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TR6 J Type overdrive relay

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I am rewiring my 1973 TR6 which has a J type overdrive and was just wondering if it is necessary to have a relay in the circuit, there seems to bee contrasting views on this subject?

If it is a requirement could anyone supply/point me in the direction of a wiring diagram for the relay. I have a basic idea but would like to confirm these details.

Kind Regards


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No need for a relay Andrew. The J type solenoid draws only around 2 Amps which is well within the capability of a normal switch.  

Are you intending to have a 'logic unit' ?  Many users consider it worthwhile; as the name suggests it makes the operation rather more logical. There have been several alternative circuits posted in previous threads. 

Edited by RobH
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Normally when you switch the overdrive on, it stays on even if you change gear (between 3rd and 4th on a J type).  With it on you find yourself going from 2nd to 3rd OD to 4th OD changing up and from 4th OD to 3rd OD to 2nd when changing down.  

With a logic unit the overdrive is disconnected at each shift,  so you go from 2nd to 3rd to 3rd OD to 4th to 4th OD - like having a closer ratio gearbox.  On the down change you go from 4 OD to 3rd so you get greater acceleration.  If you want overdrive you have to switch it on again after each gear change.   Instead of the standard on-off overdrive switch you can use a 'momentary' one - a flick switch - some people fit a push-button on the gear knob. 

There are some (rather expensive) ready-made units available like the one from Revington, or if you are happy dabbling with electrics you can make your own using a few readily available bits at a fraction of the price. 

Here are a couple of earlier threads:




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See my article in TR Action 203 (May 2005) for a description of Revington's offering.

Members of the Register can view all copies of TR Action (and Newsletters) by logging in to the Register's website.

Ian Cornish

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