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TR4a Gearbox reversing light switch


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Welcome to the forum TM.  

I believe the answer is going to depend on whether your car still has its original gearbox and what the top cover looks like as there were several variations.  The later boxes had provision for fitting a reversing switch in one of two positions.

This link is about overdrive inhibit switches but there is some info about the reversing switch too:

https://static1.squarespace.com/static/5c6dec53b10f25d4edf0b3f7/t/5d0195838ed02e0001596bc0/1560384900296/OD+Isolator+Switches.pdf

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AFAIK, and can remember, the reverse switch is opposite the blanking plug in the top cover where the OD joins the GB in your picture above.

As I have the top cover off I can have a better look tomorrow, unless I'm beaten to it....

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Hi Roger, thanks for confirming with your photo

  • what is the correct profile on the selector fork I am looking for?
  • what is the thread size I need to cut into the gearbox cover to accommodate the new switch?

jpmf, If you could post the picture of your gearbox cover that would be helpful to. 

Thanks

Rupert

 

Edited by TractorMechanic
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9 hours ago, TractorMechanic said:

Hi Roger, thanks for confirming with your photo

  • what is the correct profile on the selector fork I am looking for?
  • what is the thread size I need to cut into the gearbox cover to accommodate the new switch?

jpmf, If you could post the picture of your gearbox cover that would be helpful to. 

Thanks

Rupert

 

Hi Rupert,

I can't find a pic with the fork profile but it would be obvious when you get access to it.

As Peter states the tap used is  16mm x 2mm (12.7tpi) - this works well but not the actual size.  It should be 5/8"UNF x 13tpi.  However this is a special size not readilyt available.  At least one restorer has the correct size.  Why did they use a thread size that is odd!!!

 

Roger 

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Hi Rupert, I have just done this. 12mm drill and M16 tap. When you drill the hole go carefully, the reverse selector rod is just underneath; I slid some thick gasket paper over the selector rods to capture any swarf and provide a little protection. The M16 tap works fine with the thread of the isolator switches. The reverse selector rod has a small indent cut into it so that in fwd gears the ball bearing of the switch sits in this cut-out, when reverse is selected the full profile of the rod depresses the ball bearing and makes the switch. Easy to check that works ok when installed, with an resistance meter

Hope it goes ok

David

gearbox isolation sw.JPG

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After fitting the switch, wiring it in will need some thought. If it is to go to Earth, then the reversing lights will need a live feed and an insulated return to the switch.

I chose a different option, I assumed that the switch would be earthed, but so too would one side of the reversing lamps (fewer wires running from rear), so I added a relay controlled by the switch that supplied the live feed to the lamps.

TT

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Hi Tractor,

on Roger's photo you see all switches have spades (bended) sideward,

if you buy a new one they are probably indicating up, so bend them carefully down

before you @€&?!()@ with the gearbox cover.

You can call me Marco, Ciao

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