Bonnie Mods – Adding a Tach

Possibly the most infuriating thing about the non T100 model Bonnevilles is the lack of a tachometer.

The factory Triumph tach is available, for roughly $300… and full disclosure; I’m probably going to end up buying it eventually.

However, until that day comes there is an aftermarket electronic tach on the market that works properly with the Triumph tach signal. You can pick it up from BK Rider for $50. This tach is normally used on Harleys, but it works properly on Triumphs as well. The tach is made by DBI in Taiwan and comes in a generic cardboard box.

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DBI electronic tachometer from BK Rider
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Tach comes packaged with in-depth instruction sheet, handlebar bracket, and “triangle” bracket.
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rear view of tachometer and its triangle bracket.

I wanted the install to look relatively factory, so I decided to craft a bracket for it.  Parts were purchased at my local hardware store.

I acquired a sheet of plate steel, a 2′ PVC coupling, a large o-ring, a can of VHT High Temperature “Wrinkle Plus” paint, (2x) stainless steel screws 5mm longer than the factory screws that hold the “idiot light” cover on to the factory gauge cluster… If I recall they were M6, (2x) neoprene washers that fit around previously mentioned screws, (4x) neoprene washers that fit the tach base mount studs… I believe that they were M4’s, and (2x) stainless lock washers for the tach mount base screws.

To begin I took off my headlight and my headlight bucket, and removed all of the wiring from the bucket.  This may not have been necessary, but it was easier to have everything out of the way.

Then I removed the 4 allen bolts holding the gauge cluster to the triple tree and fed the gauge cluster wiring out past the retaining clip.  Once I removed the gauge cluster from the bike I proceeded to remove the warning light cover.

My plan for the bracket is to attach to the cluster via the two warning light cover mounting screw locations for stability, and to hold the tach in the curved recess of the factory cluster. Once the cover was removed I set it down on the sheet of steel and traced it with my Sharpie, remembering to leave space on the steel for where I intend for the gauge to be placed eventually.

Once I traced the initial outline, I cut it out with my jigsaw, and then began shaping with my 4.5″ grinding wheel. After the initial rough dimensions were just about correct, I cut in the notch to clear the bump on the warning light cover with my grinding wheel and then I drilled the two holes by taping the warning cover to the bracket and drilling down through the existing screw holes.

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Initial tracing of bracket on steel plate.
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Jigsaw cut, and rough shaping with grinding wheel.
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Rough dimensions and screw holes drilled, test fit in to place.

After I had a good rough cut, I laid on the PVC coupling (which I will use as a gauge cup) and traced that as well.  More cutting with the jigsaw and shaping with the grinding wheel followed.

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Tracing the PVC coupling.
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Rough cut bracket trimmed and shaped to size.
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Test fit of bracket.

After I had the bracket cut to shape I then drilled the holes for the two tach mount studs and the pass through for the tach wires.

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Bracket drilled for tach.

 

 

 

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Bracket drilled for tach and ready for paint.

Once the bracket was ready, i used my sanding wheel to knock down the inside diameter of the PVC coupling until it was just a snug fit around the tach and then both the bracket and the PVC coupling got coated with the VHT Wrinkle Plus. After about 12 hours of curing time, I baked the bracket in my oven at 200 degrees for about an hour and a half.

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You can see the reflection of my “BEEF CAKE” T-Shirt in the wet pain on the bracket.
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Don’t forget to sand the inside diameter of the coupling until it fits the tach snugly.
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Baking off the bracket. Don’t do this when your wife / girlfriend / mom is home.. they won’t approve.

The bracket came out from the oven looking almost like a factory piece. (Maybe not the Triumph factory, but some factory somewhere…)

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So wrinkly! So nice!
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So wrinkly! So nice! …On the other side!

Now the hard work was out of the way, all that was left was to assemble it.  To keep it similar in appearance to the Triumph factory speedo, I slid the large o-ring up to the face of the tach and then slid it in to the PVC coupling as a “gauge cup” I opted NOT to enclose the bottom of the gauge cup because this tach has a vent on the bottom for condensation, etc to evaporate out of, so I figured it would be a bad idea to cover it up. Then I fed the wiring through the central hole, placed one neoprene washer on each mounting stud, mounted the tach, placed another neoprene washer, and then the locknuts, a drop of blue loctite, fit the nuts and tightened it all up. I then put the long screws through the mounting holes and held them in place with neoprene washers (all of the neoprene washers are for vibration insulation), looped the tach wire through the warning light cover, and then screwed the cover back down to the original gauge cluster. Then I used some 3M Super 33 electrical tape to tape the tach wires to the original cluster wires.

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o-ring fitted to tach
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Tach and cup mounted to bracket.
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Tach and cup mounted to bracket.
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bracket mounting screws and isolation washers.
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Tach and bracket mounted.
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Tach and bracket mounted.
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Tach and bracket mounted.
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Tach and bracket mounted. Looks like it could have come like that.

After I bolted the cluster back to the triple tree, all that was left was wiring.  There are four wires that come from the tach; blue – tach bulb power, Red – tach main power, Green – tach data, and Black – ground.  Blue and red from the tach can be tied together since they both need voltage all the time.

Blue+red and black can be connected to pretty much any ignition switched 12V on the bike, several people use the voltage going to the warning lights. I used the unused connecter in my headlight bucket that used to go to the parking lamp on my headlight, after I added the Cibie european spec bulb it was no longer in use.  The only downside to this is that the tach will light up and sweep if I ever put my key in the “park” position – but, I never put my key in the “park” position, so it is a non issue for me.

That leaves the green tach data wire. On Bonnevilles (and Thruxtons) the tach signal wire is already present in the gauge cluster wiring in the headlight housing (frame side of the harness).  If you look at the harness connector and see the following three wires in order together; black, green/red, red – in positions 7, 8, and 9… the RED wire is the tach signal wire.  I went a few inches back into the wire and spliced in a bullet connector that also has a wire going to my aftermarket tach, that way the tach signal wire is still untouched at the connector when I install the factory tach kit later on down the line.

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Headlight bucket full o’ wires

After I bolted everything back together I took out my phone and took a video of the first key turn with the new tach, and tested signals, etc to make sure that I hadn’t disturbed any other wiring.

Success!!  And, a parting shot of my messy garage!!

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Job done! (Lets get rid of those awful turn signals soon, eh?)

 

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