TR2/3/3A OD Wiring from Switch to isolater switches.

[Redoakboo]

I have a 1957 TR-3 with a TR-4A overdrive transmission. I recently changed the clutch, and now the Overdrive does not operate? It appears that the yellow wire came apart at the splice to the dash switch. It appears that it made contact with the back of the start switch and melted most of the wire. Do they make a harness to replace the two wires that run from the dash switch to the switches on top of the transmission? I would love to find a wiring diagram to make sure I get all the contacts in their proper place.
Could a bad relay keep it from operating?

Any help would be greatly appreciated.

Thanks, Dick

 

[Geo Hahn]

Basically those yellow wires are providing a ground path to operate the relay - which then powers the solenoid:

Yes, if you think about it you can test directly rather than relying on the relay and, of course, you can test the relay itself.

 

[CJD]

https://www.britishcarforum.com/bcf/showthread.php?108693-Recipe-for-a-TR2/page3

Starting at post #93 you can follow along with my OD harness rebuild. The harness is not available...but the wires are available online

 

[TR3driver]

I believe the TR3 was wired a bit differently than Geo's diagram shows (although certainly either one will work). At least TS39781LO was, when it came to me; and the wires looked original.

Bad relay is a definite possibility, I've had several fail myself and seen failures on several other cars. But if you've got a burned wire, I would certainly start there.

Adding a diode to the relay really made a big difference for me. After replacing something like 4 relays in 2 years (including a fairly expensive repop); I finally added a diode to suppress the kickback from the solenoid. And now the "Made in China" brand relay I used has outlasted that car and served another 8 years on my current TR3.

PS, while you're in there, you might want to consider adding some in-line fuses. The fuse holder partially visible at the top of this photo was for the OD.

 

[JerryVV]

Randall, you frequently write about using a diode. I don't know anything about diodes. What do I buy and where? How specifically do I wire it into the system? And I think it needs to be oriented a certain way. Kindly detail all of this as my guess is others would also like to know.
I'm still using my original relay from the late sixties when I installed my OD in my 66 TR4A so maybe I don't need a diode.

 

[TR3driver]

I agree, the original relay probably does not need a diode. Mine lasted a long, long time with no diode; but it did finally give up the ghost after some 30+ years of service (and a whole lotta miles). The problem comes in with the replacement relays. My guess is that the original relay had special contacts that were designed to stand up to the kickback from the solenoid (which is a very typical 50s solution to the problem); but replacement relays (even repops) don't use the same material.

There are lots of different diodes that will work in this application. The one I used is part number 1N4004 (that's an industry standard part number, made by lots of manufacturers). I got mine at Radio Shack (back when they had a store on every corner); but here is another source https://www.amazon.com/SODIAL-1N400...coding=UTF8&psc=1&refRID=7RXM39VBC5NVQVSG82YW

The diode will be connected between the relay output terminal and ground. Here's a typical diagram (originally supplied by Rob Michigan)


It does have to be oriented a certain way, which depends on how your battery is connected. If you look at the body of the diode, there should be a white (or silver) stripe near one end. If your car is wired negative ground (original for a 4A), the end with the stripe should connect to the relay terminal. If your car is wired positive ground (or earth if you prefer), then the end with the stripe should connect to ground.

Geo Hahn once posted a nice photo of his TR4 installation, but I can't seem to find it now. Perhaps he will post it again.

 

[Geo Hahn]

Not only was I unable to find that photo - I couldn't even find where I put the diode. Finally spotted it and took a pic.

This is the diode I used:

I miss my local Radio Shack though the fact that I would often go in, shop for 20 minutes, and spend 87¢ probably contributed to their demise.

I connected the diode between the Yellow/Purple wire (powers the solenoid) and a ground. I think this could be done at the relay (C2) or at the solenoid - but I just changed a single-bullet to a double bullet and did it on the harness (I apparently put a male bullet on one end of the diode and a wire to ground on the other). Note that the orientation of the diode (see silver band) is for positive ground.