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Fitting an overdrive


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I've finally found an overdrive!

 

Picking it up tomorrow afternoon.

Once I get the box home, what else do I need to fit it? I understand I need the column switch and escutcheon but are the rest of the bits part of the 'box?

Also, I've heard talk of logic units. Do I need one of these and what do they do?

Despite this being my second TR, I've never had an overdrive!!!

 

Paul

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Hi Paul,

 

Is it just an OD or a complete gearbox? If its just an OD then you need a mainshaft for your gbox. Wiring-wise you may need an additional switch on the selector cover to allow 2nd gear OD (which you really should enjoy), then its just the column switch and a relay...if you use a 6RA Lucas one, make sure its the right 'sub-type'...there are many 6RAs.

If the car is a TR4A IRS the OD should be the cushioned-engagement type....in any case check that the diffmounts are in good condition.

If the OD is from another car model you may need some special rear gearbox mount.

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Paul,

 

I think you will need to check exactly what you have,

maybe quote some serial numbers and what it came

off, and get back to us.

 

When you get it working, you will kick yourself for all

those lost years of non-o/d driving.

 

AlanR

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Paul, the logic box is a nice little extra to have but not completely necessary, basically without the logic box you have to use your overdrive switch to turn the overdrive on or off, with the logic box once you've turned the overdrive on...in third for example, the moment you move the gear stick out of third the logic box drops it out of overdrive as well which wouldn't happen normally, in effect you end up with a seven speed box by splitting the gears between 2/3 3/4 and then 4+.

 

Hope that makes sense if not ask again and I'll try and explain better!

 

As Alan says above - once you've got it running - I think you'll appreciate just how good they are.

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with a mechanical switch the overdrive dis-engages when shifting gears and engages again in the new gear. this can damage the OD.

the logic unit is an electronic switch which acts like a relay. when shifting the gear the logic unit switches the od off, when in new gear you have to manual switch the od on again.

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To explain the above a bit more -

 

The overdrive wiring is based on an earth that runs

through the switches on the top of the gearbox, with

a selector switch for each gear (2nd, 3rd & 4th)

 

If you are in 4th o/d, and shift down, the selector switch

will momentarily disengage the o/d then re-engage it

when 3rd is engaged.

 

Seems the logic unit (I don't use it) will NOT re-engage

3rd gear in the scenario above.

 

This also explains why, if you take the o/d earth wire

directly to earth, you will get overdrive on ALL gears.

O/d on 1st puts too much strain on the unit, o/d on

reverse will seize up the whole o/d unit as it's NOT

designed to run in reverse. If you do run it in reverse,

(and you would certainly NOT be the first) you will

knacker the o/d.

 

If it's coming from a TR4, you should be OK, but it

depends on whether it's a true TR4 gearbox or a

later/saloon gearbox that has been fitted, in which

case you will need to adapt the rear mounting.

 

AlanR

Edited by TR 2100
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Hi Paul,

I think you need to take and post a bunch of pics along with any serial numbers you can find.

 

The logic box is a convenience for quick selection of gears/OD.

If you believe you will be whizzing up and down the gears then fit one - after you have got the GB/OD fitted and working correctly.

If you are touring and maybe requiring quickish gear changes then you do not want one - something else to go wrong.

 

If you do fit the logic box then you will also require a momentary ON switch - ie it does not lock into the ON position but returns to OFF after selection - these are not cheap.

 

Roger

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If you do fit the logic box then you will also require a momentary ON switch - ie it does not lock into the ON position but returns to OFF after selection - these are not cheap.

 

 

 

That depends upon the type of logic box you fit. My home grown one doesn't need a momentary switch, nor would it work with one. You get used to off and on to reengage when you've forgotten to disengage O/D.

Edited by peejay4A
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Hi Paul,

 

I've just been through this process myself. Just to add a few things to what others have already said ....

 

As well as the relay you can purchase the required extensions to the wiring loom. The loom extension is generally sold as two items: switch to relay and relay to gearbox. I bought mine here:

 

http://www.autosparks.co.uk/product_info.php?products_id=4466

http://www.autosparks.co.uk/product_info.php?products_id=4467

 

The steering column mounted overdrive switch is available on eBay and other places. For a TR4 it should be a shorter switch with a chrome stalk. The 4A has a longer stalk with a black covering. The former is a little more difficult to find. This seller on eBay was able to sell me one (even though it wasn't listed):

 

http://stores.ebay.co.uk/948s/

 

Depending on your set-up, you may need a longer speedo cable as the speedo drive comes from the overdrive and is on the opposite side to the non-overdrive box. On a TR4, the cable is routed through the floor and gearbox tunnel using the hole in the handbrake gaiter and connects directly to the overdrive (no angle drive). On a TR4A I believe you'll need an angle drive.

