TDM - TRX - XTZ : Gear indicator


Reliable, cheap... fits all TDM, TRX & XTZ.

I did not find the usual digital gearbox indicators very useful. In my humble opinion, it would be nice to have a top gear indicator, only to avoid looking for the 7th gear. All other indications are not so great, but hey ! if it's fun to look at, easy to assemble and costs nearly nothing, why not give it a try ?

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The usual commercial products are all digital-display based, with entries for speed-sensor, rpm-sensor, supply voltage input. You have to connect all wires, spend some time doing the set-up, realize it does not work when engine don't run or bike don't move or simply goes nut when de-synchronizing. Not to mention the price ! All in all, nothing very attractive imho.

Theory

Looking at the TDM850-4TX gearbox sensor, it's easy to see that this sensor not only senses the neutral position (like any other gearbox-sensor), but it also senses the 2 & 3 gearbox positions and sends the info to the ignition unit. Actually the neutral sensor has connections for a full six gears box !

The shift-cam ends with a small metal rod, making an electrical connection to the ground when the cam is locked in gear position. This is a perfect gearbox position sensor. Just add a display and job's done, you have a perfect, precise and reliable gearbox indicator.

I personally prefer to use an analog display, with six LEDs : the gearbox position is easy & fast to read, there is no possible confusion between the 5th and the 6th gear requiring a longer look at the display. Other pros are that the electronic design is simpler, you may find separate LEDs brighter than the usual digital displays.

I also designed a simple digital gear display, for those who prefer this kind of display.

The sensor

The original gearbox sensor is the same piece of equipment on all 10 valves engines since the XTZ750. It has 6 connections only on the TDM850-4TX. I do not know about the TRX850, its ignition unit does not need any gear info.

The gearbox sensor only senses the neutral position on the TDM900, TDM850-3VD & XTZ750.
With the above mentioned bikes, you'll need to :
1 - either apply some surgery to the gearbox sensor to add the required connections - not very difficult.
2 - or find a genuine 4TX sensor, new (expensive) or used (cheaper) as this part is very rarely broken or worn-out.

1 - Modifying the gearbox sensor from a TDM900 (or TDM850-3VD - TRX - XTZ)

The sensor has to be removed from the engine, then adapted to receive the six missing electrical connections.

First get the sensor :
• Remove the engine left-hand side plastic panel.
• Remove the speed sensor casing.
• Remove the castle nut of the speed sensor.
• Remove the speed sensor panel to access the gearbox sprocket.
The gearbox sensor is located above the sprocket, the casing is white.

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• Remove the small neutral. It is easier if the upper chain-guide is removed. Remove the wiring connector screw, then the two bigger screws, pull & remove the sensor. Carefully remove the O-ring without damaging it.

Looking down the internal side of the sensor, a circular track may be seen. This track is made by shift-cam rod.

The track has deeper marks where the shift-cam is locked. These are the places where connections has to be made. First drill a small hole with a thin hot pin. Drill holes with a thin drill first, then use a bigger one. Be precise !

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Use screws about 4mm diameter. If the heads of the screws won't fit side by side, they may be removed later.

I made some kind of mistake on my first try : screws were too thin . Don't do the same mistake otherwise no electrical connection could be made in case the screws are not very precisely inserted.

Very important : Before inserting the screws, carefully flatten the tip with a file or sandpaper. This is the part of the screw which makes the electrical connection with the steel ball. When you insert the screws, be careful not make them level on the internal side : the shift-cam rod path must stay flat.

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There it is, the modified gearbox sensor is ready to use.

2 - Using a gearbox sensor from a 4TX

This gearbox sensor is ready to use as it has the 6 terminals ready.

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Moding the gear sensor on a TDM850 MkII (4TX)

The gearbox sensor is used to sense the 2 and 3 gear positions. The two corresponding wires are connected inside the connector and therefore should be modified to avoid displaying a wrong gear position. An interesting mod is to cancel the white wire going to the TCI box : it results in a better acceleration in 2 & 3 gears.

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Original circuit 4TX
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Modified circuit 4TX

Two diodes (1N4001 or equivalent) must be added as indicated on the diagram. One may also wonder if this wiring to the ignition unit is actually required : the TRX850 does not have it.

The gear sensor on a TDM900 / TDM850 MkI (3VD) / TRX850 / XTZ750


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Original circuit 900
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Modified circuit 900
Wiring of the gear connector #6 is not required on the TDM850 MkI (3VD) / TRX850 / XTZ750 !

Mounting the sensor

Solder the wires on the screw heads, or use terminals and bolts. Be careful when soldering not to apply heat for a long time as the sensor is plastic-made and may be deteriorated.

I found an old PC mouse-cord which was perfect to use : 6 wires inside and a wonderful small & flat connector on one side. Any other thin wiring will do the job (alarm wiring, UTP network wiring, etc). Don't use telephone wiring, these are mono-filament wires and may not sustain vibrations.

Protect the connections from water projections, dust and vibration with a good layer of hot-melting glue. A plastic cap may also be added (mine comes from an old photo film casing, right diameter). Fit the cap, drill a hole on top and inject glue. I had the neutral connection relocated on the top with a screw, this is just a suggestion.

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Apply some grease inside the gearbox sensor and on the O-ring.
Replace the gearbox sensor on the engine, added wiring goes out to the rear side & above.

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There it is, the modified gearbox sensor is ready.

Encoding & Display

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Wiring diagram TDM900
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Wiring diagram TDM850 - TRX850 - XTZ750

I recommend SMD LEDs because they require less room, although they are difficult to solder. LED1 to LED5 are yellow, LED6 is orange. Choice of colors is yours, you can play with any other color. Using different colors renders the display much more readable.

Use LEDs bright enough to be visible with full sunlight but not too bright for night driving. There is no brightness compensating circuit, as for the dashboard indicators.

Encoding unit

Encoding unit and display unit are separated. I did it like that, this is not a must-do.

I had an old piece of printed circuit board to use, it had a useful connector matching the connector of the wiring. I soldered the resistors R1-6 and diodes D1-5 on this board.

Don't forget to wire the circuit to the +12V found inside the dashboard connector.

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Circuit is finally covered with hot-melt glue to protect it from corrosion & vibrations, then glued under the dash.

Display unit

Display unit is made with a small U-shaped aluminium part : 50mm x 12mm x 10mm. This is just an example.

Solder the LEDs to the wiring from the encoding circuit, then glue them with cyanoacrylate glue on a thin & rigid plastic mount.

Insert the mount inside the display unit. Cover it with a small sheet of translucent plastic (from a coolant container...).

Display unit is then glued on the dash.

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Digital display (alternate design)

You may either use a BCD encoding circuit or a full diode array circuit :

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BCD encoding circuit Diagram
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Full diode array circuit diagram

The full diode array is simple but requires no less than 27 diodes - neutral display not included - while the BCD encoding requires only 9 diodes - neutral display included (6 diodes without neutral display).

The latter circuit uses a CMOS CD4511 to perform the decoding for the seven-segment display. The CD4511 includes an auto-test function which may be used at power-on to check the display (refer to the datasheet). Supply voltage for the CD4511 must be 5V to 15V, a voltage regulator should be used. You'll have to connect the neutral wire to the circuit if you want the display to show a "0" when in neutral.

The seven segment display is a common anode model for the full diode array design and a common cathode model for the BCD encoding design.

I give no example for the practical design - everybody will find his/her own way to do it - except those two pics : first one is from TDM_eats_bandit / Carpe-TDM (UK), it uses a full diode array. The second pic is from TDM-friend forum (Germany) and is based on a microcontroler circuit.

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First design
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Another design
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