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[terry paget]

X84GNB 2.5 petrol manual estate
I have changed the crankshaft position sensor on above car, and cured its failure to start. Thanks to all for their good advice.

I have read on this forum ( I think) that what fails is the cable, not the sensor. I thought I could replace the cable. So I stripped off the insulation from either end of the cable and tested for continuity. I discovered: from socket to wires near socket - 3 wires continuous; socket to sensor end wires- continuity from socket all 3 wires. So cable is OK. Cable to sensor is more puzzling. There are three wires, naked copper, white insulated and black insulated. Between naked copper and the other two there is open circuit. Between black and white insulated there is 600 ohms. So sensor seems OK too. But car would not start using this sensor. A new crank sensor cured the fault. I am perplexed.

[05omegav6]

Duff sensor

[AndyStobbs]

Third wire is shielding against interference. Eventually it would be grounded in the ECU or near to it. Does your sensor itself have a continuity or is it open circuit? Open circuit would suggest failed sensor.

[terry paget]

The failed sensor has open circuit. I found a spare sensor in a bag labelled 1238228 an checked its continuity. Pins 1-2 open circuit pins 2-3  4000 ohms pins 1-3 100,000 ohms. So reckon Al is right, my sensor has gone, not the cable.

I was expecting the opposite, as when I changed the exhaust manifold I cut the cable in two to faciltate oil pipe removal. I later soldered it back together again and re-used the sensor. It has lasted 4 months and 3000 miles, and failed.

I have changed 6 crank sensors now. I believe some members carry spares. Trouble is they seem to vary from car to car.

[AndyStobbs]

Are you saying you've changed 6 crank sensors on this car? If so, what brand are you fitting? I certainly wouldn't class them as consumables. Cutting and soldering is usually ok, just try to preserve the sheilding or if wires are ran as a twisted pair try to preserve the twist.

[terry paget]

No, it's the first on this car. I run six Omegas for my family, and since I switched from Senators to Omegas I have changed six crank sensors, not to mention quite a few cam sensors on the four pot Omegas. I have 3 V6 Omegas and 3 4 pot Omegas. It is curious that V6s eat crank sensors and 4 pots eat cam sensors.

 

[Andy B]

.... It is curious that V6s eat crank sensors and 4 pots eat cam sensors.

I've had 2 changed on mine in 11 yrs ..... 

[henryd]

.... It is curious that V6s eat crank sensors and 4 pots eat cam sensors.

I've had 2 changed on mine in 11 yrs ..... 

I had to do mine and I only owned it 18 months or so,you had much better VFM than me

[philhoward]

My engine came with a dead one...Cheapy ebay one fitted to test but genuine one in the glovebox..

Really should swap them over!  Pretty sure it was a Bosch one I removed (oval plug) yet I have a Bosch and Siemens spare (both with oval plugs).  I recall it being critical on a 2.5 but never heard of a problem with the 3.0 - can I use either?

Sorry for the post hijack..

[terry paget]

Can anyone explain to me how these sensors work? I imagine that 12v is applied across a coil inside the sensor, causing a steady current to flow.  The head is near a rotating wheel with alternate highs and lows, like an ABS sensor, which causes fluctuations in the current flowing, by induction. The ECU uses this fluctuation to determine rate of rotation, and there is a missing high to tell it top dead centre. So why the third wire, with its 100Kohm connection to one end of the coil?

I wonder how they fail. I imagine a coil of lacquered wire potted in epoxy in a plastic case. Do the connections fail? Do the wires break?

[AndyStobbs]

On the two wire type it is essentially a generator in itself. They have a permanant magnet and a wire wound coil within the sensor. The action of the reluctor wheel passing by the end, causes oscillating magnetic fields inside the coil - the reluctor itself is not magnetic, but in simple terms it passes through the magnetic field of the sensor's internal magent and as the coil is also in the same magnetic field an EMF is set up in the coil. The sensor output is AC. Can be viewed with referance to ground on either wire using an oscilloscope but half of the amplitude is lost. Must be viewed just across the two wires for full amplitude.

The breakdown occurs when the coils start to short out and so the EMF is lowered due to some turns being bypassed. A complete breakage in the coil is ofcourse shown as open circuit when checked for continuity.
Typical output voltage anywhere from 0.5 - 5 VAC. Amplitude increases with speed upto a point, as the flux is being cut more quickly. Output is a sine wave.

A true 3 wire device is a hall sensor, that is fed with 5v, and the reluctor wheel either allows the voltage to flow through the sensor or not. So giving a square wave in DC. The three wires would be 1. +5v supply, 2.Ground 3.Sensor output, it is the output wire that exhibits the square wave.

If you want to learn more and see some example waveforms, go to picoauto.com and download the picoscope automotive software - its free and included demo waveforms - you don't need a scope to view the demo waveforms. Loads of back ground info on how things work on and around the car.

[terry paget]

Thanks Andy. That explains a lot. The website you referred me to kept locking me out saying 'Risky website', but I get your gist. It does look as though what fails is the sensor itself, through internal insulation failure or wire breakage. The cables do not fail, which is what I thought before. So no hope of repairing my old sensor by replacing or repairing the cable, something I have done in the past on front wheel ABS cables.

[2woody]

you can get a fairly good idea by using a multimeter across the two output wires of the sensor, but only if its set to Volts AC.

The faster it goes, the more Volts.

I use this trick a lot for setting the reluctor gap on ABS sensors

[Kevin Wood]

On the sensors that have failed on my car (2 of them) in both cases, the sensor itself has gone open circuit (I cut the wire close to the sensor and measured the resistance).