electronic speedo

[tigercub84]

Hi I have a tiger with a electronic speedo, which has stopped working I found out that it was the magnets on the prop shaft have moved. There are two magnet. Do they have to be placed 180 degrees to each over and how do I calibrate it. Thanks

['The Gaffer']

Best thing would be to try and download the manufacturers user manual.

Use metal glue to stick the magnet/s. Calibration is usually done by measuring wheel circumference and applying that to settings in the unit.

Dependent on gearbox, you could look at trying to find a mechanical cable to digital converter, all depends on your speedo's spec though.

[Iancider]

Older ones used magnets but newer installations just sense ferrous metal and work by sensing either the propshaft bolt heads or the hub bolts on the wheels.  Assuming you stay with the magnetic sensor you have got then two magnets are not really enough because at low speed the speedo would pulse badly.  Three or four magnets would be better.

To gain miles per hour, the speedo has to count the pulses per mile travelled.  If you have 4 magnets evenly spaced, then you will get 4 pulses per rotation of the prop.  You will need to know you diff ratio to translate that into wheel rotations.  The distance travelled for each rotation is the wheel rolling radius squared times 3.142 to get the rolling circumference.  So one rotation might be 1.614 yards and that would be 1760/1.614 rotations per mile or 1090 rotation per mile.  If the diff reduction is 3.8 - 1 then that is 3.8 x 1090 propshaft rotations per mile and if you have 4 magnets that equals 16,568 pulses per mile. (set 2% below this to make sure it over-reads slightly = 16,236)

The rolling radius is from the centre of the wheel to the contact point in the road which is less than the centre to the top of the tyre due to the tyre's distortion.  It is easiest to do that in metric and then convert to yards per rotation.  Metres to yards is times 1.0936. 

The speedo is calculated by setting the number of pulses in a mile travelled.  Typically there is a rubber button on the front of the speedo which you press and hold as you turn the ignition on it will then display a number which is the number of pulses per mile.  If you press again and hold it will scroll up until you reach the number you want.  You should set your pulse per mile slightly lower to make sure the speedo over-read slightly - typically 1.5-3% - this keeps you legal if your tyres are soft or low on tread.

Having said all of that it should be working in some way already and if not at all check that it has a power supply and a ground connection - it wont work without power.

If you want to switch sensor which is wise then go for the passive types - enquire at the manufacturer.  They sense the hub bolts - typically four of them per wheel rotation.  This has the advantage that no magnets have to be glued-on and therefore do not fly-off and it is best to sense a front wheel to avoid wheel-spin causing the speedo to over-read.

Final check - find one of those speed indicating road signs (not the yellow one with the flashy camera!) and compare the speed indicated with the speedo reading.  Those signs are very accurate and can be used as a calibration.  If you are reading 10% higher then you could increase the pulse per mile by 8% say.  Try it again to prove the point - job done!

The passive sensors are far better than the magnet type and need to be within 2mm of the passing bolt-heads

Good luck
Ian

[paintman]

There you go.....simple!!  Lol....

[Gary RH7]

After that I think I will stick with my mechanical one even though it bobs around a bit.

[benchmark51]

My speedo cable is currently broken so I'm guessing it based on the tacho'. Still the speedo is accurate at the traffic lights!

[tigercub84]

Thanks for that I will try and find my diff ratio out soon

[dikkie]

Another approach to calibrating, if you're lazy like me...

take a guess at the calibration,
put your sat nav on to get an accurate speed and go for a drive,
adjust the calibration up or down accordingly
Repeat

The calibration unit is usually distance travelled per rotation, (or in your case you have 4 magnets so per 1/4 rotation) which should help to give you an initial estimate. It will also be linear, ie if speedo is reading half the speed of the gps; double the calibration unit...

[peterw]

Stuck my 2 magnets on the off side drive shaft, that way it doesn't matter if i change the Diff ratio. Rolling circumference was 6ft. 2 yds per wheel rev with 2 magnets = 1760 pulses per mile, which was spot on according to the IVA test...