets quiet, and noticeable deceleration stops.

As to whether fuel is ever completely cut I don't know, but it is clear that when the torque converter is locked up, the brakes are not applied, and the speed is just right, there is indeed a fuel cutoff mechanism in place.

 

Thanks for the info mike!

Sometimes I just wished I had a 6mt
Or at least a way to change the way the TC behaves because I really hate that 'slipping' feeling with the automatic.

 

The TC used in these cars is a relatively high stall unit, though pretty dang efficent. This is why you see the rpms jump up so high when the rpms are low at the start of throttle application. The TC also incorporates a slip-lock function which allows the TC to be somewhat locked under certain speeds or throttle applications. Most TCs don't employ this function and is why the automatic Gs and Zs are much more responsive and faster that your typical automatic equipped car. This function also allows the fuel cut to be used until getting to very low speeds (typically 10mph or less). Next time you drive and your approaching a stop (off throttle, entirely braking), take notice of the rpms slightly bouncing upwards as the rpms approach about 1100rpms or so.

FYI, when you release the throttle at speed and the rpms drop, the tranny is still engaged. That's why the rpms still hang in the 1200-1300rpm range (unless the TC is fully locked).

 

not knocking anyone's theories... and self proclaimed knowledge...

but there still has been no explanation for the injector cycle while off throttle in the data logs I posted... and the other two members that claimed they would post data-logs still have not.

I'd like to see some real world application (other than what I posted).

 

^I got bored/sidetracked

 

^^^ unfortunately there was lots of back and forth; "I'm right, you're wrong" going on... but only one set of real world data.

 

I'll get to it if/when I feel like it. Truthfully it's not a high priority. Especially since I need to get the car to the body shop

 

Have you guys noticed(not to get off topic here) that when you put the car into MM from D it makes a very high pitched whistle? Almost like a real loud electronic hiss but you cant really hear it until your next to a wall (garage, drive through at mcdonalds lol, etc)

Ive noticed it lately and it seems like when in MM my car feels actually sluggish. But getting back on topic my 2005.5 5AT does what the above video does when cruising at like 55MPH+ Steady when in D then in MM it the RPM start to fall when letting off the gas.

 

decel fuel cut is documented in the service manual - 1400 rpms I believe is the rpm for the stock ECU to re-engage the injectors. I looked it up to set my Haltech at the same threshold rpm for decel fuel cut (helps to not throw codes when trying to pass emissions).

 

^^^ that's already been addressed earlier in the thread...

 

Yes. It's very apparent in a garage, car in run position, but the engine not running.

 

I am still trying to understand what this slip, slip-lock and lock modes are.

first of, I shouldn't be using cruise control to test this correct?

so if I am driving at lets say 70mph in Drive mode with little throttle at 3k rpm, after letting the foot off the gas, the revs should stay the same? The car is expected to do the same in manu-matic mode too or is that a little different?

confused! Will appreciate your feedback!

 

Just tested this the other day. If you're in auto mode and you let off. The RPM doesn't dip down right away and you hold engine speed for a wee bit. Then it eventually dips down slowly.

Do the same in manumatic mode and you'll notice the RPM drops normally rather than holding for a bit.