Citroen XM Forum

Inductive (but compromised) sensors often work adequately at low and medium frequencies, where the connected electronics can make sense of a waveform which lacks maximum output (voltage output drops off significantly as the distance between the sensor tip and the passing peg or tooth increases). Sensor output voltage can also drop off if the sensor has been damaged, chipped or partially demagnetised.

At higher frequencies (ie as engine speed increases) the output of an inadequate sensor becomes increasingly difficult for the electronics to 'read'. In a worst case, instead of the upstream electronics seeing a series of well-defined pulses, it just sees a 'ripple', which can even 'flatten out' enough to become d.c.

On ABS, such a sensor would throw up a fault condition. Driving a speedo or tacho, the needle would rise, but then begin to flutter/waver, and eventually slump to zero. On electronically-controlled ignition, triggering of sparks at the correct moment would become erratic or impossible, I guess.

That advice is helpful Chris and does encourage me to think that the flywheel sensor may be the issue. I suspect at the very least the length of the tip may be reduced and I assume the thickness of gap against the surround is critical. It would certainly seem likely that at higher speeds readings become less accurate with a consequential effect on performance. With regards, Matthew T.

Personally, I would only expect the crank sensor too affect the spark activation, not mixture strength.

Yes Will but if the spark is in the wrong place would that not cause some sooting of the plugs with a similar effect? I would not say that the mixture appears excessively rich and it is more the poor running and slight hesitation on revving up. Regards, Matthew T.

The symptoms do not seem to indicate an ignition or timing issue. Given the history of the car, I wonder if one of the wires from a sensor to the ECU has been damaged?

Thanks Mike but I do think it might simply be a damaged flywheel sensor tip. There is normally quite a small gap between the sensor and starter ring and when the latter became loose it was flapping every which way! The starter motor also seems to have been a victim of the failure and it is remarkable no more damage was done, although I received some warning of the problem and quite quickly stopped using the car! The access to the sensor is a little awkward but I shall have a look tomorrow. Regards, Matthew T.

I think if there was a broken sensor wire I would also see the engine light. Regards, Matthew T.

Also forgot - the angle at which the sensor is mounted can affect its performance and output. Rotating them can affect their sensitivity/output. Remember what John has written about ABS sensors.
The smaller the distance between sensor tip and peg/teeth (which is it on your engine, