Overdrive?

From my 740 Owner's Manual (a couple of years older than yours):

"The overdrive can be engaged in 4th gear only. To engage, depress the push button in the top of the lever. To disengage, depress the button once again. The clutch pedal should be depressed slightly when operating the switch to facilitate smooth changeover.

"The overdrive is automatically disengaged when downshifting.

"We do however recommend disengaging the overdrive before changing down.

"Use the overdrive as much as possible at speeds in excess of 45 mph (75 km/h) for good fuel economy."

Specifically, an overdrive unit is an extra bit added on the end of the gearbox, usually engaged electrically. When engaged, the output shaft will rotate faster than the input shaft. In practical terms, it adds an extra gear ratio, though this is usually restricted to top gear (some cars enable it for 3rd gear too), and in effect makes a 4 speed box into a five speed.

Although the overdrive unit has its own internal type of clutch, and is designed so that you can't crash the gears, it is a good idea to relieve stress on this by dipping the clutch pedal as advised.

The light on the dash is simply a reminder that overdrive is engaged. Remember that if you change from 4th + overdrive to 3rd, you are effectively changing down two gears at once.

I've never really understood why an overdrive unit is fitted to a car which has a perfectly good 5 speed option.

Thanks for that. I guess it's the same logic for an automatic is it?

On the automatic, that's just a way to lock the transmission's top gear out. The transmission was just a 3 speed unit when designed, they added on 4th gear, aka overdrive. It was cheaper (I presume) to add the electric control circuit rather than redesign the valve body. Normally you just leave the button alone, so the transmission shifts through all

4 gears. When the arrow is on, then the transmission functions as a 3 speed, this is useful if you don't want the transmission to shift into top gear for whatever reason.

Thanks for that Mike.

I really can't think of a reason why I wouldn't want to go into 4th gear (unless it's to do with towing large caravans/boats etc).

Regards,

Richard

On Tue, 08 Feb 2005 13:24:05 +0000, the illustrious Richard Setford favored us with the following prose:

I sometimes find the overdrive button useful when overtaking/passing at high speeds. When on a two-lane highway at 80-90km/h and you have to pull out into the oncoming lane to pass you can hit the button and get more direct power to the wheels to get more responsive accelleration at that speed.

Just remember to switch it back once you finish passing. Also, even with an automatic, it's not a bad idea to give it some gas and raise the engine RPM before dropping the top gear because at higher speeds the the transition from 4th to 3rd adds a couple thousand RPM all at once.

Also, if you're in a particularly hilly area you might find that with the OD off (arrow lit up on the dash) the engine remains more engaged when coasting and will help when attacking steep climbs and also improve control on the descent. In my 4-cylinder 240 Turbo this keeps the car at a fairly constant 70km/hr (downhill) and supplies power to the wheels for better control. Haven't tried it out on the V6 760 yet.

blurp

1983 240Turbo 320,000km and counting! 1988 760 GLE 320,000km and counting! DISCLAIMER: I am likely less experienced than many of the users on this newsgroup and can only provide suggestions based on my own experience with my car. Many of my problems have been solved by the more knowledgeable members of this community.

In the absence of facts I will speculate wildly.

Ah. Auto! I must read the post before replying.

I didn't know there was an auto + overdrive option on a 740; I thought that was only on the 240s. It seems strange that I've got a 4 speed auto, but they were still offering 3 speed + o/d on later models.

When I've driven automatic Volvos I've occasionally used it on a hill where the transmission kept shifting in and out of 4th, easier to lock it down to

3rd when it can't make up it's mind.

Thanks a good idea. Thanks for that one.

Maybe we should set up a competition, 'Find the best use for overdrive in an automatic'. The prize is a weeks holiday in Iraq!

I also use it for extra drag on the same sort of hills going down. Maybe I'll get the second prize: two weeks holiday in Iraq.

Mike