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TR4A - 'J' type overdrive


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Hi Guys,

I have just  got around to swapping out my non OD gearbox in  my 4A for a 'J' type OD gearbox rebuilt during the summer.

The OD gearbox I rebuilt was an ex US re-import cica 1974.

This had the OD switch for 3/4th located at the rear of the top box. Unfortunately , the top box was cracked and had to be substituted for an earlier top box without the rear tapping for the OD switch. However it has the OD switch tapping at the front left in line with the 3/4th selector rod. I am currently undertaking follow up testing in the car on axle stands prior to refitting the tunnel etc. 

My question is that the  the OD switch is live when both 3rd gear are 4th gear are engaged. However when going from 3rd to 4th gear if in OD, the OD  momentarily disengages as there is a notch in the sector rod between 3rd and 4th as it slides across the OD switch  button. This notch seems to be correct as per workshop manual  diagrams

Is this normal?

Thanks 

Rich

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Yes that is normal Rich.

It's that momentary break that allows the overdrive logic boxes to work, so you can go from 3rd OD to 4th instead of to 4th OD.  

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Hi Rich,

I've got the same J-OD configuration in my TR4.

Just in brief, YES it is quite normal !

Anyway whenever changing gears in OD position ( 3/4 or v.v.) it is recommended to disengage the OD switch before shifting.

The OD's cone clutch wont be overstressed by shifting under load.

Hope this helps

Wolfgang

(Austrian member)

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Hi,

How do you go from 3rd OD to 4th?

The OD is new to me, so far i did switch it on and off on the same gear (3rd or 4th, never tried 2nd) but i never tried to shift gear with the OD engaged. I thought that if you do that it goes from 3 OD to 4 OD?

Cheers,

Laurent

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Hi Laurent.

Yes you are correct.

The full benefit of overdrive is felt when say ;-

Whilst driving in O/D Third, you switch out of O/D as you move from Third to Fourth, then on into O/D Fourth.

Leaving the O/D engaged, or just forgetting to disengage, means that you do not get the benefit of the 'Kick'

though it will work, and it will not cause any harm.

Finally the accessory available, of the Logic Switch, is simply that should you move between gears whilst in O/D then the Logic will automatically disengage the O/D for you, leaving you to switch O/D in again when next appropriate.

John.

 

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1 hour ago, John Morrison said:

Finally the accessory available, of the Logic Switch, is simply that should you move between gears whilst in O/D then the Logic will automatically disengage the O/D for you, leaving you to switch O/D in again when next appropriate.

John.

 

Or when you really need a good burst of power and you go from O/drive 4th and change down the logic box drops it out of overdrive and you go to direct 3rd. I wouldnt be without my logic box.

Stuart.

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4 minutes ago, rvwp said:

Thanks Guys, so tell me more about a 'Logic box' and where to buy? Rich

There are a number round, my preference is for the Revingtons one, not cheap but works very well.https://www.revingtontr.com/search.asp?for=logic box you do also need to convert your stalk switch to a momentary from a straight on/off.

Stuart.

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Depending on your aptitude at electrics Rich, it is pretty simple to make your own. It just requires a couple of cheap relays and a suitable switch.  If you are so inclined, a search on the forum will unearth a number of options for circuits. This thread for instance has several :

 

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20 hours ago, stuart said:

Or when you really need a good burst of power and you go from O/drive 4th and change down the logic box drops it out of overdrive and you go to direct 3rd. I wouldnt be without my logic box.

Stuart.

 

17 hours ago, stuart said:

There are a number round, my preference is for the Revingtons one, not cheap but works very well.https://www.revingtontr.com/search.asp?for=logic box you do also need to convert your stalk switch to a momentary from a straight on/off.

Stuart.

Love mine too. 

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With logic Overdrive, rather than continuing to use the long, dangly wand, install a momentary action switch which brings the lever next to the fingers of one's right hand and requires almost no movement, so no need to take that hand off the steering wheel.

With some ingenuity, one can mount a momentary action switch in place of the wand - I have two momentary action switches mounted on the right of the steering column, one for Logic Overdrive (like Stuart, I have Revington's version), the other for the horn.  Why for the horn, you ask?  Because, the wood-rim steering wheel with button in the centre, would sound the horn whenever it felt like it - but seldom when I needed to use it.  Very embarrassing!  The navigator also has the original "Panic" horn button at the bottom of the left side of the dashboard.

Ian Cornish

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