what input pin is it using?

Joes:
look at the 2 rear wheel speeds and pick the fastest travelling one and compare it to the speed of the propshaft and if it is going slower execute it into another piece of code to lock the diff up.

That seems unnecessarily complex. Unless your diff has broken you know that the propshaft speed is proportional to the average of the two wheel speeds so all you need to do is look at the ratio of the two wheel speeds.

If the car is not going in a straight line the outer wheel will turn faster than the inner wheel.

I have been assuming that you have done at least a few of the example projects that came with your IDE. They do require some hands-on to teach basics.

Suppose you hook up the sensor and wheel you have and see what you can get from that?
The sensor is digital right? It's either on or off, LOW or HIGH, 0 or 1?

So write code to read the sensor and if it is HIGH you turn a led on (UNO has built-in led on pin 13, I dunno about the MEGA) and if it is LOW you turn the led off. Then turn the wheel and see how the led flickers. If it does not then it's time to check the wiring and the code.

All that does is to ensure that you can read the sensor and get meaningful data.

That seems unnecessarily complex. Unless your diff has broken you know that the propshaft speed is proportional to the average of the two wheel speeds so all you need to do is look at the ratio of the two wheel speeds.

i disagree the car is normally a front wheel drive car, when slip is detected it will lock up the rear diff (as it doesnt have a centre diff) to transfer the power to the back wheels. in its original state the car will decide how much lock to put on by looking at the outside temperature, steering angles, throttle position, G force meter and overall speed. so this will be a simple system compared to the original

If the car is not going in a straight line the outer wheel will turn faster than the inner wheel.

yes it will this is why i want the arduino to pick the fastest moving wheel

I have been assuming that you have done at least a few of the example projects that came with your IDE. They do require some hands-on to teach basics.

yes i have so my knowlege is very limited

Suppose you hook up the sensor and wheel you have and see what you can get from that?
The sensor is digital right? It's either on or off, LOW or HIGH, 0 or 1?

So write code to read the sensor and if it is HIGH you turn a led on (UNO has built-in led on pin 13, I dunno about the MEGA) and if it is LOW you turn the led off. Then turn the wheel and see how the led flickers. If it does not then it's time to check the wiring and the code.

yes i would of thought so i will have to try that
but i am on the understanding that it is a hall sensor
i will do that and let you no what i get

Be aware that there is more than 1 kind of hall sensor. Some are digital switches (yours probably is) and some are analog sensors (that can be read digital too). All react to magnetic fields.

This is time to play and try different things to see what works and what comes out. You will gain knowledge that you can be sure of just by experimenting.

Be sure to put a resistor in the path between 5V to sensor to digital pin unless the sensor already has one. A 1k resistor should keep your Arduino pin safe, even 2.2k is not too much for digital.

Be sure to put a resistor in the path between 5V to sensor to digital pin unless the sensor already has one. A 1k resistor should keep your Arduino pin safe, even 2.2k is not too much for digital.

so i still need to put a pull down resistor to the digital pin?

Yes still have a resistor pulling in the opposite direction to the sensor.

There are in fact two types of digital hall sensor, one which switches when it sees any magnetic field and the other that outputs one state when it sees a south pole and changes state not when the magnet is removed but when it sees a north pole.

Joes:
i disagree the car is normally a front wheel drive car, when slip is detected it will lock up the rear diff (as it doesnt have a centre diff) to transfer the power to the back wheels. in its original state the car will decide how much lock to put on by looking at the outside temperature, steering angles, throttle position, G force meter and overall speed. so this will be a simple system compared to the original

For that to make sense, the drive to the rear wheels must be very different to a conventional prop shaft and diff. Do you have a clutch in the prop shaft? (I remember there was a very nice hydraulically controlled one from Land Rover a few years ago.)

Maybe it's a posi?

For that to make sense, the drive to the rear wheels must be very different to a conventional prop shaft and diff. Do you have a clutch in the prop shaft? (I remember there was a very nice hydraulically controlled one from Land Rover a few years ago.)

yes it is a hydraulically controlled

i tride that sensor out to day and could not get it to work the way we discussed so i tock the 10K pull down resister out and put a pot in then tork the pot down to 1.3k and it did work but very tempermental
the sensor seems to work between 1.0-1.3k
what do you think?

Joes:
i tride that sensor out to day and could not get it to work the way we discussed so i tock the 10K pull down resister out and put a pot in then tork the pot down to 1.3k and it did work but very tempermental
the sensor seems to work between 1.0-1.3k
what do you think?

I think you need to find out for certain what sort of sensor you have and what sort of signal it outputs. Is it definitely a Hall effect sensor? I would have thought that an MVR sensor was more likely.

If it's a Hall switch then you use it like a manual switch/button.

I think you need to find out for certain what sort of sensor you have and what sort of signal it outputs. Is it definitely a Hall effect sensor? I would have thought that an MVR sensor was more likely.

how can i test to see if it is a mvr sensor?
mite be easier to just get a now one so we no what we have then somthing like this?
http://www.amazon.com/Inductive-Approach-Proximity-Sensor-Detection/dp/B004U4GCCW/ref=pd_sxp_grid_pt_1_0

but would be better in 5v

Joes:
how can i test to see if it is a mvr sensor?

If it's an MVR sensor then the sensor will be slightly magnetic and it will produce a weak alternating electrical signal when ferrous material is moved towards and away from it. The magnitude of the signal is determined by the speed of the movement. This type of sensor is commonly used to detect the position of an iron/steel toothed wheel.

Hall effect sensors are not magnetic but require a magnet in the part being sensed.

this sensor is very magnetic on the end so might be, is that easy to be used with the arduino or am i better off getting a hall sensor ?

i thought it must be a hall sensor as a lot of the cars had a hall sensor looking at a steel trigger wheel for the engine rpm, so i thought these might be the same but obviously not

Joes:
a lot of the cars had a hall sensor looking at a steel trigger wheel for the engine rpm, so i thought these might be the same

They probably are the same, but those other cars you're thinking of aren't using Hall effect sensors either.

You need to measure the voltage level over the range of speeds you need to support when the sensor is installed. You will almost certainly need to do some signal conditioning to take your variable voltage incoming signal and clean it up to a square wave.

so i will be easier to put some hall sensor on it like in link then?

It could be as simple as a magnet in a coil. When a gear tooth passes close across the axis of the coil it will induce current in the gear tooth (eddy current) and the coil, the latter which can be picked up as a pulse with strength in proportion to the speed of the gear tooth. Such a sensor would not be affected by dirt or wet the way an light and sensor would. Unlike a Hall sensor, it would not require any magnets placed on the turning wheel.

If it is, don't try feeding it directly to Arduino until you know what directions make +V and -V.

Why buy when you already have something that must work or the car won't?
Can you get a part # on the sensor? Maybe it would be in a shop manual for the vehicle.

ahh yes there is a pos and neg symbol just above the pins on the sensor that would explain why they are there so how do i intergrate that with the arduino?

trying to find out information about it is near enough impossible ans manufacturers keep that information to themselves

Joes:
so i will be easier to put some hall sensor on it like in link then?

I would have said that an MVR sensor is the best one for the job - that's why they are so commonly used for this. But you need to cope with the fact that the amplitude of the output signal varies with speed, so you need to apply some conditioning before you can pass it to your input pin.

i just looked on my altodata and that says 60rpm 65 mV ac min
does that mean much to you or any help?