Is a Triumph overdrive tranny all that?

[RonMacPherson]

Okay, 74's had four speed synchros. Synchronizer used on all forward gears.

The forks are shift forks, the brassish looking things hanging down from the rods on the trans upper cover.

The upper cover will have flat spots on either the top or side of the cover where the switches mount. The switches work by enabling a ground to pertinent circuit whenever that gear(reverse, 3-4 is engaged). With the cover off move the selector into reverse, observe the movement of the rod against the reverse switch. It should close it. You want to emulate the same affect when you select 3rd and 4th. So look at those rods and see where the mounting pad(s) are. That is where you will drill, tap and mount the switches.

The switch on the column, when activated, is the signal to the solenoid, completed through the selected switch to ground. So make sure your vehicle has a VERY good ground from the tranny/engine to chassis. I run a supplemental ground at the rear tranny mount, in addition to the one at the bellhousing bolts, on mine. Your supplemental wire harness, which is easy enough to make will provide power from the fuse box to the o/d solenoid on one lead, and the other lead will be from the selector switch to the 3/4 switch to ground. comprenedez-vous?

 

[2wrench]

Thank you, gentlemen. I have copied and collated
this information and sent via e-mail to my tranny
mechanic.

Geo, I did note the previous picture, but could not
appreciate its value at the moment except to say
I made a mental note that I likely would be referring
back to it again, as is the case. Thanks again for
the repost.

My mechanic's name is Steve so let me refer to him as
such.

Steve mentioned the switch he was installing, he
thought, protected against damage that may occur
in the event the overdrive switch is on and the
car is put into reverse. On my '74 TR6, does one
switch protect damage which may occur if it is put
into 1st; 2nd and reverse or is there yet another
switch I must buy to protect other gears? Is it a
one-size-fits-all type of protection; ie., 1st, 2nd
and reverse?

Thanks,

 

[Geo Hahn]

The switch in question (there is only one) closes when 3rd or 4th is engaged.

On an A-Type its closing it completes the ground for the relay that powers the OD.

Since J-Types do not use that relay I am thinking that the switch provides power directly to the big solenoid on the OD itself.

Either way -- it is more of a 'lock-in' switch rather than a 'lock-out' switch... only allowing the OD to operate in 3/4 and thus not requiring anything to sense when the shift is in 1,2 or reverse.

Sort of a (almost) fail-safe design since a failure of that switch to operate would most likely mean 'no OD' rather than 'OD in all gears'.

 

[TR3driver]

For a 74, you'll be installing a J-type. The usual wiring for a J-type is the single isolator switch in series with the solenoid on the OD, then the solenoid is grounded at the OD. But of course Ron's scheme will work just as well.

I have seen some top covers where the factory put the isolator switch somewhat off-center to the cast round pad; so my preference would be to start the hole from inside the cover. Of course that means you'll need to strip the rods, etc. out of the cover (after marking the center of the 3-4 fork when in neutral). In this diagram P/N 127387 is the 3-4 shift fork. Hopefully you can see the notch on the top of it that operates the OD switch. You want the hole for your switch to be centered in that notch (both along and across the shaft) when the fork is in the neutral position. I don't believe it needs to be very precise, probably 1/8" in any direction won't hurt anything too much, so just "by eye" should work.

 

[RonMacPherson]

Randall provides you a very accurate schematic for the o/d. That's the way my first 6 was wired, and kept having problems with the o/d engagement, it would popout-popback on, whenever it had a mind to.

So I wired it the way I described. I prefer to keep the power feed plain and simple, less switches, less potential resistance buildup and amperage drop. Granted the 6 solenoid takes less amperage than an A, but every power drop is some that takes away from what should be applied to the user. So all of MY wiring, wherever possible, I run switches on the ground side. As you will find more and more manufacturers have gone that route also. It makes it easier to maintain and easier to troubleshoot if a problem occurs.

Most non-pros's still use a test light when checking for power flow connection. Difficulty there is the bulb may light, but you do not have any idea if ENOUGH power is getting to the user. If you run a simple power feed from the fuse box to the solenoid you have only two connections and no switch to worry about. On the ground side, only the switch and selector(gear) switch and a continuity tester is easy diagnosis on those. IF a problem occurs. Revington used to offer a wire harness setup that they said would protect the system if reverse was engaged improperly. Dunno if they still do. But if you are concerned that might be something to check out.

