91 Camry Automatic Overdrive

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Old 02 Jun 2006, 01:08 am   #1 (permalink)
jg
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Default 91 Camry Automatic Overdrive

Does anyone actually know how it works?
Owners manual says OD off simply prevents it using the OD gear - it affects
the whole gearbox range so that ain't right.
The condescending repair manual (which doesn't even have a photo of it),
says it stops the torque converter slipping so much - that ain't right
either because the motor revs faster with OD off even down hills.
My guess would be a planetary system devoted soley to OD and controlled by a
hydraulic solenoid, but I've never seen inside one. Scarey the manuals
simply don't know... what other errors are in them?


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Old 02 Jun 2006, 07:31 am   #2 (permalink)
m Ransley
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Default Re: 91 Camry Automatic Overdrive

All I know is it doesnt work till apx 110f, I just leave it on always.

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Old 02 Jun 2006, 12:55 pm   #3 (permalink)
Daniel
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Default Re: 91 Camry Automatic Overdrive

jg wrote:
> Does anyone actually know how it works?
> Owners manual says OD off simply prevents it using the OD gear - it affects
> the whole gearbox range so that ain't right.
> The condescending repair manual (which doesn't even have a photo of it),
> says it stops the torque converter slipping so much - that ain't right
> either because the motor revs faster with OD off even down hills.
> My guess would be a planetary system devoted soley to OD and controlled by a
> hydraulic solenoid, but I've never seen inside one. Scarey the manuals
> simply don't know... what other errors are in them?

==========================
Not sure I understand your question, but basically, you seem to be
mixing two concepts. Overdrive is simply a gear ratio of less than one.
It means if the input shaft turns once, the output shaft turns
something more. There is also a locking torque converter that improves
fuel economy by removing the normal slippage of the torque converter
drive at low throttle positions by mechanically holding the torque
converter so that on the highway, it's like you had a standard
transmission with the clutch engaged - no slippage.
If you're going up a hill on cruise control, and watch the tachometer,
as the transmission downshifts, first you'll see a slight rise in RPM -
that's the torque converter lock up disengaging, then you see a larger
jump in engine speed as the transmission down shifts to the next lower
gear.
If you are in overdrive, and the lock up torque converter is engaged,
you should see the same engine speed at the same road speed, regardless
of whether you're going up hill or down hill, but it doesn't take much
throttle for it to automatically disengage.
The actual fourth gear is a planetary system with a hydraulic clutch,
the lock up in the torque converter is a separate hydraulic clutch.
If you block over drive, it's like you have a three speed transmission.
I don't know if the lock up converter works in that event.

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Old 02 Jun 2006, 04:50 pm   #4 (permalink)
jg
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Default Re: 91 Camry Automatic Overdrive


"Daniel" <nospampls2002@yahoo.com> wrote in message
news:1149270950.096876.86860@i40g2000cwc.googlegro ups.com...
> jg wrote:

..........
> Not sure I understand your question, but basically, you seem to be
> mixing two concepts. Overdrive is simply a gear ratio of less than one.
> It means if the input shaft turns once, the output shaft turns
> something more. There is also a locking torque converter that improves
> fuel economy by removing the normal slippage of the torque converter
> drive at low throttle positions by mechanically holding the torque
> converter so that on the highway, it's like you had a standard
> transmission with the clutch engaged - no slippage.
> If you're going up a hill on cruise control, and watch the tachometer,
> as the transmission downshifts, first you'll see a slight rise in RPM -
> that's the torque converter lock up disengaging, then you see a larger
> jump in engine speed as the transmission down shifts to the next lower
> gear.
> If you are in overdrive, and the lock up torque converter is engaged,
> you should see the same engine speed at the same road speed, regardless
> of whether you're going up hill or down hill, but it doesn't take much
> throttle for it to automatically disengage.
> The actual fourth gear is a planetary system with a hydraulic clutch,
> the lock up in the torque converter is a separate hydraulic clutch.
> If you block over drive, it's like you have a three speed transmission.
> I don't know if the lock up converter works in that event.
>

Thanks, that explains how the lockup works. The only thing is, the motor
seems to rev consistently higher in all gears & gradients (there's no tacho)
with OD off and I'm fairly sure there are still 4 ratios with it off.


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Old 03 Jun 2006, 09:22 am   #5 (permalink)
Daniel
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Default Re: 91 Camry Automatic Overdrive

jg wrote:
> >

> Thanks, that explains how the lockup works. The only thing is, the motor
> seems to rev consistently higher in all gears & gradients (there's no tacho)
> with OD off and I'm fairly sure there are still 4 ratios with it off.

==========================================
The torque converter lock up is primarily designed to work with the
overdrive gearing to conserve fuel on the highway.
When you say there are four gear ratios with the overdrive off -- I
don't see it that way. There are a total of four gear ratios, and in
fourth gear, (over driven) the torque converter lock up function is
also available which gives that fourth gear ratio kind of a dual
function by reducing engine speed at low throttle openings when all
slippage is mechanically removed from the torque converter.
Toyota Transmission Gear Ratio's
.... Automatic Transaxles, 1st, 2nd, 3rd, 4 th.
Ratio. A140E, 2.810, 1.549, 1.000, 0.706
As you can see, if you take away the fourth gear, (overdrive at .706
ratio) there are only three gear ratios remaining.
The A140E is for my '94, so your '91 may have a slightly different
transmission but the principle is the same.

