Here's a technical one for you.

The Focus uses a hall-effect sensor mounted in the gearbox. This is used to measure crownwheel speed, which is converted to road speed.

My question is whether any kind of magnet is mounted on the crownwheel to trigger the switch? I cant imagine it's just counting teeth, is it?

 

The teeth will create the pulse in the same way a crank sensor or abs sensor would work.

 

What exactly are you trying to achieve bud?

 

The CW will have a toothed ring on it which the sensor reads off.

Same as any similar rotational speed sensor.

 

I have a Quaife dog box in the car. It's an ST170 with the 6-speed Getrag.

The Quaife crownwheel is about 1cm larger in diameter than the standard. This means the VSS sensor wont fit in the hole as it is fouled by the crownwheel. Quaife was blissfully unaware of the problem before now.

My plan is to machine the sensor mounting eccentrically (ie move the centre of the mounting boss) so I can get it back in there. The sensor is already offcentre and I can take advantage of that without damaging the sensor - I dearly hope.

But I had a sudden heart-in-mouth moment that the Hall-effect trigger might be an embedded magnet rather than just the admittedly ferro-magnetic teeth.

Meanwhile I was thinking of putting 12v across a sensor I have on my bench, attaching an LED and waving various bits of metal at it to see which might be right.

On the other hand, if you know anyone who has a similar dogbox, I'd love to hear from them...

Cheers.

 

Again, what are you trying to achieve ?

just a speedo reading ?

 

I'm trying to get the warning light to go off, and the speedo to read. No need for accuracy - but both of these are now MOT fail points. And mine is due....

 

make a new trigger elsewhere.

It could be the brake disc bell/bolts, CV bolts if they are bolt in units, attach a new toothed wheel to a driveshaft, etc etc

There are many options.

If you need it to be accurate, once up and running you can buy DIY kits to build a circuit to +/-100% the numder of pulses so you can calibrate it to the speedo

Or you could weld a boss or install a new sensor directed at the toothed wheel, assuming this new crownwheel still has this provision ?

 

double post

 

Most Hall sensors rely on a metal toothed wheel (or similar) and static magnet to generate a variation that is measured.
See: http://en.wikipedia.org/wiki/Hall_effectSo moving the sensor as you suggest is worth a try.