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Overdrive logic box --- a different approach.


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Hello All,

 

 

About 20 years ago I had a 1950’s Rover 90 and I modified the overdrive wiring so that to engage the overdrive I pressed a momentary “On” button. This operated a relay which self latched through one make contact, and pulled in the overdrive solenoid via another make contact.

 

To disengage the overdrive I gave the clutch pedal a very slight tap. I’d fitted a microswitch to the clutch pedal that operated in a similar way to a brake light switch on the brake pedal, except that when the clutch pedal was pressed the latching circuit of the relay was broken, the relay dropped out and so did the overdrive. The overdrive would not engage again until the momentary “On” button was pushed again.

 

The clutch pedal switch was adjusted so that the slightest movement made the switch work.

 

What this meant was that if I wanted to drop the overdrive whilst in a gear, I just taped the clutch pedal, and if I was in overdrive and wanted to change down a gear the overdrive would drop out automatically because I’d be pressing the clutch pedal to change down anyway.The same with changing up a gear, if I was in overdrive.

 

I found the system very easy to use and never had any problems with it. Also the wiring was simple (Just one relay, if I remember right.)

I intend doing the same with my TR, now that I’ve come to the wiring loom part of my rebuild.

 

Just wondered if anyone could see any potential problems doing it this way.

 

 

Charlie D.

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Hi Charlie,

that is clever thinking.

I have made one and the difficulty I found was making the column switch (momentary ON) both the On and the Off.

Your system uses two switches but nicely positioned to give the best operating.

 

Go for it.

 

Roger

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Bob,

 

I think it needs a different relay because you need TWO make contacts. One to operate the solenoid, and one to do the latching.

 

Seeing as you and Roger both approve of the idea I’ll draw up a circuit sometime next week and post it here so if anyone wants to try it they have the map. It’s dead easy really. The fiddly bit is fitting the pedal switch, but no harder than fitting an electrical brake switch, which I know some people do.

 

 

Roger,

 

I was thinking of a momentary push switch mounted on the gear knob.

 

Looking through past threads on the subject of overdrive operation I noticed that some people said there were occasions when they needed to change up or down while still in overdrive. With it on the knob all you need to do is keep the button pressed in as you change gear and that will over ride the operation of the clutch switch.

 

Charlie.

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Hi Charlie,

I think changing from 3OD to 4OD is not a common selection so I wouldn't panic about that.

Bearing in mind that changing from 4th to 4OD is virtually instantaneous when the button is hit.

 

I appreciate that there is little between 3OD and 4th but it does work.

 

Roger

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Hi guys

 

I have OD on a simple on off switch on the dash. (Not the big lollipop one).

I like the idea of the logic OD as I struggled between quick changes whilst doing sprints and hill climbs. Especially changing down.

Its a big jump between - say OD 3rd down to second but I dont see how the logic switch works on the way down the box.

On the way up its nearly sequential

OD 2nd use clutch to 3rd use OD switch to OD 3rd etc.

But on the way down under heavy braking all too easy to drop a gear and a half

 

But I think the clutch switch may help this a bit like driving with a pre-selector box.

 

Would that be right ?

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Bob,

 

I think it needs a different relay because you need TWO make contacts. One to operate the solenoid, and one to do the latching.

 

 

 

Charlie.

Why two ? just connect the solenoid operating contact via a diode to the side of the input coil you have the button connected to (other side to IGN switched supply) The diode prevents the solenoid getting any volts until the relay has energised.

The gearbox isolator switches connect from the other end of the input coil to the clutch release switch, & then to ground.

 

post-12009-0-68649100-1509984029_thumb.jpg

 

Bob.

Edited by Lebro
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Hi Hamish,

The logic box takes you out of OD no mater whether you are going up or down.

 

So if you are in 3OD and change down to 2nd then you will be in second until you select 2OD

 

The difference between 2OD and 3 is so small why not simply go 3OD to 3.?

 

Whatever system you choose to use you need to become very familar with it and use it to the best of its facilities.

 

Roger

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Yeah I know what you mean.

I think my confusion and timing is using the 2 systems on the way down. Whilst hurtling to the hairpin.

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Hello Bob,

 

Your circuit is a bit too much for me after a couple of glasses of wine….

 

To defend myself I must point out that when I left school I became an apprentice at the GPO (telephones), in 1965. We were taught EVERYTHING about relays. BUT Diodes had not been invented then…………(Well… only just…)

 

OK, I do think you are right.

