What does the overdrive do?

I just bought a 1989 Volvo 740 GL and it has that button for the
overdrive. What does it do? and when should I use it? I know this
is probably a stupid question, but I would still much appreciate an
answer. I'm sure whatever it is, it's only half as endearing as the
hand cranked sunroof.

 

In an earlier contribution to this discussion,

It's an additional 2-speed gearbox behind the main gearbox. When it's Off,
the main gearbox output drives straight through to the axle. When the
overdrive is engaged, it provides a higher gear than normal top gear -
useful for quieter cruising and improved fuel economy. I don't know anything
about the overdrive used by Volvo but overdrives in general can usually only
be engaged when the main gearbox is in top gear (plus maybe the next one
down).
--
Cheers,
Roger
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It is an extra little gearbox with one gear and a little clutch outside the 4
gears-gearbox. It can be engaged only when you are in forth gear. The 4 gears
are shifted mechanically, the overdrive is controlled by a solenoid and oil
pressure when you press the button in 4th gear. It is practically a 5th gear.

You use it like a 5th gear as soon as you have reached the rpms when you would
change gear with a 5 gears manual gear box. Be sure to use the clutch before
pressing the button. When you want to shift back to gear 4, press clutch again
and press button again. When you are in gears 1, 2, 3 the overdrive can not be
engaged. The overdrive is also controled by a relay near the fuses.

did you mean enduring? I have one with 376.000 km. The only thing I have
replaced is the relay some time ago.

http://en.wikipedia.org/wiki/Overdrive_(mechanics) gives more info.

Viktor

 

Hi,

As already mentioned, the OD is sort of a 5th gear. IIRC the manual
suggests not to apply (too much) power while engaging and slightly
press the clutch while disengaging th OD for smooth operation.


Roland

 

As others said it is nothing more than another gearbox added to the
back of the standard transmission. The Laycock de Normanville unit
has been used in a wide variety of cars.

I'm confused by this statement. The overdive unit is in my experience
very reliable. And the hand cranked sunroofs in two of my earlier
cars were quite reliable too.

 

Thought for the day: Older Volvos have a way of combining endearing and
enduring into one.

 

If you desire a "normal" fuel economy it is only used when
towing, climbing hills and possibly passing. Keeping it on
will lower your gas mileage since you will be in a lower gear..

hope helps...have fun....sno

--
No matter how dangerous nuclear power may or
may not be.....
Is it any more dangerous then what we are doing
now.....???

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SLNG (Silly Little Nuclear Generator)

 

Since this car is an '89 and has overdrive, it has an automatic transmission
so the other replies you've had so far are not correct for this car and only
apply to pre-'87 manual transmission cars.

The overdrive button on your '89 disables the 4th gear on the transmission.
When you engage the overdrive lockout, a yellow arrow will illuminate in
your instrument cluster and the transmission will function as a 3 speed
unit, never shifting into the top gear. This is useful occasionally when
going up long hills or when towing a trailer where the transmission may
hunt, shifting back and forth between 3rd and 4th with annoying frequency.
99.9% of the time you'll just want to leave the overdrive lockout
disengaged.

 

The overdrive button on your '89 disables the 4th gear on the transmission.

I am getting confused.

I have an 88 GLE automatic. I run with it all the time without the
Yellow up arrow illumenated in the dash. Is that the way, it should
be?

I should have the arrow illuminated while climbing a mountain or going
up a steep hill tough, right?

 

You got it....think of it as downshifting to a lower gear in a manual
when you hear the engine starting to lug....it does the same thing..
and the arrow will be on at this time...

hope helps....have fun....sno

--
No matter how dangerous nuclear power may or
may not be.....
Is it any more dangerous then what we are doing
now.....???

This tag line is generated by:
SLNG (Silly Little Nuclear Generator)

 

In an earlier contribution to this discussion,

I've no doubt that you are right - but it seems very odd to call this
'overdrive' when it is, in fact, exactly the opposite! Or is it actually
labelled 'overdrive lockout' or somesuch - on the basis that the 4th gear is
considered to be an overdrive, and this button prevents it from being
engaged?

My previous reply related to the conventional definition of overdrive,
usually - though not exclusively - fitted behind manual transmissions.
--
Cheers,
Roger
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Hi,

Well, since we had also a 760GLE long time ago, I remember now the
button.
I think, it wasn't labelled at all... ;-)


Roland

 

Overdrive, fourth gear, fifth gear, another gear,... call it what you want.
Whatever you call it, it's engaged by default when you start the car
(assuming all is functioning as designed) and can be disengaged by
pressing the button or romping on the gas.

Presumably, this 'on by default' helps score better EPA/mileage ratings.

Although I've never heard or read the same about Volvos, Chevy used to
recommend disengaging their OD when putting around town as (their) OD
slows everything down in the transmission, including the pump, and could
cause excessive heat build up.
ymmv.

 

It's not labeled as anything, it's a plain black button, and pushing it
illuminates the yellow arrow symbol and disables the automatic overdrive. I
forget what they call it in the owner's manual, but it does describe what it
does and how to use it.

Overdrive, by definition, is a gear in which the output shaft of the
transmission rotates faster than the input shaft, the term has no bearing on
whether the transmission is automatic or manual or how the mechanics of it
function.

 

In an earlier contribution to this discussion,

I have no problem with that.

What I *do* have some difficulty with is the concept of a so-called
'overdrive' button which *prevents* the overdrive from functioning!
--
Cheers,
Roger
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So, you have a problem with a button that disengages the overdrive when
you push it... but no problem with a pedal that disengages the
transmission when you push it?
Ok...

 

In an earlier contribution to this discussion,

I may do if it were called a 'drive' pedal. <g>

I guess it's a case of usage. The only overdrives I've come across in the UK
have had a button to *engage* them - the default when not pressed being 1:1
drive. So the concept of having the overdrive engaged by default, and
needing a button to *disengage* it is somewhat foreign to me.
--
Cheers,
Roger
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"Overdrive lockout" or "overdrive defeat" is perhaps a better term. You
could also call it 4th gear lockout, semantics aside, it's a feature common
to virtually every automatic transmission made in the last couple decades.
Some you push a button, some you move the gear selector from D to 3, it all
really does the same thing, limiting the highest gear the transmission will
shift into. I haven't seen a non-overdrive slushbox since the late 1970s.

 

What peeves me about it is it's on by default but when it fails, it
fails in the disengaged mode. The old four banger really sings when it
drops out of overdrive at 80 mph.

Anyway, it's all about "fuel economy."
The manufacturers can get a better highway rating if it's on all the time.
I remember when the Z1 Corvette first came out with it's fancy 5 or 6 or
7 speed manual. GM built in a system that forced you to shift from first
to third (locked out second gear) unless you wound it past a certain RPM
in first.
GM got a better MPG rating and drivers got pissed.
Funny story: When Car and Driver tested the new Vette and discovered
this 'feature', they asked one of the GM techs that was there for the
test how to disable it.
Of course, they weren't allowed to reveal how to disable it but they did
offer that 'there's a wiring harness coming out of the transmission and
whatever they did, they should not cut the red one'... or words to that
effect.
It's been a lot of years.