Overdrive

[Stuart Wilson]

I've just aquired a gearbox with overdrive to go in the Moss. I've never had a car with an overdrive and never had anything to do with them before, so these may seem very basic questions to some.

I'm guessing you have a positive feed up the gear stick switch, this then goes through the inhibitor switch then to the solenoid? Or does it go through inhibitor switch first? Do you need a higher current for the solenoid via a relay?

Now don't laugh at this question, how do you use the overdrive when on the road? I know it's in 3rd or 4th gear. Is it just a case of flicking the switch? Do you lift off the power, press the clutch?

Thanks in advancr

[Ridgetone Triumph]

The electrical side of your question is not for me to answer, unless you want your car to go on fire 😀
As for selecting overdrive, this a matter of just flicking the switch, no need to depress the clutch, lift off or brake. Overdrive kind of is automatic, you hit the switch and the rest is elementary my dear Watson  

[Ridgetone Triumph]

oops I mean Wilson  

[Nick Jones]

Wiring question answer varies according to overdrive type.

D type (or A type on the bigger cars) must have a relay.
Relay coil circuit:
positive supply from ignition controlled source > fuse > inhibitor switch > gearstick or column switch > relay coil > earth.
Power circuit:
positive supply from ignition controlled source > fuse > relay contacts > overdrive solenoid coil > earth.

J-type OD doesn't have to have a relay (though you can use one if you want)
positive supply from ignition controlled source > fuse > inhibitor switch > gearstick or column switch > overdrive solenoid coil > earth.

overdrive can be switched in at full power without using the clutch.  More thought is needed when switching out. Don't over-rev engine by switching out too soon when slowing down and switching out when already braking heavily (for a corner) can cause more rear wheel braking than you wanted......

Hope this helps

Nick

[Nick Jones]

3rd and 4th only is normal for small chassis Triumphs.  Reverse switch is not involved in the OD wiring as the inhibitor switch means it can only engage in the 3rd/4th gate anyway.

Nick

[RobPearce]

I don't know about the full set of possible cars, but I was fairly sure the D-type wiring doesn't connect the relay coil to earth. On all the examples I've worked on it was:

  Ignition switch  -  fuse  -  relay coil  -  gearknob/column switch  -  inhibitor switch  -  ground

[Hogie]

Hi Stuart,
              To clarify the use -
When in the correct gear you can select OD without too many problems. Don;t be going too slow when you do this

Coming out of OD is more interesting. If you take your foot off the throttle and disengage OD the back wheels will try to lock up. Worse still the OD will try to act like a brake and it will probably break.
You MUST increase the revs slightly then drop out of OD and it will be as smooth as a babies rear end.
You could also drop the clutch and change down - but this is for cowards.

Or you could install a simple logic box that will bring you out of OD when you change gear (up or down).

I'm sure on the TR cars that the switched cable is earth. If running your wires through the gear stick then earth wires would be safer.

Although the inhibit switches are there to stop you selecting anything other than the correct gears things can go wrong.
Never select reverse gear with the OD switched in (just in case)
If when you reverse you feel a resistance to going backwards DO NOT put your foot harder on the throttle - if you do achieve reverse in OD you could easily destroy the OD casing

Roger

[piman]

Hello Rob,

it matters not which order the various switches are connected, just connect in the most logical route. In your example the relay is permanently live with the earth being switched, opposite to Nick's but it is irrelevant to the operation.

Alec

[RobPearce]

Alec,

I'm well aware of the details of electrical operation, thank you. I was merely correcting Nick's statement from "it will work if done like this" to "this is how the factory did it".

As Roger said, it's safer not to pass a live wire through the gear stick. In fact, I'd probably advise swapping the switches on my previous post, so that even if the wiring to the gearknob does fray and short out, it won't try to engage OD in reverse. When my brother first bought (what is now) my GT6, it had a bit of insulation missing on the wire from the gearstick to the relay, and would rattle the OD solenoid as you drove because this kept tapping the side of the gearbox.

Rob

[Richard B]

Quoted from Stuart Wilson

I'm guessing you have a positive feed

Make that an ignition swtiched feed to the inhibitor and then the O/D switch (to earth the relay). That way the thing is turned off with the key. A GT6 I know of had a permanently live feed to the relay and was draining the battery down if the gearstick was left over to the R/H side by 3rd/4th.

[Stuart Wilson]

Thanks for all the replies so far.

No doubt when I actually come to fit it I'll be back with more questions.
I've a feeling I'll be making s modification to the tunnel on the Moss, as I don't think it has a removable cover. But nothing a jigsaw can't cure

[nang]

Having seen a D type shaft twisted at the splines, I'm a believer in using the clutch for every o/d change up or down to eliminate shock loading.
Just my thought.
Tony.

[JohnD]

Mechanical sympathy is all, but not as far as using the clutch to change in and out of O/drive.
A gentle feather of the throttle, more for o/d out, less for in, is all that is required.
Becomes second nature after a while, like rapid gear changes, and your fair passenger won't know you've done it.
John

[heraldcoupe]

Quoted from RobPearce

I don't know about the full set of possible cars, but I was fairly sure the D-type wiring doesn't connect the relay coil to earth. On all the examples I've worked on it was:

  Ignition switch  -  fuse  -  relay coil  -  gearknob/column switch  -  inhibitor switch  -  ground

Some are wired this way, others have the switching in the feed side of the circuit. A  lesson learnt when intending to use a Mk3 Spitfire overdrive loom on a Vitesse...

Cheers,
Bill.

[Stuart Wilson]

Reading all the possible wiring options, it looks like there is no set way. As long as it goes through the inhibitor switch.

I'll have a little play with the gearbox on the floor and a battery

[Pete Lewis]

D type need a relay as the solenoid pull in current is around 10amps, once the solenoid has energised the armature inside hits a switch in its  end  ( under the boot), and drops a 2nd  hold in coil with a current of 0.5amps

if you do a test take off the small plate and check when fired thesolenoid pulls a lever a small distance and a  hole in the valve lever aligns with a hole in the case , use a 3/16 drill as a gauge , adjust the tapet nut if its wrong.

Pete

[Nick Jones]

Quoted from RobPearce

As Roger said, it's safer not to pass a live wire through the gear stick.

Don't see how you can avoid that one.....   No power, no work.....  Could use a column switch.... that'd do it 🙂.  If on the earth side it just means it'll engage the OD if it shorts out rather than blowing a fuse.  I prefer the fuse blowing.

Nick

[Stuart Wilson]

I think what they mean about not pass a live through the gearstick is to switch the earth rather than the live