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.

Reply to
VladicRavich
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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).

Reply to
Roger Mills

Hi,

snipped-for-privacy@gmail.com schrieb:

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

Reply to
Roland Messerschmidt

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.

Reply to
Roadie

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

Reply to
Pat Quadlander

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

Reply to
sno

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.
Reply to
James Sweet

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?

Reply to
Hameed

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

Reply to
sno

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.

Reply to
Roger Mills

Hi,

Roger Mills schrieb:

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

Roland

Reply to
Roland Messerschmidt

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.

Reply to
clay

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.

Reply to
James Sweet

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!

Reply to
Roger Mills

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...

Reply to
clay

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

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.

Reply to
Roger Mills

"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.

Reply to
James Sweet

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.
Reply to
clay

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