How to identify overdrive?

cartoonhead · Feb 14, 2014

Hi All,

I am trying to work out what overdrive is fitted to my 89 3.5 efi classic, I know nothing about it and neither does the bloke I bought it from.. It automatically kicks in at exactly 55mph every time and drops the revs down to under 2k at 60mph which is nice.. I can't find any info about such a overdrive online though, I wasn't even aware you could fit them to automatics.

Has anyone got any ideas?!

Cheers

3.5 efi · Feb 14, 2014

auto?

wammers · Feb 14, 2014

There is no overdrive, that is the torque converter locking up.

Musicmaker · Feb 14, 2014

wammers said:
There is no overdrive, that is the torque converter locking up.

The 3.5 classic I owned dropped revs at 55mph which made me think it had an overdrive. Wammers, what do you mean by a torque converter locking up? Does it act like an overdrive?

MrGorsky · Feb 14, 2014

The torque converter is a fluid coupling between the output of the engine and the input to the gearbox. You could say it takes the place of the clutch in a manual gearbox setup.

There will always be some losses with a fluid coupling, so at steady throttle openings (like when you're cruising), when the difference between the input and output sides of the TC are small, a clutch is used to lock it to stop these losses.

When you accelerate again, the TC will unlock and it will operate as normal.

The drop in revs happens when it locks because the input speed will now match the output speed. It's not really an overdrive, just an efficiency drive. Older auto boxes didn't have it, so you were always losing some engine power to the TC, even when cruising.

cartoonhead · Feb 14, 2014

Cheers lads thats interesting stuff.. I did think it was all a bit suspect but the bloke I bought it from assured me it had a overdrive!

Oh well lol

zen · Feb 14, 2014

yes all true..but if you got standard tyres then at 65 you wont be getting under 2000 rpm..no way..32 mph per 1000rpm???.not unless you got 2.83 diffs.....which is unlikley..

i get 30mph per 1000rpm with 31's and 1:1 lt230 in 5th...so with 205's 1.2 transferbox its impossible.

so you may have an overdrive.(you describe operation of gkn o/d with electric operation)...but thats unlikley too as they have a switch/lever to operate and i think they drop out at 35..and on a bw transferbox i dont think you can even fit one..

to be sure crawl under car and take a photo of transferbox and post it on here..

jamesmartin · Feb 14, 2014

zen said:
yes all true..but if you got standard tyres then at 65 you wont be getting under 2000 rpm..no way..32 mph per 1000rpm???.not unless you got 2.83 diffs.....which is unlikley..

i get 30mph per 1000rpm with 31's and 1:1 lt230 in 5th...so with 205's 1.2 transferbox its impossible.

so you may have an overdrive.(you describe operation of gkn o/d with electric operation)...but thats unlikley too as they have a switch/lever to operate and i think they drop out at 35..and on a bw transferbox i dont think you can even fit one..

to be sure crawl under car and take a photo of transferbox and post it on here..

you cant fit anything to bw t/box

How to identify overdrive?

cartoonhead

Active Member

3.5 efi

Well-Known Member

wammers

No longer with us, RIP

Musicmaker

Active Member

MrGorsky

Well-Known Member

cartoonhead

Active Member

zen

Well-Known Member

jamesmartin

Well-Known Member

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