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Old 03 Jun 2006, 03:50 pm   #6 (permalink)
jg
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Default Re: 91 Camry Automatic Overdrive


"Daniel" <nospampls2002@yahoo.com> wrote in message
news:1149344522.947253.248870@g10g2000cwb.googlegr oups.com...
> jg wrote:
> > >

> > Thanks, that explains how the lockup works. The only thing is, the motor
> > seems to rev consistently higher in all gears & gradients (there's no

tacho)
> > with OD off and I'm fairly sure there are still 4 ratios with it off.

> ==========================================
> The torque converter lock up is primarily designed to work with the
> overdrive gearing to conserve fuel on the highway.
> When you say there are four gear ratios with the overdrive off -- I
> don't see it that way. There are a total of four gear ratios, and in
> fourth gear, (over driven) the torque converter lock up function is
> also available which gives that fourth gear ratio kind of a dual
> function by reducing engine speed at low throttle openings when all
> slippage is mechanically removed from the torque converter.
> Toyota Transmission Gear Ratio's
> ... Automatic Transaxles, 1st, 2nd, 3rd, 4 th.
> Ratio. A140E, 2.810, 1.549, 1.000, 0.706
> As you can see, if you take away the fourth gear, (overdrive at .706
> ratio) there are only three gear ratios remaining.
> The A140E is for my '94, so your '91 may have a slightly different
> transmission but the principle is the same.
>

Just feels to me like there are still 3 distinct changes with OD off (1st
gear making 4 ratios), and they all seem different to with OD on. And though
the manual agrees with what you say about the lockup, there doesn't seem to
be much point since there is effectively no slip in the torque converter at
low load anyway.


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Old 04 Jun 2006, 02:28 pm   #7 (permalink)
Daniel
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Default Re: 91 Camry Automatic Overdrive

jg wrote:

there is effectively no slip in the torque converter at
> low load anyway.


======================================
There's always going to be more slippage in a fluid coupling than a
direct mechanical connection.
Think about the torque converter at idle - that's a "low load"
condition - yet you know there is slippage because the engine is idling
with the wheels not moving and the car in gear.
Some cars have a button to change the shift pattern in the
electronically controlled transmission - aka "ECT" switch. That one
would change the behaviour of the shift pattern for every gear, but
"overdrive" is just the name of the top gear by virtue of its low
ratio.
If you have a four speed transmission and take away the fourth gear -
how can you still have four gears? When I went to school, 4 - 1 = 3,
not four.
=========================================
Here's another way to look at it.
I have a Toyota truck with the manual transmission - 5 speed. The fifth
gear is like an overdrive in your automatic transmission. It is the
"taller" "high" gear.
I once drove with someone who had a Toyota with a 4 speed manual
transmission. His top gear was 4th. So the engine was revving higher on
the freeway at the same speed.
That would be like driving the automatic with the overdrive switched
off. You have locked out access to high gear, so you would always be in
the next lower gear - with the four speed automatic, that would be
third gear, according to my records, a 1:1 transmission gear.
========================================
For all practical purposes, you just leave the overdrive on all the
time.
That's the benefit of an "automatic" transmission, it automatically
selects the most appropriate gear for all conditions using a
sophisticated array of sensors - even retarding the ignition timing
under most conditions to smooth upshifts.
The only recommended time to turn off overdrive is towing uphill, or
when the transmission is at an awkward speed continually shifting
between gears under rare conditions.
========================================
Best idea, to borrow a phrase from Ron Popeil - "you just set it and
forget it."
========================================
When I used to experiment with the overdrive off button, the only real
use I found was in city traffic, around 40 - 45 mph, when I wanted to
waste fuel, and have more power for passing temporarily, I would switch
off the button, then press down on the throttle to save the millisecond
of time it would take the transmission to automatically downshift, but
even that really isn't worth the bother.

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Old 04 Jun 2006, 03:42 pm   #8 (permalink)
jg
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Default Re: 91 Camry Automatic Overdrive


"Daniel" <nospampls2002@yahoo.com> wrote in message
news:1149449291.860554.201560@j55g2000cwa.googlegr oups.com...
> jg wrote:
>
> there is effectively no slip in the torque converter at
> > low load anyway.

>
> ======================================
> There's always going to be more slippage in a fluid coupling than a
> direct mechanical connection.
> Think about the torque converter at idle - that's a "low load"
> condition - yet you know there is slippage because the engine is idling
> with the wheels not moving and the car in gear.
> Some cars have a button to change the shift pattern in the
> electronically controlled transmission - aka "ECT" switch. That one
> would change the behaviour of the shift pattern for every gear, but
> "overdrive" is just the name of the top gear by virtue of its low
> ratio.
> If you have a four speed transmission and take away the fourth gear -
> how can you still have four gears? When I went to school, 4 - 1 = 3,
> not four.
> =========================================

Yes I understand the theory, but it didn't seem to be what happened in
practice. Actually I very recently discovered the kickdown wire was way too
loose (don't know why) and it spent a great deal of time in 3rd or 4th
depending if OD was on just as you say, so it seemed to be always at a
higher or lower ratio. The torque converter fooled me into thinking it must
have changed gear. Centrifugal force "hardens" a torque converter so that
much less slip occurs at revs than at idle, but I guess the engineers have
determined there is still enough to warrant locking it. My manual makes it
sound like the locking is soley responsible for creating an overdrive
effect. Wish they had included a drawing.


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