Mosfet R/R on Triumph Thruxton

A while back the charging system on my Thruxton started failing. I found this out one morning while I was a few hundred miles from home in New Hampshire, my bike wouldn’t turn over. Luckily the place I was staying had a battery charger (dumb luck) and a few hours later my battery was back to full power. My battery then began to die slowly but it got me through the rest of the trip.

At first I didn’t get what was going on. Really I just through my battery was dying but after poking around in some forums I got a tip to fire up my bike and check the voltage that my charging system was putting out. It should put out around 14 volts but it was only putting out 11.5V.

This is a sure sign that either the magnets on your stator is bad (this never happens from what I’m told) or your regulator / rectifier (R/R) is dead. A little more research turned up that the stock R/R on Triumphs are rubbish and generally fail after a few years. Replacing the stock R/R is a pretty standard upgrade on lots of bikes.

There’s a number of after market R/Rs to chose from. I found a few guys who had good results with a Msofet R/R. There’s even a guy that sells kits for the upgrade at roadstercycle. The instructions on the roadstercycle site aren’t very detailed so I figured it might be useful to have a detailed howto (aka lots of pictures) for those like myself new to doing electrical work on their bikes.

Installation

The installation has two main parts:

• First is preparing all the wiring: attaching the connectors from the kit to the existing wires from your stator and the power/ground wires provided in the kit as well as running the power and ground wires to your battery.
• Second is finding a place to mount your new R/R on your bike. Depending on your bike this may be very easy or very difficult so YMMV.

I started with the wiring because mounting stuff on your bike is usually pretty permanent. This typically involves drilling / cutting and if something goes wrong and the new R/R doesn’t work you’re stuck.

Wiring

So the kit ships with these really fancy waterproof connectors that match up with the connectors on the R/R. There are two of them and they both accomodate 3 wires but one has the center hole plugged. This one is for the power and ground that will attach to your battery. The connector with the three holes open is for the three wires from your stator.

I started with the stator wires. They’re located at the engine casing on the right side of the bike. Follow them up to the connector where they hitch up to the wiring harness. On the Thruxton this is a 3 pronged connector and the three stator wires are bundled together in a black casing.
Sorry for the picture quality (focus is way off). I’m still trying to get the hang of of this new phone.

Cut these three wires right at the connectors to keep as much wire available as possible. You’ll have to run these to the new R/R wherever you decide to mount it. Once the old connector is off, you’ve got to put the new connector on.

The kit provides a bunch of crimp-on connectors:

You’ve got to:

1. Strip about an inch off the wires casing
2. Slip one of the provided water seals on the wire. This will be used to plug up the hole in the back of the connector.
3. Slip on a bit of shrink tube. My stator wires were too thin for a good seal with the plugs so I put on some shrink tube to fill the gap.
4. Fold the exposed wire in half to give yourself enough surface to crimp the connector
5. Crimp on the connector, you can even throw a bit of solder on too if you’d like but it’s not necessary.
6. Slip the shrink tube up over the crimp and shrink it down
7. Slip the water seal up over the shrink tube

Do this for each wire

Then slip the three stator wires into the back of the connector. Make sure to press each water seal into the connector. If the fit is tight grab some silicon and lube up the rubber plugs.
I’m not sure I put the rubber plugs on in the right direction but the seal will work either way. I switched the direction when I put together the plug for power and ground.

Then do the same thing for the power and ground wires. These wires are much bigger (10 or 12 gauge) so I didn’t use shrink tube this time. Also the rubber stoppers were much more difficult to fit into the connector so keep your silicon handy.

Next you’ve gotta wire the fuse provided in the kit into your new power line and put the provided ring connectors on both power and ground. Nothing special here.

By this point you should have your stator wires set up with the new plug, the power and ground wired into the other plug, and the new power and ground wires ready to hook up to your battery.

Mounting the R/R

With your wires done you can start thinking about where you want to mount the R/R. Be sure you’ve got enough wire to reach the R/R wherever you mount it. BUT FIRST you should test the R/R to be sure the thing works.

Plug in those connectors you just put together, connect the power and ground to your battery and fire up your bike. The voltage across your battery should be between 14V and 14.5V.

If the voltage is right get ready to mount the R/R … if it’s not, well you screwed something up. Sorry. Break out your debugging gear and get to work. Hopefully you just screwed up a connection and you’ll find it quickly.

Once you’ve verified the R/R works pick out your mounting location. I installed a fender elimination kit (FEK) from newbonneville.com a few years back. They still sell a kit but it’s significantly different from the one I bought. Regardless the battery box from the kit has a piece of metal hanging off the bottom to act as a sort of mud guard. The inside (toward the engine, away from the rear wheel) of this is just the right size and location to mount the new R/R.

The Mosfet kit doesn’t come with mounting hardware so you’ll need to pick up some up. I got mine at the hardware store down the block. I think the bolts were 6mm but I can’t remember. Just bring the R/R into the hardware store with you and try the hardware before you buy it. I picked up some lock-washers too for good measure.

Next I drilled the holes in my battery box. It’s impossible to get a drill into the right place without pulling off the wheel and going in through the rear. Remember, measure twice and drill once. Then mount the R/R, run the power and ground wires up to your battery and you’re done.

If you want to get fancy you can secure them with some zip ties. In the pictures you’ll see I haven’t got around to that yet.

Clean up

The stock R/R on the Thruxton is mounted on the front of the bike below the headlight on the same bracket as the blinkers. This is pretty much on the opposite end of the bike as the battery and the stator wires. This means that the stator wires run into the wiring harness back by the battery and extend to the front of the bike to plug into the R/R. The power and ground wires then come out of the R/R to the wiring harness and run back to the other end of the bike where they plug into the battery.

This is completely insane IMHO. You’ll notice I didn’t use any of the existing wire in the harness. All I did was snip the stator wire and tape up the old wire leaving the harness alone. Cutting into the wiring harness should be avoided at all cost. You can do the same to the stock R/R on the front of the bike. Just cut it off and tape up the wires.

Good luck.