Transmission Shifting (was Gas Tanks)

D

dave lowry

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Jul 7, 1998
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Arch; Your posts last Friday prompt the following:

It sounded like your tranny downshifts from third to second when ever you
get below 42, irrespestive of throttle position. Is that right?

I've got the opposite problem. Mine will upshift to 3rd below 30 under very
light throttle, but won't DOWNSHIFT under hard throttle until I either get
down below 35 or yank the selector. I know the kick down switch is working
and sending juice to the modulator, but nothing happens. I've been told
that I'll need a new modulator, but now I wonder. Could low or no vacuum
cause a downshifting as well as upshifting problem? I'm going to have this
problemd fixed, but I need to be armed with facts to avoid getting burned.

Input from the collective wisdom of "GMCnetters" would be greatly appreciated.

TIA
 
Dave --

Only commenting on what little I do know...

> Could low or no vacuum
> cause a downshifting as well as up shifting problem? I'm going to have
this
> problemd fixed, but I need to be armed with facts to avoid getting burned.

The vacuum modulator's primary purpose is to control the tranny shift points
in relation to engine vacuum.

Light throttle acceleration produces higher engine vacuum, this forces the
tranny to shift up.

Heaver acceleration has low engine vacuum, this delays the up shift, or
forces a down shift.

To answer your question more directly, low engine vacuum would cause the
coach to hang in second without up shifting until the mechanical tranny
governor forces the up shift.

That's why the failure of the vacuum modulator produces the
'stuck-in-second' symptom. This is a fairly common failure for this tranny.

People have written about adding a second manually controlled foot operated
push button to force the tranny into a lower gear using the electric shift
down circuit. Early shifting of these THM (turbo hydra-matics) has always
been a common complaint.

It sounds like Arch's answer to the problem is getting very close to a
working solution.

Hope this helps,

Mark
 
Thanks Mark (and Arch). If low or no vacuum causes it to hang in second,
then I know that's not my problem. But now I've got an even dumber
question; if the amount of vacuum to the modulator controls shift points,
what does the current from the kick down switch do? Is the modulator a 12V
or vacuum solenoid?

BTW, I have added the manual overide switch, but still no downshift.

Always learning,

___________
Dave (& Dege), '76 Royale /_][__] [_] | "SR JAMES"
Santa Barbara, CA *0-------OO--* (our hobby)
 
Someone recently raised the interesting idea of installing a floor-mounted
switch to force downshifts.

I assume this could be a second "headlight dimmer" type of switch wired in
parallel with the kick-down switch that closes when you stomp on the accelerator
pedal. Is this correct?

According to the service manual the kick-down switch energizes a solenoid within
the transmission to cause the downshift.

Would this method of selecting second gear at part-throttle have exactly the
same effect as using the shift lever?

Is pressure within the transmission increased in the same way?

Any problem driving for 10-15 minutes at a time with the solenoid energized?

This would be a useful add-on for those of us who drive in the hills and like to
keep the revs up to climb and use engine braking when coming back down.

Richard
 
Good idea Richard.

The kickdown switch is just that, a switch. I just replaced mine. I couldn't
kick down, i.e. switch stayed open.

I think the opinion against may be that you'll forget to switch it back, or
rather off.

I'll be listening intentlly re comments to the downside of shifting via the
solenoid vs lever.

Heinz

.
- -----Original Message-----
From: Richard Guthart
To: gmcmotorhome
Date: Monday, June 07, 1999 2:30 PM
Subject: Re: GMC: Transmission Shifting (was Gas Tanks)

>Someone recently raised the interesting idea of installing a floor-mounted
>switch to force downshifts.
>
>I assume this could be a second "headlight dimmer" type of switch wired in
>parallel with the kick-down switch that closes when you stomp on the
accelerator
>pedal. Is this correct?
>
>According to the service manual the kick-down switch energizes a solenoid
within
>the transmission to cause the downshift.
>
>Would this method of selecting second gear at part-throttle have exactly
the
>same effect as using the shift lever?
>
>Is pressure within the transmission increased in the same way?
>
>Any problem driving for 10-15 minutes at a time with the solenoid
energized?
>
>This would be a useful add-on for those of us who drive in the hills and
like to
>keep the revs up to climb and use engine braking when coming back down.
>
>Richard
>
>
 
