Transmission Question

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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Some like one of my cars has a six speed double OD automatic.

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snipped-for-privacy@lycos.com wrote:

Can't give you a good answer without more info. Suburbans come / came with about 6 different trans' depending on transfer cases, axle ratios, and engines.
What is the trans code?
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Very good thread guys - thanks for the replies and interesting reading...
It's a 97 GMC K1500 Suburban - don't know tranny codes or axle ratios off hand... I live in eastern Washington and talked to a tranny shop owner who says he frequently rebuilds trannys of people who drive over Snoqualmie Pass and abuse their overdrive. Having had my last Suburban tranny rebuilt, I tried to learn what I could from that experience. In my own observations, the ones who are asking for trouble are those who leave their cruise control set at 70mph up a grade and just sit there on the way up conversing with passengers, as the tranny kicksand out of passing gear (3rd), as the vehicle tries to maintain that cruise control speed setting of 70mph up the grade. From what I now believe to be true, that's definitely a tranny killer so I'm now generally kicking the cruise control OFF during those uphill occasions and settling back to a comfortable climbing speed of 55-60mph when climbing a fairly steep grade. I think experienced drivers know when they're putting their vehicle at risk and increased stress by kicking it in the butt with passing gear and forcing it to maintain that 70 (or better) up a steep hill. All that said, I don't want to end up in a tranny shop again if I can ever avoid it in this lifetime... lol
I don't know a lot about how torque converters work (lock vs unlock) and how that impacts the hill climb but a big question I *have* had is, if I'm going up a grade starting in overdrive at the bottom of the steep grade and as my speed slows, depress the accelerator gently to get it into 'passing gear', is there any reason at that point that I can't just pull the transmission handle into 3rd gear (while in 'passing gear')? I would normally DO this while holding my constant speed and without letting off the gas. So in essence - for a given speed, just pull the tranny handle into 3rd to get it out of OD and save the transmission. OR... does it matter? In other words, if I'm climbing the grade at a comfortable 60mph, no cruise on, engine temp is good, at approx 2300 RPM in overdrive though 'passing gear' that we've hashed about, is there indeed any difference?
thanks again guys for the replies... It's very informative. :-)
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You drive that stretch a bunch, so smoothing a pattern out should be easy. Hit the grade at normal speed with cruise on and in OD. You should know by now when the tranny will want to kick down. Just go ahead and shift the tranny into 3rd at about that point. Use the set/ coast button to lower rpm's if desired and just let the truck keep on going. As you level out over the pass, shift back into overdrive and bump up your speed to desired level with the resume/accel slide.
If you know for a fact that the tranny will only downshift once, on a grade, don't even bother with anything. Depending on the grade you can even use the set/coast button to lower your speed a bit at a time and the tranny won't downshift, but this is dependent on the vehicle and how good you are at hearing what your truck is telling you. It generally has a certain hum to it when it is ready to downshift and you just tell it to slowdown a bit and this will delay the shift.
Now, this isn't recommended for long or steep grades, or grades you are not familiar with. I generally don't like my truck downshifting and blowing the extra fuel out the pipe for a measly 5 mph, so I'll just drop the speed down a bit, and if the cruise control is being extra sensitive I just flick it off and let my foot keep the tranny from downshifting. If I drop below 55, or traffic speed, It's blow the fuel time, though.

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