 

[TR3driver]

As always, different strokes for different folks. The two circuits are electrically equivalent, excess resistance in wires or switches will be a problem no matter whether they are on the hot side or the low side.

Using a non-powered 12v test light (like a taillight bulb with wires soldered to it) will quickly point out any high resistance problems; just use the car's battery as the power source and see if the bulb goes full bright or not. But I can't imagine why any Triumph owner wouldn't have a DMM, even if it's just a $5 cheapy from HF.

To my mind though, there is one big advantage of the factory wiring scheme : Having a short to ground cannot cause the OD to activate in reverse.

 

[2wrench]

Thanks again. I'll likely be making reference back,
once again, when Steve has completed his work;
I have the tranny along with the ordered activation
switch. Got to have him get past the lockout
install, then I can actually move forward.

Actually, sorry, but I'll have to have my engine
back from the machine shop and rebuilt as well.
Whew, I can get dizzy.

BTW, my rocker assembly should be done right about
now. I'll have to call them on Monday.

 

[2wrench]

Update from Steve (mechanic) on rebuild/overdrive upgrade sent to me via e-mail Monday, January 21, 2008 as follows:


"Dennis,

Good morning,

I have located a gearbox top cover that I can copy for the location of the switch. Also the location of the indent in the shafting fork rod.
I will keep you informed.

Steve Miller"

I didn't understand his question/comment regarding
the location of the indent in the shafting fork rod;
but, hey, if he knows what it is and where it's at
now, I'm good. I mean, that's why I'm paying him.

Suppose all is well until further notice regarding the
tranny.

 

[Opa]

shafting fork rod.....sounds like a weapon of mass destruction...

seriuosly though, he's talking about the shift rod and the detent is were the ball falls into for overdrive switch

 

[2wrench]

Thanks for helpin to decipher the hyroglyphics,
Casey.

And, yeah, I was thinking to call Homeland Security.

You saved me some headache.

 

[TR3driver]

I didn't understand his question/comment regarding the location of the indent in the shafting fork rod;

Sounds like he got ahold of a later top cover, with the OD switch much farther back and riding directly on the shift rod; rather than the earlier location riding on the shift fork. I think that means he will also have to cut the notch in the rod.

 

[jjbunn]

I have what is perhaps a silly question about the J O/D. If one removes the 2nd gear inhibitor switch on the transmission cover, would that allow the O/D to be used in 2nd gear? If so, what are the downsides?

 

[skucera]

Second gear with overdrive engaged isn't halfway between 2nd and 3rd. It's more like "gear 2.25." However, the torque stresses on the gears and the layshaft are multiplied, resulting in increased wear and a shorter lifespan.

Remember that the later single-rail transmission is a "reduced cost" design over the earlier three-rail design that was intended to be just enough for the 1500 engine under standard operating conditions. It works really well when you use it the way the engineers expected you to (no lugging the engine and not hurrying the synchros) and lasts pretty well too, but it wears pretty quickly if you try to drive it like you had an American V-8 attached to it.

Scott

 

[TR3driver]

I have what is perhaps a silly question about the J O/D. If one removes the 2nd gear inhibitor switch on the transmission cover, would that allow the O/D to be used in 2nd gear? If so, what are the downsides?

It's the other way around, you would have to add a switch to enable operation in 2nd gear.

The main disadvantage IMO would be the very slow OD engagement; which would also cause fairly rapid clutch wear if you tried to do it under power. But if you always backed off the power & waited for the OD to shift, I think it would be OK. Might need to turn the pressure up a bit to handle the increased torque, but again if you didn't shift under power, I think you'd get away with it.

 

[TR3driver]

Second gear with overdrive engaged isn't halfway between 2nd and 3rd.

Depends a lot on which gearbox it's behind, and which OD you're talking about. On a TR3 (A-type OD with stock .82 ratio; 3.7 rear axle), 2nd direct is 7.4 overall; while 2nd OD is 6.07 and 3rd direct is 4.9. That's pretty doggone close to exactly halfway.

Another way to look at it : 5000rpm in 2nd direct is 50 mph, and when you flip the switch, rpm drops to 4100. Then when you get back to 5000 rpm in 2nd OD (at 61 mph), and shift to 3rd direct, it drops back to 4026 rpm. (Assuming you have the 3.7 axle and tires the same diameter as the original bias ply Dunlops ... numbers taken from Practical Hints 2nd edition.)

Not that those are necessarily the optimum shift points, just examples.