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

On Mon, 24 Aug 2009 22:18:19 -0700 (PDT), jcage (AT) lycos (DOT) 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.

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

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.

Il mittente di questo messaggio|The sender address of this
non corrisponde ad un utente |message is not related to a real
reale ma all'indirizzo fittizio|person but to a fake address of an
di un sistema anonimizzatore |anonymous system
Per maggiori informazioni |For more info
https://www.mixmaster.it

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


"ben91932" <benteaches (AT) gmail (DOT) com> wrote

On Aug 25, 5:37*am, PeterD <pet... (AT) hipson (DOT) net> wrote:
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).

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?

<jcage (AT) lycos (DOT) com> wrote

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

jcage (AT) lycos (DOT) 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?

On Tue, 25 Aug 2009 19:56:40 -0700 (PDT), jcage (AT) lycos (DOT) 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.