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Immobilizing Overdrive


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Last year when I picked the car up from voluntary MOT, the Overdrive switch was in the on position and the car had been reversed into parking bay. The switch may have been accidentally knocked or used as lights/indicator. Would the inhibitor switch prevent damage, although I would rather not risk it.I really don't want to give a toolbox talk on the workings of a TR when I take it this year, and was thinking if it is possible to pull wires on relay switch,  I have two red, a  white and green, can anyone advise if this is possible and what wires to pull.

Any advice greatfuly received 

Mick 

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Hi Mick,

the system comprises of a -

switch on the steering column

Then inhibit switches on the gearbox.

You can only get into OD by having the Switch ON and in 3rd or 4th gear to allow the inhibit switches to allow it.

If the switch is OFF and you are in 3rd or 4th then the OD is not working

If you have the switch ON and in 1st, 2nd or rev gear then the OD is not working

Don;t panic

Don;t pull any wires

 

Roger

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The two or three gearbox switches are a path to earth, .... if a wiring fault allows the O/D switch to go directly to earth the the O/D solenoid will operate ... (overdrive in all 4 gears and, when the switch is accidentally left On, overdrive in reverse!,  but only briefly then .... BANG!!!) .... Ask me how I know this!!   .... so check that the O/D switch only operates when the relevant gears are selected ie for A type:-  2nd, 3rd, &4th ....and J type :- 3rd & 4th .

Cheers Rob  

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The inhibitor switch will prevent damage and lock out the O/D in reverse if correctly wired. Mine wasn't when I first got the car. You can get the result you want by pulling the feed to the O/D from the relay. it should be a yellow/ purple wire but yours might have been rewired.

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23 hours ago, Rob Salisbury said:

The two or three gearbox switches are a path to earth, .... if a wiring fault allows the O/D switch to go directly to earth the the O/D solenoid will operate ... (overdrive in all 4 gears and, when the switch is accidentally left On, overdrive in reverse!,  but only briefly then .... BANG!!!) .... Ask me how I know this!!   .... so check that the O/D switch only operates when the relevant gears are selected ie for A type:-  2nd, 3rd, &4th ....and J type :- 3rd & 4th .

Cheers Rob  

Rob is spot on. When we bought our 4A there was a handwritten note in the owner's handbook to the effect that the o/d switch was shorted out and o/d was operating in reverse! That condition cannot have persisted for long and was sorted before we acquired the car. If the yellow/green wire from the overdrive switch shorts to earth at any time that the ignition is on then the solenoid will operate and engage overdrive, whatever gear is engaged. If the black wire from the switch shorts with the ignition on and the overdrive switch in the on position the same thing will happen. Whenever any work is done that involves stripping the steering column it is very important to pay attention to the insulation on these wires from the o/d switch and be careful they cannot chafe on the column harness cover.

Tim 

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The wire that switches the O/D on earths through the switch and gear interlocks, when this happens  the relay switches the 12V  feed to the O/D on.

Doesn't affect what you want to do. Disconnect the 12V feed to the O/D solenoid (originally yellow/purple) and the switching can be  in any position but the solenoid can't energize.

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Normally in the drivers side footwell at the front at  about 1 o’clock on the gearbox tunnel cover there will be a Yellow and purple wire leading to the inhibitor switches and solenoid if you are that worried about a fault occurring then disconnect this wire and you then have no electrical supply  to the gearbox system the solenoid cannot engage and therefore the box is safeguarded. Or put a big note on the switch saying do not touch at risk of death? 

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