# Transmission/Torque converter



## dreworz5 (Oct 28, 2013)

So yesterday while having my Cruze in "Drive" I had the brake applied but then touched the gas but only revved the RPM's to about 1100-1200 for about 1-2 seconds tops. Admittedly not understanding how an automatic transmission worked but smart enough to realize that it can't be good for it i quickly got curious and started googling and reading about it. Just a day later I feel like I have the general concept down. However, I have a few questions...

To start, can doing that at even those relatively low RPM's still cause damage?

Also, I understand that the torque converter will "slip" at 600 rpms at a stop light as it is deigned that to do so, but will it also slip if the brake is applied and given gas or will the additional power overwhelm it and force the transmission to spin while the car is stopped... or is that not even possible and it will just "slip" but the higher spinning rate just heat up the fluid and degrade it quicker causing future wear?

I don't want to sound dumb but an honest question I have been trying to answer in general about transmissions is that are the wheels on the road connected enough that both the wheels and the transmission move in equilibrium at all times... for instance, if i'm coasting up to a red light then apply my brakes, does me doing so also stop the transmission as well as the wheels or did the transmission stop once i took my foot off the gas and started coasting?

I highly doubt that my simple mistake of my foot slipping and slightly hitting the gas while also still on the brake could have cause damage but i also can't help but be curious. Are there exact components on a torque converter can be damaged by doing that and cause complete failure? or is it just general stress overall that helps wear it out faster? I've just become interested in understanding all of this stuff as a result.

Thanks for any help in understanding!


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## Robby (Mar 1, 2013)

Transmission is in motion anytime the wheels are turning, regardless of throttle position.
Torque converter is simply a fluid coupling.....the body of the converter is bolted to the engine and spins with the engine.

To simplify a torque converter, visualize two electric fans.....one plugged in and running, and one facing it, not plugged in.
The air being moved by the running fan, contacts the blades of the unplugged fan and in so doing, makes it spin as well.
In the case of a torque converter, the plugged in fan is the engine side, the unplugged on is the transmission input shaft.
The medium. in a converter, is fluid as opposed to air.

If we hold the blades still on the unplugged fan, it is the same as you holding the brakes on your car.
The fluid is trying to turn the input shaft, but you are preventing it with the applied brake.

Short term, no harm at all.....however, if you were to hold the throttle to the floor while holding the brakes (known as brake torque) after about thirty seconds you will begin to overheat the fluid due to the tremendous heat being generated within.

Does that help at all?

Rob


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## dreworz5 (Oct 28, 2013)

Yes it certainly helps! Thank you. So I shouldn't be worried at all about what happened?

I was being careless and with car in "drive" and my foot on the brake I turned around and reached into the back seat and accidentally shifted my foot. In hindsight, I really should have put the car into park before doing so for safety reasons. I did react quickly enough though that when I turned back around I was able to see that the rpms only hit about 1200 so I really barely touched to gas but still it got me to start thinking about how the transmission and such works.


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## obermd (Mar 3, 2012)

As long as you don't hold the RPMs high while the brake is engaged you won't overheat the transmission fluid. 1200 RPM isn't high.


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## carbon02 (Feb 25, 2011)

Robby-

I've often wondered how the transfer of power worked on a torque converter. That was one really cleaver, consise, easy to follow explanation. 

Thanks-


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## NickD (Dec 10, 2011)

If I can go back a few years, was common practice to use the torque converter as a dynamometer. You would block the front wheels, apply full pressure to the service brakes, have a tachometer connected to the engine so you could read it. Put the transmission in drive, then floor the gas pedal. The manual would state what should be the minimum rpm that tach should read to let you know your engine was in good tune.

But this is when transmissions and drivetrains had a strong so-called reserved factor. Not recommended anymore with these newer transmissions, would burn them out if a half axle, differential, etc. doesn't break first. All this started when the only way to improve fuel economy was to reduce vehicle weight.

Transmission housings use to be made of heavy cast iron, today as thin as possible aluminum, to save weight as the excuse. Get concerned about the parking pawl, been cases where people would leave their vehicles in park, get bumped when park to come back to a cracked transmission case. Could leave the transmission in neutral and use the parking brake. But due to safety reasons and unreasonable claim from people running over their own kids, can't even remove your ignition key unless in park. And then have to apply the service brake to release a solenoid to get out of park. Not too many lawsuits in this respect, but changed the entire industry.

But you still can apply your parking brake for extra assurance, and be darn careful when parking diagonally that you don't hit the curb with locking steering wheels. Steering wheel will lock up, but you maybe never to unlock it with all that pressure on the lock.

So there you have it, your safety, thief proof, and to save weight.


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## au201 (May 18, 2013)

The two electric fans analogy is how I learned how a torque converter worked at first. Very good analogy. To answer a few of your other questions...you asked if when you're stopped, does the converter keep slipping at 600 rpm...basically, yes. However, the cruze is a little bit different than almost all other torque converter autos in that it shifts in to neutral at a stop, and therefore the torque converter is basically disengaged from the gears and allowed to spin freely. (That is if my understanding is correct. Still learning myself). However in most cases on most cars the torque converter will be slipping at 600rpm at a stop. 

