Transmission Question

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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'. During this past weekend, I was going through a mountain pass, held it in overdrive (in passing gear) throughout this ordeal. I was traveling 60mph in OD and in passing gear, 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. 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.
In short - isn't overdrive passing gear the same as running in 3rd gear? I always heard it was but thought I'd try putting it into 3rd gear on the way home and was surprised to see the same engine RPM with a lower speed and a hotter running engine (as per my temp gauge).
Anyone have info on this? I guess my question is, isn't overdrive passing gear the same as 3rd gear??? If this assumption is correct, nothing should happen when I'm in overdrive passing gear on a mountain pass and pull the transmission lever into 3rd gear.
TIA for any info on this...
John
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On Mon, 24 Aug 2009 22:18:19 -0700 (PDT), snipped-for-privacy@lycos.com wrote:

There is no such thing as 'passing gear'. What you call 'passing gear' is just a downshift into (probably) third gear.

It was either held in OD (not possible) or was downshifted into third.

Third gear.

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)?

Just above you said it was the same, now you say different, so which was it?

Third, then as things slow down, second. IOW, the transmission simply downshifts to allow the engine to achieve higher RPMs.

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Thanks Peter - I assumed that but wasn't sure...

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.

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).
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On Tue, 25 Aug 2009 19:56:40 -0700 (PDT), snipped-for-privacy@lycos.com wrote:

Sounds to me that although you thought the grade was the same of less it was actually more. Otherwise you would not have felt the need to put it into third and even then had difficulty maintaining speed. The increased grade caused the engine to run hotter. It can be very easy to be fooled about how steep a grade is, there are places where people swear water is running uphill because they are certain it's an up grade when in fact it's a downgrade.
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On Tue, 25 Aug 2009 19:56:40 -0700 (PDT), snipped-for-privacy@lycos.com wrote:

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.

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?
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wrote:

Might want to check the oil quality in this case. Maybe a head gasket leak 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.
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3rd gear is passing gear...nothing more than a downshift. Overdrive is to lower the rpm for cruising. 3rd gear is usually 1 to 1, and overdrive appx .7 to 1. HTH, Ben
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WTF is a kickdown gear? Does it just float around and jump in as needed?
Yes, if you downshift the engine will run faster and not lug. There is a range that gives the most power and this varies with the design of the engine.
I'm surprised no one has mentioned the torque converter yet.
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Well the TC lock should be engaged so thats why its not mentioned.
If the lock is not engaged, then it could cause overheating.
'On the way home' means different driving conditions so thats why the speeds are different. Also, at altitude his mixtures will be different. So it depends at what altitude he started out from. He probably was running a little leaner on the return trip, resulting in higher temps.
Standard gears are 1,2,3,4 for a 4 speed OD.
Cheers
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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?
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I just checked this for the 460LE trans and the TC lock up can only occur in 4th(OD). This is an 95 model so I'm not sure about yours. However, if there is a 4-3 downshift then the TCC is released. So driving in OD and down shifted into 3rd is the same as running in 3rd. And the TCC is disabled in either case.
I see some mention of lockup in 3rd on some forums, so that may be the case is later models. Either way the same conditions exist in OD(downshifted) or 3rd. Lockup is primarily to increase economy.
Cheers
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On Tue, 25 Aug 2009 19:58:37 -0700 (PDT), snipped-for-privacy@lycos.com wrote:

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.
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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?
Ever drive a car that unlocked the TC with no/less throttle applied? Kind of a surprise when you realize that there is no engine braking when you were expecting some the first time through. :)
wrote:

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On Wed, 26 Aug 2009 09:07:40 -0500, "Nightcrawler"

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.

They all do. I think it would be hard to find one that doesn't.

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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. 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. 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.
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.
wrote:> That would lower gas mileage. They have been like that (unlock on

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On Thu, 27 Aug 2009 10:55:40 -0500, "Nightcrawler"

You could try this, but you may find there are downsides to it when maximum power is required. I don't think you will get maximum performance if it (the TCC) is 100% locked.

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.

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!)
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'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>

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wrote:> Again, an idea in thought that doesn't translate to practical in the

Hehe, that's my problem. Grew up with manuals and old T 350/400's. When you let off of the gas you were slowing down unless you had enough throttle on. I like to utilize that live axle.

The GM's will do this too, depending on which Allison tranny you have. 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.

It's not that the brake system would know, it's that one should not have 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. :)
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On Thu, 27 Aug 2009 14:50:44 -0500, "Nightcrawler"

Absolutely Fords! I noticed that too when I was *forced* to use a Ford 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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Maybe it's the exhaust system. I don't know. I do recall that my Silverado seemed to have more engine braking before I put the cat back system on it. It's still better than a Ford, but I still think it's bunk that the TC won't give a good hook-up until you are in OD when it really doesn't matter anymore. When I mash down the peddle I want the ponies going to the rear wheels, damn it! :)
Honestly, I don't know if I've got a dog (I have a '99 2500 with the 6.0), but when you stomp on it you have to wait a long time before she decides that she wants to scoot, once she's up to around 4500 rpm she screams, but that's a bit odd to me. Zero tire chirpage, whatsoever. Of course she being a dog has nothing to do with the TC, just something to bitch about.

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On Thu, 27 Aug 2009 14:50:44 -0500, "Nightcrawler"

The reason you slowed down in those was because they did not have a 4th gear overdrive. Their TCs didn't have any lockup capability. Drive your car in third gear and you while have the same thing today as you had back then.

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