Diff speed sensor info

[Pond]

Really struggling to find the answer to my question on t'internet, so here goes.....

I need to find out the number of 'trigger points' for the speed sensor inside the diff on my car.
I think it is a Torsen diff (3 litre Z3)
I think the diff ratio (final drive ratio?) is 3.07

But I need to know the number of pick up points that the sensor translates into pulses. I am leaning toward just one, but can't find the answer.

Anybody?

 

[Rudyrov]

Google Image Result for https://i.ebayimg.com/images/g/vOUAAOSwN81WD2Np/s-l400.jpg

images.app.goo.gl

looks like 9 pins

 

[Pond]

Google Image Result for https://i.ebayimg.com/images/g/vOUAAOSwN81WD2Np/s-l400.jpg

images.app.goo.gl

looks like 9 pins

Thanks but that confuses things even more!
I don't have an LSD and don't think (but could be wrong) the M54 diff is 'open'.

 

[Delk]

I think they are all the same number of blades or whatever you want to call them.

There are several diameters to fit different diffs and covers but the numbers are the same per rotation

 

[Pond]

I think they are all the same number of blades or whatever you want to call them.

There are several diameters to fit different diffs and covers but the numbers are the same per rotation

Thanks.
It looks like there are nine 'trigger points' on all the different sizes, as you say.

I need to program a new speedometer and the calculation I have been given is:
number of rotations of the rear wheel per mile X number of trigger points for the speed sensor x diff ratio

So if the number of trigger points is nine I get:

797 rotations X 9 X 3.07 = 22,021 pulses per mile. Sounds a lot!

 

[Duncodin]

I probably wrong but I thought rhe z3 speed sensor was on the diff output? If it is then you should be able to calculate pulses per mile based on the circumference of the tyre. ie how many rotations of the tyre will equal a mile.

I'd have thought the diff ratio would only be relevant if the speed sensor was on the gearbox output.

 

[Pond]

I probably wrong but I thought rhe z3 speed sensor was on the diff output? If it is then you should be able to calculate pulses per mile based on the circumference of the tyre. ie how many rotations of the tyre will equal a mile.

I'd have thought the diff ratio would only be relevant if the speed sensor was on the gearbox output.

The sensor is plugged into the rear of the diff, so you may be right.
I suppose it all depends what the ring that triggers the sensor is connected to?
I need to find out for sure, as the number of pulses is vastly different if I remove the 3.07 ratio.

I am purely guessing but as the sensor points toward the prop, I would think the ring is rotating at 90 degrees to the driveshafts, so could be picking up prop speed rather than driveshafts? If so, then I could need my diff ratio figure. I dunno.

It's either 7,173 or 22,000 odd.

Having said the above, if I get it wrong it will be bloody obvious, so I can then just re-program the numbers.

 

[Delk]

The ring is attached to the right side of the diff and rotates at wheel speed. Gear ratio has nothing to do with it and doesn't change the speed.

 

[Duncodin]

So go to willtheyfit.com and key in your tyre size. That'll show you the rolling circumference. Divide a mile by your rolling circumference to get the number of rotations per mile - if that's what your speed calibration needs - multiplied by the number of clicks per rotation that is.

 

[Pingu]

If it's the ETB speedo, you can programme it over a measured mile. This is what I did, and it's very accurate - so accurate that I did it again to make it read slightly faster.

It's easy to measure a mile on an OS map - 32mm on a 1:50000 map.

 

[Pond]

If it's the ETB speedo, you can programme it over a measured mile. This is what I did, and it's very accurate - so accurate that I did it again to make it read slightly faster.

It's easy to measure a mile on an OS map - 32mm on a 1:50000 map.

Yes I saw that was one way to do it..."auto calculate" but I don't want to do it that way (too many reasons to get it wrong and too complicated).
If the diff ratio can be taken out of the equation, then it is simply 63,360 inches divided by wheel circumference times 9.
I heard that the speedo has a 'built in' over compensation of around 2%, so if I reduce the pulse number by 2% the speedo (and odometer) should read almost spot on.

 

[Pond]

So go to willtheyfit.com and key in your tyre size. That'll show you the rolling circumference. Divide a mile by your rolling circumference to get the number of rotations per mile - if that's what your speed calibration needs - multiplied by the number of clicks per rotation that is.

Done all that previously. I know the circumference of the new tyres (79.4488" ), I know there are 63,360 inches in a mile and I (now) know there are 9 triggers per rotation.
It was only whether the diff ratio should be used, but it would seem not.

If I have it wrong, the indicated speed will be around 300% out, so will be easy know! I can then just re-program the speedo with the right number.

 

[Pingu]

Top tip. The best way to measure the circumference is to get the valve to the very bottom (or draw a chalk mark on the tyre), roll the car one wheel revoultion and measure the distance. Better average to roll for three revolutions and divide by three.

This method removes any error caused by tyre pressures and car weight.