Gene,

Yes, we have a push button switch that does what the WOT downshift switch
on the gas pedal does. We got the idea from one of the articles in an old
issue of GMCMM which gives the details of wiring/operation. We have it
floor mounted so it can be pushed and held with the foot when the lower
gear is desired. Pushing the switch will not allow for engine braking
while going down hill as someone else desired, however. The trans goes
into a quasi-neutral while going down-hill with the button pressed until a
high enough speed is reached that the trans shifts into third (around 65
mph for us). It then has engine braking in 3rd as it normally would. The
switch works very well for us for holding lower gears longer while starting
from a stop and downshifting more easily/safely while going uphill or
accelerating.

Zak

>-- [ From: Eugene Fisher * EMC.Ver #2.5.3 ] --
>
>Me toooo
>
>I have never liked the "punch it down" in the GMC, in fact, I am not
>sure mine works. I would like to shift into second with a switch rather
>than the lever. I am never sure I am going down just one position or up
>one position with the sticky lever/selector. I would like a push button
>on the steering or some thing like that. I think I remember Zac saying
>he has this with out causing problems.
>
>While we are on shift points. I was following a SOB and talking on the
>CB. We started up a hill he said --- just went out of over drive ---
>ok so we don't have an over drive.
>
>He said---- just shifted down ---- Mine never did shift down, I pulled
>it down at 2000 rpm manually and then put it back at 3000 rpm.
>
> Does the shift kit fix these shift points ?? and what are they before
>and after ?
>
>I would like my transmission to keep the engine between 2000 and 3000
>all the time.
>gene
 
>The trans goes into a quasi-neutral while going down-hill with the
>button pressed until a high enough speed is reached that the trans
>shifts into third (around 65 mph for us). It then has engine braking in
>3rd as it normally would.

Have you ever noticed or know what might happen if after you (gracefully)
downshifted into 2nd (without the 4 barrels dumping fuel into the intake)
you just moved the shift into 2nd before the crest of the hill?

- --
"I do whatever my Rice Krispies tell me to..." John said, from inside a
1974 Glacier.
 
Well Rick,

Guess I'll just have to continue to row the shift lever.

Richard

> While the separate "kickdown" switch may be a good idea, I don't think it
> should or can replace manually downshifting to second or even first.
> Consider the following...
>
> 1. ..................you would probably not get any engine braking from a
manual "kickdown" switch.
>
> 2. ..................As far as I can determine, the kickdown switch doesn't
increase line pressure directly. Also, the
> aforementioned bands assist the sprag clutches in transmitting torque, which
> is why I always recommend downshifting in the mountains, uphill or down.
 
John,

We haven't had to move the lever much at all once we installed the button.
From the few times we have done it while going up-hill, I think it would
work fine and just as expected. We actually don't pick up much speed with
the trailer on even when in neutral. The aerodynamics of the trailer seem
poor enough that a couple of applications of the brakes are all that is
needed for even large hills. We are, however, comfortable enough with the
handling that we allow it to get to 75-80 (in third gear with engine
braking) before we really would think about trying to slow it down. We
would never want to put it in second and be forced to slow down to 55-60 to
keep the engine RPMs in an acceptable range. We actually like that the
button does the quasi-neutral (going down-hill with no throttle) because we
often use it to allow coasting down smaller hills. It would be a pain to
shift into neutral all the time to do this.

Zak

>>The trans goes into a quasi-neutral while going down-hill with the
>>button pressed until a high enough speed is reached that the trans
>>shifts into third (around 65 mph for us). It then has engine braking in
>>3rd as it normally would.
>
>Have you ever noticed or know what might happen if after you (gracefully)
>downshifted into 2nd (without the 4 barrels dumping fuel into the intake)
>you just moved the shift into 2nd before the crest of the hill?
>
>--
>"I do whatever my Rice Krispies tell me to..." John said, from inside a
>1974 Glacier.
>
 
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