Also, something else you may or may not know, there is a thing called a torque converter clutch. This clutch engages a lock in the torque converter to almost completely lock the "two fans" together so that they spin at the same speed and do not slip. This engages whenever you are traveling at a speed somewhat above idle and you are "firmly in gear." This keeps the fluid from overheating during constant slipping when driving and also keeps you from wasting gas by losing energy in that slipping. The cruze locks the torque converter very quickly compared to many other cars for fuel economy reasons. Automatic transmissions are, in my opinion, very interesting and quite complicated. 


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## obermd (Mar 3, 2012)

au201 said:


> Also, something else you may or may not know, there is a thing called a torque converter clutch. This clutch engages a lock in the torque converter to almost completely lock the "two fans" together so that they spin at the same speed and do not slip. This engages whenever you are traveling at a speed somewhat above idle and you are "firmly in gear." This keeps the fluid from overheating during constant slipping when driving and also keeps you from wasting gas by losing energy in that slipping. The cruze locks the torque converter very quickly compared to many other cars for fuel economy reasons. Automatic transmissions are, in my opinion, very interesting and quite complicated.


This is the old "overdrive" you used to see in automatics and usually only engaged once you were in your top gear. I suspect the Cruze will engage it in 4, 5 and 6 when driving at steady speeds.


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## jblackburn (Apr 14, 2012)

obermd said:


> This is the old "overdrive" you used to see in automatics and usually only engaged once you were in your top gear. I suspect the Cruze will engage it in 4, 5 and 6 when driving at steady speeds.


Overdrive was usually just a gear that was < 1:1 ratio. Before they introduced "overdrive" gears, your top gear was usually a 1:1 ratio, and depending on the axle ratio, sometimes you'd be screaming along at 4000 RPM at highway speeds. 

Most 3 and 4 speed automatics would shift to a tall top gear and then lock up the torque converter once it could maintain a sufficient RPM that would not lug the engine. Some would do it in 2nd/3rd gear as well when not going uphill. My past 2 automatics would lock the converter in top gear somewhere between 45-55 mph under light throttle. It often feels like an extra shift. 

Our 6-speed Toyota starts aggressive lock up shenanigans in 2nd gear. Shift, lock, unlock, shift, repeat. It feels very busy. I have not yet driven an 8 or 9 speed automatic from ZF, but I really hope they are programmed better. 

It's been over a year since I've driven a Cruze auto, so honestly, I don't remember when that starts locking up.


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## Robby (Mar 1, 2013)

Cruze auto, normal from stop acceleration, combines T/C lockup as it upshifts to third.
The converter stays locked through the remaining upshifts as long as throttle position is maintained.

Any deep throttle position will cause a unlock/and downshift......it'll lock up again once you stop doing the happy dance on the skinny pedal.

Rob


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## au201 (May 18, 2013)

Robby said:


> Cruze auto, normal from stop acceleration, combines T/C lockup as it upshifts to third.
> The converter stays locked through the remaining upshifts as long as throttle position is maintained.
> 
> Any deep throttle position will cause a unlock/and downshift......it'll lock up again once you stop doing the happy dance on the skinny pedal.
> ...


I was under the impression that a momentary unlock of the TCC was required for a shift? You're saying it stays locked through all the shifts? That would explain the harsh 2/3 upshift and the firm (though smooth) shifts from then on.


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## Robby (Mar 1, 2013)

au201 said:


> I was under the impression that a momentary unlock of the TCC was required for a shift? You're saying it stays locked through all the shifts? That would explain the harsh 2/3 upshift and the firm (though smooth) shifts from then on.
> 
> 
> Sent from AutoGuide.com Free App


Your obsevation is correct.....the 2/3 has more things going on than any other shift......and, as long as throttle position is not abrubtly changed, lockup is maintained through the remaining shifts.
The slight harshness has been commented on in the past by other members.

Rob


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## NickD (Dec 10, 2011)

Hmmm, a seven solenoid valve transmission, accustomed to seeing only four. Valve number 3 is for torque converter lockup, is a clutch between the driving turbine to the driven turbine that is engaged to lock them up.

In terms of reading the shop manual, worthless for informing when the toque converter is locked up. With a four speed, recall a Ford where lockup occurred at 37 mph, try to hold <37 mph in traffic, torque converter was constantly engaging and disengaging wearing itself out. To get around this, moved the lever from OD to D to prevent this. GM typically used 45 mph.

Cruze shop manual could have specified the speeds and when torque converter lockup occurred, but left this important bit of information out.

Been reading shop manuals for over 65 years, Cruze shop manual get five stars from me for being the worst most disorganized, least informative shop manual I have ever read. Extremely poor index, and can't even cover one subject in one page. Even something as simple as the auxiliary power outlets, scattered all over three huge volumes. Tell you to do a diagnostic check first with no page reference. So you go to the diagnostic section with no index, and have to page through hundreds of pages to find the correct one.

Its no wonder why the dealers don't know what they are doing.


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