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TR2/3/3A TR3A Overdrive Relay

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Tinkerman

Tinkerman

Darth Vader
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This afternoon after a bit of a search in the correct spots I realized that I didn't have an overdrive relay. Probably one of those things I put off buying because it was so expansive.
I have two questions about the relay and one about the lockout switches.

1. Where does it mount?

2. Has anyone found an inexpensive place to buy one? Like at radio shack or some such place.

3. I have a rebuilt TR4 tranny and it has three lockout switches. it's been so long I don't remember why. Getting old..................Ahhh. I​s it because it is a full syncro tranny?

As always, your thoughts appreciated.

Tinkerman
 
OD relay usually mounted on the bottom rear (cockpit side) of the battery box.

I use the old Lucas metal can relay, but I see know reason why a modern 40 amp (?) relay wouldn't be able to do the job.

This pick shows the 2 lockouts the OD requires, if you have a reverse switch perhaps that is the 3rd you see:

ODTop_zps94c20774.jpg


Of course the 'lock out' switches actually permit the OD to operate by proving a path to ground -- so you should see continuity at the respective switches when the gearbox is in that gear.
 
My experience is that a cheap modern relay will work fine, as long as you add a diode to absorb the inductive kickback from the solenoid. Without the diode, even an expensive reproduction relay only lasted a couple of years for me.

Geo did a much neater job with his, but here's a shot of mine, repleat with el-cheapo relay.


That photo was taken a long time ago, on my previous TR3A, and even before I re-did the wires from the relay to the OD. Probably around 1988. But the same relay (and diode) is on my current TR3 and still working fine (which is more than I can say for the lockout and operating switches). The diode is inside the black heat shrink tubing, and is a common 1N4004 type, readily available at Radio Shack, Amazon, etc. Rated at 1 amp, 400 volts (although I'm sure a 50 volt, 1N4001 would have worked just as well; the 4004 was what I had on hand).

PS, the diode installation is polarity-sensitive. It will work with either polarity, but you have to reverse the diode connections. For a negative ground system, the end with the stripe goes to the relay terminal. For positive ground, the end with the stripe goes to ground.

PPS, while you're at it, I recommend an in-line fuse in the wire from the ammeter or fuse holder to the relay. An AGC 5 fuse should do. If the diode fails shorted, the fuse will blow, preventing further damage to the wiring. It also will blow and protect the solenoid, if the pull-in contacts fail to open. You can see my fuse holder at the top of this shot

 
Hi Dick,

The relay on mine was mounted on the back of the battery box. You can see the studs here on the old box



And one from inside the car.



Cheers
Tush
 
I just installed one in the 56, I used the dash brace on the drivers side. Holes already there. Dick I have a couple of o/d relays. Pm me if you want one.

Marv
 
Tush, I hadn't noticed those studs till you posted your pics. Sure enough, I went back through my pics and the overdrive cars had studs on the battery holder. Cool!

null_zps8efa91b8.jpg
 
Hi john , I know that I need to make provisions for the studs on my new battery box that I installed as it didn't have them :cool:

Cheers
Tush
 
Hi Marv, you should have a PM on the way, might be there now.
Dick
 
I just looked at mine and it has no studs.
I like the dash brace, cause it is easier to get at.
Thanks to all of you I now have the info I need to move on!

Thanks all, Dick
 
I don't believe they were studs, at least not on all cars. Just bolts inserted from the battery side. The workshop manual talks about drilling the holes when adding OD to a non-OD car.
 
Randall's right! I was curious so went to check it out on the old hull. They are not studs, but rather slot head countersunk screws that go in from the battery side of the box.
 
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