Overdrive switch ?

About the Granvia or about whatever....
ibex32
Posts: 5
Joined: 18 Dec 2017, 17:35

Overdrive switch ?

Postby ibex32 » 27 Jan 2018, 11:54

I'm a very pleased new owner of a 1996 diesel automatic Granvia, but I'm confused about what I read here about an overdrive switch. I haven't found one ! I do notice a "bit of a "bounce" through the throttle pedal at higher speeds on the motorway, which may be overdrive kicking in ? But I haven't yet noticed that the engine revs drop appreciably when that happens. I'm getting 29MPG, but always looking for a way to get more miles to my gallon. If I can find that overdrive switch (if I have one) it may help ?

V6vanman
Posts: 130
Joined: 06 Dec 2010, 19:55
Location: Abingdon

Re: Overdrive switch ?

Postby V6vanman » 27 Jan 2018, 17:13

Its on the end of the gear selector.
2000 3.4 V6 Granvia - camper conversion

jarmmo
Posts: 64
Joined: 30 Apr 2011, 06:43

Re: Overdrive switch ?

Postby jarmmo » 27 Jan 2018, 18:57

Hello Ibex
The bump you feel at motorway speeds could be the torque converter locking up at approx 56mph. When the overdrive is disengaged there is a warning lit up on the dashboard to warn you. If you are getting 29mph you are doing very well indeed, the best I have ever got is 27.5 mpg on a long run.
Jarmmo

PUSHKIN
Posts: 54
Joined: 04 Sep 2017, 02:53

Re: Overdrive switch ?

Postby PUSHKIN » 28 Jan 2018, 03:41

We petrol owners can only dream of getting 29mpg, maybe coasting downhill :-) :-)
Tony - 1999 3.4 V6 4wd Auto Camper located in Chorley Lancashire.

ibex32
Posts: 5
Joined: 18 Dec 2017, 17:35

Re: Overdrive switch ?

Postby ibex32 » 28 Jan 2018, 12:23

Found it ! Thank you V6vanman. And thanks Jarmmo & Pushkin for your input. From everything I'm told & read on this very useful site, I'm going to do the following :-

1) Leave the overdrive switch pushed in (on) to maintain my relatively good MPG, unless I need more power up hills, or I require engine braking for reducing speed down hills without using the brake. When "on" the overdrive will then automatically cut in when the speeds are right.

2) Leave the ECT switch off, unless (again) I need more power up hills, because then I can rev higher in the gears before the gearbox changes up.

3) I can't control the torque converter. That automatically cuts in, at about 56mph, to save fuel at around motorway speeds, by providing freewheeling when my foot's not on the throttle.

4) Driving normally like above, there should be no warning lights on the dash.

Have I got that about right ?

Thanks. Ibex32

V6vanman
Posts: 130
Joined: 06 Dec 2010, 19:55
Location: Abingdon

Re: Overdrive switch ?

Postby V6vanman » 28 Jan 2018, 13:26

Sounds good to me.
2000 3.4 V6 Granvia - camper conversion

jarmmo
Posts: 64
Joined: 30 Apr 2011, 06:43

Re: Overdrive switch ?

Postby jarmmo » 29 Jan 2018, 11:13

Hi Ibex
The torque converter does not freewheel, it just unlocks the torque converter so it will act like any other torque converter which does not have a lock up function, at 70mph when you let your foot off the throttle the engine will not drop to tickover and will still provide engine braking to a certain degree. It is illegal to freewheel in the UK as this will affect things like power steering and brake servo vacuum, create brake fade and also you do not have the same control over the vehicle as well. Back in the 40s and 50s there were cars that did have a one direction clutch in the drive that provided a true freewheel but they were soon phased out for safety reasons.
Jarmmo.

ibex32
Posts: 5
Joined: 18 Dec 2017, 17:35

Re: Overdrive switch ?

Postby ibex32 » 29 Jan 2018, 11:56

I'm from that era (40's & 50's), which might explain how daft I'm getting, but I still don't understand what the torque converter does and when it cuts in and why.

I understand about the ECT switch.

I understand about the overdrive switch.

But not the torque converter ! I really would like to understand it !

Cheers, ibex32

User avatar
72degrees
Posts: 22
Joined: 19 Jan 2012, 20:52

Re: Overdrive switch ?

Postby 72degrees » 29 Jan 2018, 13:09

Saab were still using a selectable freewheel mode in the early 60s - but that was on their 3 cylinder two stroke!

I find it worth flicking the overdrive off when on a long gradient on the motorway, before the revs drop too much, then flick it back on once the rise has been crested. The ECT becomes useful when I'm towing a trailer (2 motorcycles) in hilly areas with twisty roads and wanting to "make progress".

I assume it's also a good idea to flick the overdrive off and unlock the torque convertor if faced with a long, steep, downhill stretch (particularly if signed with an "engage low gear now" warning).
Granvia 3.0TD 4WD Camper
"Do not adjust your mind there is a fault in the reality"

jarmmo
Posts: 64
Joined: 30 Apr 2011, 06:43

Re: Overdrive switch ?

Postby jarmmo » 29 Jan 2018, 20:47

Hello Ibex
A torque converter is very similar to a fluid flywheel, ie an impeller driven by the engine and a turbine connected to the gearbox, a torque converter also has a stator positioned between the two which redirects the fluid with more force onto the turbine when needed ie when pulling away, as the car speeds up and the impeller and turbine are turning close to the same speed the stator also spins as it is attached to the center shaft via a one way clutch thus no longer multiplying the torque and acting as a fluid flywheel. The lockup that you are talking about is a device which locks the whole torque converter thus giving 100% efficiency, ie input and output shafts are turning at the same speed, before the torque converter locks up it will be operating at around 95 to 98% efficiency, not all cars have this facility.
Please see attached link.
https://auto.howstuffworks.com/auto-parts/towing/towing-capacity/information/torque-converter2.htm

Regards
Jarmmo


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