 

But simple coils and contacts are easier to understand than one-way streets.

 

 

Charlie.

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Hi Hamish,

The logic box takes you out of OD no mater whether you are going up or down.

 

So if you are in 3OD and change down to 2nd then you will be in second until you select 2OD

 

The difference between 2OD and 3 is so small why not simply go 3OD to 3.?

 

Whatever system you choose to use you need to become very familar with it and use it to the best of its facilities.

 

Roger

Correct and the same for od 3 to none od 4,its job is to save time to switch off and strain on the overdrive should you forget it is in,you will gain no time on the track.

Edited by ntc
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My TR6 had the Revington logic box. The best upgrade I ever put on the car.

 

It also had a dash warning light to remind that the o/d was engaged, as with a mom switch there is no visual indication.

 

As regards losing the o/d when changing down, this is more of an issue with the J type as it does not immediately re-engage, but losing the o/d on a downshift happens with or without the logic box.

 

One thing I thought to do, given that the mom switch only needs a dab, was to put a second switch on the trans tunnel by the left knee.

And one could put one on the gear lever as well. The switch only send a pulse, the o/d doesn't know how many switches there are, nor care where the pulse comes from.

Those adept with their cheeks could put one in the centre of the seat cushion.

 

Ivor

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Bob,

 

With a clear head I can see that your circuit is the easiest way to do it. Uses original relay and very little change to the original wiring.

 

The only slight disadvantage that I can see is that you can’t “Over ride” the action of the clutch pedal switch. (Some people may want to stay in OD occasionally as they change up or down.)

 

Using 2 contacts on the relay allows you to do so by keeping the gear knob button pressed as you change gear. (Circuit attached. If you see anything wrong, or an improvement, please let me know)

 

However…

 

All I ever used it for on my Rover was a quick way (lazy way) to get out of overdrive from top gear, so I think I will be using your circuit on my car. (I take my hat off to your use of diodes…)

 

 

 

Charlie D

 

 

post-14075-0-51174400-1510047377_thumb.jpg

Edited by Charlie D
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Roger,

 

Thanks for that , circuit now changed.

 

(I thought that when in first or reverse that the circuit was "Broken", I now realise that when in second , third, or fourth the circuit is "Made".)

 

"... I take it that the gear knob switch is momentary ON not a toggle ON/OFF..."

Correct.

 

 

Charlie

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I should point out that the inhibit switches on the gearbox would normally only be carrying the current drawn by the relay coil, not the solenoid.

(my circuit above also tweaked to show isolation switches in parallel)

 

Bob.

Edited by Lebro
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TRs are not quiet cars and it can be difficult when motoring fast to determine from engine note whether or not one has overdrive engaged.

The up/down selector switch, whether of the very neat sidescreen type or the wand used on TR4 onwards, gives that information.

However, once one abandons the use of a switch, I consider a lamp (or similar indicator) is necessary to avoid engine damage from inadvertent de-selection at high engine speed.

The Revington kit allows for this, and I have an extra lamp in the dash where it's easily seen.

Ian Cornish

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Hi Ian,

fully agree. The original switch provided a very useful reminder where you are.

Also by quirk or design the tacho needle and speedo needle stay in sync in 4th. 2000 = 40mph. 3000=60mph etc

In OD 4th the tacho lags the speedo by quite a few degrees.

 

Having changed to a press button selector I found I needed a lamp to give me that extra warning. It also needs to be a dimmable lamp for night driving.

Finding anywhere on the TR4/4A dash to drill a hole for a LED is a nightmare. I ended up drilling a 1/4" hole in one if the switch covers immediately behind

the steering wheel.

 

Roger

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Hi Bob,

the standard (or Standard TRiumph) version is perfectly practical for normal touring/driving.

The racing boys have other needs.

 

However to the inquisitive it is a source of meddling .

Why have a switch when you can have relays, timers. LP filters, acres of Vero board and a black shiny plastic box. Hmmm, black and shiny :ph34r:

 

No contest.

 

Roger

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Yes, I put the warning light in the dash right in front of me.

And the Revington kit was/is particularly cunning, in that the light could be selectively set to dim at night.

A light which is bright enough to be seen during the day will be dazzling at night.

It was somehow wired into the dash lighting circuit, and there was a potentiometer on the control box.

 

Ivor

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