Can anyone explain to me the transmission in my '05 Impala? It's a
base model with the 3400. Transmission just feels odd... there's four
gear positions on the shifter but I feel four shifts up then if I'm on
the highway and apply really light throttle it kind of slowly drops the
engine RPM way down, like there's either a 5th gear or else the torque
converter is real loose and I'm feeling the lockup. Which is it? I
assume the latter but don't have any experience with GM's any newer
than my dad's '73 Chevy pickup.
Also, I assume that this is good for fuel economy but it is practically
impossible to maintain this highest gear ratio as as soon as I have to
lift off the gas for traffic etc. or have to accelerate with more than
a feather touch on the accelerator it "unlocks" (or shifts down) and
RPMs go way up again. Any trick to keep it locked into the top ratio?
I can't imagine this constant shifting is any good for the tranny
either. Would a synthetic fluid be a good idea, if I can't change the
operation of it?
That's the torque converter.
"The converter is designed to provide torque multiplication during
acceleration and at slow vehicle speed, and lock-up during normal operation
for increase operating economy."
In addition to the very important job of allowing your car come to a
complete stop without stalling the engine, the torque converter actually
gives your car more torque when you accelerate out of a stop. Modern torque
converters can multiply the torque of the engine by two to three times. This
effect only happens when the engine is turning much faster than the
At higher speeds, the transmission catches up to the engine, eventually
moving at almost the same speed. Ideally, though, the transmission would
move at exactly the same speed as the engine, because this difference in
speed wastes power. This is part of the reason why cars with automatic
transmissions get worse gas mileage than cars with manual transmissions.
To counter this effect, some cars have a torque converter with a lockup
clutch. When the two halves of the torque converter get up to speed, this
clutch locks them together, eliminating the slippage and improving
really? feels odd to me, the engine drops in pitch by a fifth or more
meaning there's a significant drop in engine RPM. Also kind of oozes
into lockup, feels like there's a lot of slippage (potential wear)
going on there. Is this how other people's cars operate as well?
(can you tell I don't usually drive automatics?)
Steve Mackie wrote:
If you had a tach you could see. My car has the 4T60-E, essentially the same
as your 4T65-E, and it'll usually drop about 250 rpm at highway speeds. If
you hook a scan tool to your car, you can see the "TCC Slip Speed" which is
the difference between engine crankshaft speed and torque converter turbine
It's completely normal for the TCC for take 1 to 5 seconds to apply. My
4T60-E is now 10 years old and has 250,000kms on it, with no noticable TCC
shudder or slipping of any kind. However, if you feel there is something
wrong with your transaxle, go to the dealer and test drive another one and
see if it does the same thing.
Also.... next time you are criusing down the highway, lightly tap the brake
pedal while maintaining accelerator position, you should feel the TCC
release (engine RPM increase), and then a few seconds later apply (RPM
It releases almost instantly on brake application, full release of the
throttle, or depression of the throttle past "a hair..." also the
change in RPM is really dramatic, like I said the engine note drops at
least a fifth or more so since an octave would be 50% of the initial
RPM I guess this would be something like 70% or less of the initial
RPM? The engine really feels like it's on the verge of lugging
although I'm used to driving 4-bangers or alternately older V-8s so I
might be mis-estimating. I guess I just don't see how the converter
could be this "loose," I have not driven a car with such a dramatic
difference between lockup and unlocked, not even my old '56 Golden Hawk
which in the shop manual considered the two states to be different
Steve Mackie wrote:
Part of how GM gets segment leading fuel economy with relatively simple
mechanicals is because of all of the controlled tricker going on in both
engine and transmission management.
Drive another example to make sure yours isn't doing anything it
shouldn't. That said, everything you have described sounds normal.
Sounds exactly the way the transmission behaves in our '02 Oldsmobile
Minivan. 60k+ miles and no sign of transmission trouble yet.
BTW, don't believe GM's claims for extraordinary transmission fluid
change intervals. Our smelled quite bad and was turning brown by 30,000
miles, well before the manual says to change it. Go with a 2
year/24,000 mile maximum ATF change routine to maximize your odds of
getting good life out of the thing.
Cool, thanks. I just wanted some corroboration because it feels a
little weird. then again, I'm pretty much a stickshift snob, and if I
have to drive an automatic I would prefer that it chirp the tires when
it shifts :)
I don't :/
Is there any advantage to switching to synthetic after the car has had
a chance to break in? I haven't decided yet if I'm going to buy it out
when the lease is up; I understand that the deals offered for buying
out your own company car are often decent and it would be nice to have
a car that a) I'm not tempted to spend lots of $$ hopping up and b) has
few enough miles and/or has been maintained well enough that it can be
considered dead-nuts reliable...
Just give it a few years and over 150,000kms, it'll start chirping between
gears. Mine does it, been doing it for a couple of years now. Pretty sure
it's the common problem I hear other people talking about, but I don't want
to fix it. It gives those rice burners a little reality check because
sometimes, if I time it right, I can cruise at about 40km/h, give a little
throttle so it downshifts to first, gets the rpms up then go WOT so it
basically shifts to second at the same time the car launches and she'll let
out quite the chirp. Other than that, she'll chirp 1st to 2nd everytime from
a dead stop at WOT.
"chirp" the tires at WOT? Man, you need a real car. Or more power.
It takes two nice hundred foot patches of sticky rubber to make me grin.
(or is that one nine hundred foot patch? My old Firebird could probably
lay a 900 foot burnout with it's pegleg rear end...)
My view is that you pick up a little bit of safety margin with synthetic
transmission fluid. However, GM recently upgraded the requirements on
the conventional Dexron ATF to "Specification H". Personally I use
major brand conventional ATF and change it at least once per 24,000
miles using the poor man's flush method of removing a fluid line at the
radiator and using idle power to pump out old fluid while adding new.
Takes about 1 case of ATF to do this on our Silhouette. I case of
Chevron Dexron ATF is about $15 at Costco, so this is not an expensive
thing to do.
> I haven't decided yet if I'm going to buy it out
There are some things which maintenance does not ensure reliability of
... like the A/C condenser which is presently being replaced on our '02
Silhouette at a cost of almost $1000 (stealership pricing here in Sunny
We had made a highway speed trip through the Rocky Mountains into
Colorado when our acquired the burnt smell, so perhaps those demanding
driving conditions had something to do with it. Also, the 3.4 L powered
minivan is quite a lot harder on the drivetrain than is a Grand Prix all
other things being equal. More weight, higher revs required to get
power from the engine, more wind resistance and often much higher
passenger + cargo loads. Minivan service for passenger car drivetrains
is really pretty hard on the components.
Personally I feel that spending $15 once every 18 months or so to
replace the ATF plus around $10 for a transmission filter is a bargain
basement bit of maintenance to do. Heck, it costs almost $50 to fill
the fuel tank on our Silhoette. One of the reasons I enjoy doing my own
routine maintenance is that I can maintain the vehicle more agressively
than by-the-book while still having out of pocket expenses much lower
than those who pay to have the work done.
My experience is that automatic transmission failures are more common on
modern vehicles than are engine failures, and the cost of replacement is
very high. But hey, do whatever you want!
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