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Transmission Question
4x4 Chevy/GMC Trucks 4x4 Chevy/GMC Trucks Discussions (alt.autos.4x4.chevy-trucks)
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#11
 
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Re: Transmission Question -
08-26-2009
, 02:40 AM
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I'm surprised no one has mentioned the torque converter yet. Let's talk about that... Is there a different impact or result on what the torque converter delivers in OD when on a hill and the accelerator is depressed into 'passing gear' versus climbing a hill in 3rd gear where the shifter has been pulled from OD into 3rd? |
Yes. It's possible to be in OD with the TC locked or unlocked,
depends on how much power is being called for. Same is true for third
gear. What could have happened, in addition to the possibility that
the grade coming back was steeper then you thought, was that on the
way up it dropped out of OD but the TC stayed locked, but on the way
back when you put it in third the TC unlocked and stayed unlocked. An
unlocked TC generates extra heat and is simply less efficient then a
locked TC, so that could also be a factor in why you were running
higher temperatures. In vehicles I've driven, you can be climbing at
a steady speed with the TC locked and just lightly tap the brake pedal
and that will make the TC unlock. If it's a decent grade, it may not
lock up again until you let off on the gas.
#12
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On Aug 25, 5:37*am, PeterD <pet... (AT) hipson (DOT) net> wrote: On Mon, 24 Aug 2009 22:18:19 -0700 (PDT), jc... (AT) lycos (DOT) com wrote: I have a GMC Suburban and have a tranny question. *I was under the impression that so long as the tranny wasn't shifting up and down to maintain a speed over hills (i.e. maintain speed with cruise control set), that my transmission wouldn't be harmed. My question really seems to be about 'passing gear'. * Thanks Peter - I assumed that but wasn't sure... at 2300 RPM, the temp gauge looked great and it was "steady as she goes" through this mountain pass area. *On the way home on the other hand, and for the same hills, I down shifted to 3rd gear and was holding a steady 55mph at the same engine RPM but the engine was running hotter. Notice what you just wrote: third gear, same speed to engine RPM... How do you know the engine was running 'hotter'? Do you have a gauge other than the factory one (which is of questionable accuracy)? This was the most interesting point in my experience. In OD 'passing gear' [suffice to say 3rd] I was running at normal operating temperature at 2300 RPM at 62 MPH. On the way home and on a similar or lesser grade and at night, was running in 3rd gear (had pulled it into 3rd), was only managing around 50-55 MPH and the temperature was running hotter as per the gauge in the vehicle (which normally seems to work well, relatively speaking). In short, it's as if the OD 'passing gear' and pulling it into 3rd gear were in fact, two separate gears or settings. |
(going up the hill) downshift, is locked if you are pulling the
shifter lever to D (third, instead of fourth). TCC can be locked in
either third or fourth, but when you press the accelerator, the
transmission will unlock it when it does the downshift.
Quote:
| Interesting to note too, that on the way over the first time while in OD *passing gear, for the same RPM my speed was higher. Just above you said it was the same, now you say different, so which was it? the RPM was the same, in one case, I was in OD 'passing gear' and in the other, I had pulled the transmission lever into 3rd gear. Same RPM, different achievable speeds and different engine temperatures (not running HOT, just notably warmer for the slower RPM while manually in 3rd). |
downhill run... The engine should have been receiving minimum fuel as
it was not doing any work. That is one of the reasons that I suspected
your gauge not being accurate (or something interfering with the
gauge's reading). What engine, and vehicle? Does it have a fan clutch?
#13
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I am happy to answer your question. When you floor the gas pedal, you engage the "kickdown gear." The purpose of this gear is to rev the engine which also revs the water pump to keep the engine cool. Otherwise the engine "lugs" and this overheats the engine, causes "pinging" and can lead to piston burnthru, exhaust valve burning/warpage and rod bearing overload and melting out. I hope this clarifies the matter. |
did nothing.
