Reasons to S range

[gene]

This brings up a lost reference.

Some time ago, some one posted a note that there were 3 three things that
happened when you pull the trans down into S range rather than using a
switch or the gas peddle kick down. I did not keep the reference but I
would like to see it again. The thins were something like:

the S range used two additional clutches or bands ???
maybe this apply pressure ??

I wonder if you could explain these to me ?? I know it convinced me to
pull it down manually, now would like to know why ;>)

gene


>
>> My Tranny won't do reverse at all now (acts like neutral) and then
>> it drops out of 3rd into 2nd when on the highway as soon as you touch the
>> throttle. 1st and 2nd are OK. As long as I don't want to go over 65 mph or
>> backup I'm fine.
>
>Nate,
> Sorry to hear of your tranny trouble. From your description, it sounds
>as though your "Direct Clutch" (second one in from the drive chain end) is
>shot. This clutch is used (only) in 3rd gear drive range, and in reverse.
>That's the easy part. Figuring out WHY it failed is harder, as is deciding
>whether to do a complete overhaul.
> Preventing a recurrence might involve using heavy-duty clutch plates and
>slightly raising the apply pressure via a shift kit or whatever. Most
>automatic trannies reduce the main oil pressure in high gear to minimize
>power loss and give better gas mileage. Not really helpful in a 12,000 lb
>GMC, and another good reason to use the "S" range on long hard pulls. ("S"
>INCREASES apply pressure quite a bit.)
> Good luck. Holler if we can help.
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/

 

[gene]

Yes this was the info. The insides of that monster the transmission are
really mysterious to me. But I like the sound of what you are saying.

I would rather have an electric down shift, and I might put in a foot
switch like Zak has for those times when I am too busy driving to pull down
to S. But I also think driving in S during those pesky ups-n-downs is much
better than the wild thrashing around of putting the peddle to the metal on
the hard pulls. Now to know there are several advantages makes me confident
of pulling down into S.

Thanks for the info and I have put it up on the page so I can find it
again during my senior moments.

gene


>
>> Some time ago, some one posted a note that there were 3 three things that
>> happened when you pull the trans down into S range rather than using a
>> switch or the gas peddle kick down. I did not keep the reference but I
>> would like to see it again. The thins were something like:
>>
>> the S range used two additional clutches or bands ???
>> maybe this apply pressure ??
>>
>> I wonder if you could explain these to me ?? I know it convinced me to
>> pull it down manually, now would like to know why ;>)
>>
>
>Gene,
> Don't know if it was my post you referred to, but I have advocated using
>"S" rang in the mountains.
>
> 1. When in "S", the tranny line pressure (which applies the clutches)
is
>increased from its normal 60 to 90 psi, to a more robust 135 to 160 psi.
>This almost doubling of pressure should greatly reduce the chance of
>slipping/burning-out your clutches.
> 2. In "D" or Drive range, both the 1-2 and the 2-3 upshift are effected
>by a multi-disc clutch picking up the load from a sprag or overrunning
>clutch. This simplifies shift element timing (you needn't be sure the lower
>gear apply element releases precisely when the higher gear element applies),
>giving smoother upshifts with no danger of "flare" or overreving between
>gears. (If anyone recalls the Peter, Paul & Mary routine about "The Kid"
>drag racing "Mr. Businessman" who just had his "bands tightened", you know
>what "flare" sounds like.) ;-) OTOH, this arrangement means there is NO
>engine braking in 1st or 2nd gear drive range. Since this is hard on brakes
>in hills and traffic, GM provide the "L" and "S" ranges. In these ranges
the
>sprag clutch is supplemented by a band (actually a form of brake) which
holds
>the given gear element stationary against force in EITHER direction,
>restoring engine braking. (Of course now we must release the lower gear's
>band when upshifting, but hey, Hydra-Matic's been doing this since 1938.
>They can figure out the timing.) IMHO, these bands also shoulder some of
the
>load when accelerating forward, assisting the sprag clutches in their
mission.
> 3. Of course, "S" keeps the transmission from upshifting into 3rd gear,
>and may also raise the 1-2 shift point.
>
>HTH.
>
>Rick Staples
>'75 Eleganza
>Louisville, CO
>
Genef -- 77PB/ore/ca
GMC MOTORHOME INFORMATION
mr.erf
http://www.california.com/~eagle/