 

As others have said, it would be useful to know the numbers from your gearbox and overdrive. If fitting a TR4A gearbox to a TR4 (as I did) you may find that the TR4A gearbox has a thicker flange and so you'll need longer studs and screws to attach it. The extension, on which the clutch release bearing slides, is different lengths for a TR4 and TR4A (2 3/8" for a TR4 and 2" for a TR4A I think). The longer extension is OK for use with the TR4's spring clutch but not the TR4A's diaphragm clutch. Also, the clutch release bearing carrier is different on a TR4 and TR4A so make sure you have the correct one.

 

Also, while the gearbox is out:

 

- Check the clutch and replace if necessary as you won't want to take the gearbox out again in a hurry (make sure you use an alignment tool when refitting the clutch).

- Check the clutch cross shaft bushes and replace if needed (this is an easy job and RevingtonTR do some nice uprated ones).

- Consider fitting a roll pin to take some strain off the cross-shaft tapered pin as this is a weak point (lots of info on this forum).

- Don't forget to refit the cross-shaft tapered pin with locking wire.

- Replace the rear gearbox mount if the rubber has had it.

- Choose your gearbox oil carefully (again lots of info and many different opinions on the forum). I finally decided on Penrite GB40.

 

I didn't fit a logic unit.

 

Hope this helps.

 

Cheers,

Steve

Edited by Steve Priest
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Hi Paul,

 

I've just been through this process myself. Just to add a few things to what others have already said ....

 

As well as the relay you can purchase the required extensions to the wiring loom. The loom extension is generally sold as two items: switch to relay and relay to gearbox. I bought mine here:

 

http://www.autosparks.co.uk/product_info.php?products_id=4466

http://www.autosparks.co.uk/product_info.php?products_id=4467

 

The steering column mounted overdrive switch is available on eBay and other places. For a TR4 it should be a shorter switch with a chrome stalk. The 4A has a longer stalk with a black covering. The former is a little more difficult to find. This seller on eBay was able to sell me one (even though it wasn't listed):

 

http://stores.ebay.co.uk/948s/

 

Depending on your set-up, you may need a longer speedo cable as the speedo drive comes from the overdrive and is on the opposite side to the non-overdrive box. On a TR4, the cable is routed through the floor and gearbox tunnel using the hole in the handbrake gaiter and connects directly to the overdrive (no angle drive). On a TR4A I believe you'll need an angle drive.

 

As others have said, it would be useful to know the numbers from your gearbox and overdrive. If fitting a TR4A gearbox to a TR4 (as I did) you may find that the TR4A gearbox has a thicker flange and so you'll need longer studs and screws to attach it. The extension, on which the clutch release bearing slides, is different lengths for a TR4 and TR4A (2 3/8" for a TR4 and 2" for a TR4A I think). The longer extension is OK for use with the TR4's spring clutch but not the TR4A's diaphragm clutch. Also, the clutch release bearing carrier is different on a TR4 and TR4A so make sure you have the correct one.

 

Also, while the gearbox is out:

 

- Check the clutch and replace if necessary as you won't want to take the gearbox out again in a hurry (make sure you use an alignment tool when refitting the clutch).

- Check the clutch cross shaft bushes and replace if needed (this is an easy job and RevingtonTR do some nice uprated ones).

- Consider fitting a roll pin to take some strain off the cross-shaft tapered pin as this is a weak point (lots of info on this forum).

- Don't forget to refit the cross-shaft tapered pin with locking wire.

- Replace the rear gearbox mount if the rubber has had it.

- Choose your gearbox oil carefully (again lots of info and many different opinions on the forum). I finally decided on Penrite GB40.

 

I didn't fit a logic unit.

 

Hope this helps.

 

Cheers,

Steve

Textbook stuff this! Sums it up quite nicely.

 

Menno

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Thanks guys, I'll keep all this in mind.

Most of the stuff was done when I restored the car so clutch, mountings etc are only a few years old.

The box is the original out of a TR the same year as mine and is in working condition so once I've collected the rest of the bits I'll chuck it it.

I'm going to try it without the logic unit first. The roads here in BC are mainly open highways so I envisage using it to reduce the engine RPM on long runs more than anything else.

 

Paul

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FWIW 4a overdrive switch has a chrome stalk the same as a 4 one. Very very late 4a`s then occasionally had the black ones as they were being brought in for the 5.

Stuart.

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