#14
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On Tue, 25 Aug 2009 19:58:37 -0700 (PDT), jcage (AT) lycos (DOT) com wrote: I'm surprised no one has mentioned the torque converter yet. Let's talk about that... Is there a different impact or result on what the torque converter delivers in OD when on a hill and the accelerator is depressed into 'passing gear' versus climbing a hill in 3rd gear where the shifter has been pulled from OD into 3rd? Yes. It's possible to be in OD with the TC locked or unlocked, depends on how much power is being called for. Same is true for third gear. What could have happened, in addition to the possibility that the grade coming back was steeper then you thought, was that on the way up it dropped out of OD but the TC stayed locked, but on the way back when you put it in third the TC unlocked and stayed unlocked. An unlocked TC generates extra heat and is simply less efficient then a locked TC, so that could also be a factor in why you were running higher temperatures. In vehicles I've driven, you can be climbing at a steady speed with the TC locked and just lightly tap the brake pedal and that will make the TC unlock. If it's a decent grade, it may not lock up again until you let off on the gas. |
#15
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On Tue, 25 Aug 2009 19:56:40 -0700 (PDT), jcage (AT) lycos (DOT) com wrote: On Aug 25, 5:37 am, PeterD <pet... (AT) hipson (DOT) net> wrote: On Mon, 24 Aug 2009 22:18:19 -0700 (PDT), jc... (AT) lycos (DOT) com wrote: I have a GMC Suburban and have a tranny question. I was under the impression that so long as the tranny wasn't shifting up and down to maintain a speed over hills (i.e. maintain speed with cruise control set), that my transmission wouldn't be harmed. My question really seems to be about 'passing gear'. Thanks Peter - I assumed that but wasn't sure... at 2300 RPM, the temp gauge looked great and it was "steady as she goes" through this mountain pass area. On the way home on the other hand, and for the same hills, I down shifted to 3rd gear and was holding a steady 55mph at the same engine RPM but the engine was running hotter. Notice what you just wrote: third gear, same speed to engine RPM... How do you know the engine was running 'hotter'? Do you have a gauge other than the factory one (which is of questionable accuracy)? This was the most interesting point in my experience. In OD 'passing gear' [suffice to say 3rd] I was running at normal operating temperature at 2300 RPM at 62 MPH. On the way home and on a similar or lesser grade and at night, was running in 3rd gear (had pulled it into 3rd), was only managing around 50-55 MPH and the temperature was running hotter as per the gauge in the vehicle (which normally seems to work well, relatively speaking). In short, it's as if the OD 'passing gear' and pulling it into 3rd gear were in fact, two separate gears or settings. I suppose I should also mention that TCC lockup, which is unloced in a (going up the hill) downshift, is locked if you are pulling the shifter lever to D (third, instead of fourth). TCC can be locked in either third or fourth, but when you press the accelerator, the transmission will unlock it when it does the downshift. Interesting to note too, that on the way over the first time while in OD passing gear, for the same RPM my speed was higher. Just above you said it was the same, now you say different, so which was it? the RPM was the same, in one case, I was in OD 'passing gear' and in the other, I had pulled the transmission lever into 3rd gear. Same RPM, different achievable speeds and different engine temperatures (not running HOT, just notably warmer for the slower RPM while manually in 3rd). I really don't understand why the temp got hotter when doing a downhill run... The engine should have been receiving minimum fuel as it was not doing any work. That is one of the reasons that I suspected your gauge not being accurate (or something interfering with the gauge's reading). What engine, and vehicle? Does it have a fan clutch? |
from water to oil? No way should an engine heat up on a downhill glide
unless there is too much friction on the engine parts.
#16
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I still think that the TC should remain locked, unless shifting/low speed/stop-n-go, period. I might give a little bit of leeway for coasting, but 100% off throttle = engine braking in my book. I wonder if it is possible to change the programming for this? |
coast) for over 20 years and I'm used to it now. Just use the brakes
if you need to slow down.
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Ever drive a car that unlocked the TC with no/less throttle applied? |
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Kind of a surprise when you realize that there is no engine braking when you were expecting some the first time through.  "Ashton Crusher" <demi (AT) moore (DOT) net> wrote in message news:60m995hap4je0i2iubh5tgr7iie6eu1ho7 (AT) 4ax (DOT) com... On Tue, 25 Aug 2009 19:58:37 -0700 (PDT), jcage (AT) lycos (DOT) com wrote: I'm surprised no one has mentioned the torque converter yet. Let's talk about that... Is there a different impact or result on what the torque converter delivers in OD when on a hill and the accelerator is depressed into 'passing gear' versus climbing a hill in 3rd gear where the shifter has been pulled from OD into 3rd? Yes. It's possible to be in OD with the TC locked or unlocked, depends on how much power is being called for. Same is true for third gear. What could have happened, in addition to the possibility that the grade coming back was steeper then you thought, was that on the way up it dropped out of OD but the TC stayed locked, but on the way back when you put it in third the TC unlocked and stayed unlocked. An unlocked TC generates extra heat and is simply less efficient then a locked TC, so that could also be a factor in why you were running higher temperatures. In vehicles I've driven, you can be climbing at a steady speed with the TC locked and just lightly tap the brake pedal and that will make the TC unlock. If it's a decent grade, it may not lock up again until you let off on the gas. |
#17
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On Wed, 26 Aug 2009 09:07:40 -0500, "Nightcrawler" Dirtydeeds (AT) dirtcheap (DOT) net> wrote: That would lower gas mileage. They have been like that (unlock on coast) for over 20 years and I'm used to it now. Just use the brakes if you need to slow down. Ever drive a car that unlocked the TC with no/less throttle applied? They all do. I think it would be hard to find one that doesn't. |
#18
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I used the wrong term. Engaged should be used instead of locked. Two different concepts. Regardless, I'd prefer a TC that locks when 100% engine power is needed at the wheels. |
maximum power is required. I don't think you will get maximum
performance if it (the TCC) is 100% locked.
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I'd prefer one that will stay engaged if I let off the throttle a bit (say up to 50%) and want to use the engine to brake the vehicle. |
real world. Most (not all, but most) use a much lower value for TCC
lockup, (about 10%) but even 10% throttle will give virtually no
engine braking.
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This is beneficial when cruise control is in use since you don't have to hit your brakes, turning the cruise control off, when going down a slight grade, then have to "resume" when you level out. I don't have this problem as much with my Silverado, but I've noticed it is severe with rental cars. |
(for example a 4L80E will support this, and IIRC so will a 4L60E) and
that is an 'aftermarket' lockup switch. We use these on (diesel)
trucks and it works, but the driver must remember to switch it off
below a minimum speed or things get *really* interesting (think a
manual with the driver being unfamiliar with the use of the clutch
peddle!)
Now, as to down hill driving... My (ahem, Dodge) truck will
automatically down shift to hold speeds going down hills when the
cruise control is on, and when the 'tow/haul' switch is also on. I
believe the Alison transmission behind the Duramax will do this as
well, but I'm not 100% sure. I can say, in the mountains where I live
this is handy. You don't have to ride the brakes, manually downshift,
or do anything: the ECM/PCM takes care of it.
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It's also a nuisance in traffic when everyone is hitting their brakes just to bleed off speed. Brake lights should only indicate rapid slowing, or stopping, not a minor deviation in speed. |
intentions! At one time brake lights on some cars and trucks were
pressure activated, and would only light with significant pressure.
Keep in mind the flip side of the coin: car starts to slow down
slightly, then does a panic stop: I'd rather get an advance warning
myself... <g>
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| "Ashton Crusher" <demi (AT) moore (DOT) net> wrote in message news  tac95hsr8bao1q7b71q63la5qsikkmepk (AT) 4ax (DOT) com...On Wed, 26 Aug 2009 09:07:40 -0500, "Nightcrawler" Dirtydeeds (AT) dirtcheap (DOT) net> wrote: That would lower gas mileage. They have been like that (unlock on coast) for over 20 years and I'm used to it now. Just use the brakes if you need to slow down. Ever drive a car that unlocked the TC with no/less throttle applied? They all do. I think it would be hard to find one that doesn't. |
#19
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On Thu, 27 Aug 2009 10:55:40 -0500, "Nightcrawler" Dirtydeeds (AT) dirtcheap (DOT) net> wrote: Again, an idea in thought that doesn't translate to practical in the real world. Most (not all, but most) use a much lower value for TCC lockup, (about 10%) but even 10% throttle will give virtually no engine braking. |
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There are two things you can try if you have the 'right' transmission (for example a 4L80E will support this, and IIRC so will a 4L60E) and that is an 'aftermarket' lockup switch. We use these on (diesel) trucks and it works, but the driver must remember to switch it off below a minimum speed or things get *really* interesting (think a manual with the driver being unfamiliar with the use of the clutch peddle!) |
When you let off of the gas you were slowing down unless you had
enough throttle on. I like to utilize that live axle.
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Now, as to down hill driving... My (ahem, Dodge) truck will automatically down shift to hold speeds going down hills when the cruise control is on, and when the 'tow/haul' switch is also on. I believe the Alison transmission behind the Duramax will do this as well, but I'm not 100% sure. I can say, in the mountains where I live this is handy. You don't have to ride the brakes, manually downshift, or do anything: the ECM/PCM takes care of it. |
I know someone that had an old pole truck that had an Allison tranny
that you could dial in the max speed for down hill grades. Flip the switch,
turn the knob and presto, you're doing that speed. Well, it might take
a bit to slow down, but it worked.
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I'll make no comments about how the brake system would know your intentions! At one time brake lights on some cars and trucks were pressure activated, and would only light with significant pressure. Keep in mind the flip side of the coin: car starts to slow down slightly, then does a panic stop: I'd rather get an advance warning myself... <g |
to hit the brakes for a reduction in speed. I rented a Ford a while back
and if you let off of the gas you'd coast forever. So, instead of just letting
off the gas, while in heavy traffic, and letting the vehicle slow itself via
engine drag, you would have to hit the brakes every time. One or two
miles an hour does not seem like much, but it is enough to ride up on
someone's tail if you don't want to.
I've spent countless hours in heavy traffic behind vehicles like this and
you start to ignore their brake lights because there really isn't any apparent
change in distance between you and them, they're just trying to maintain
a certain gap between them and the vehicle in front of them. My work
trucks would only require a little lift of the accelerator, that Ford required
a more active approach.
Hmm, the Dodge Stratus I drove slowed down on its own, too. Must be them
crappy Fords.
#20
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a certain gap between them and the vehicle in front of them. My work trucks would only require a little lift of the accelerator, that Ford required a more active approach. Hmm, the Dodge Stratus I drove slowed down on its own, too. Must be them crappy Fords. |
loaner, and it drove me nuts going down the local (hilly) terrain.
You'd let off the accleerator at the top and gain 20-40% speed. Bloody
death traps!
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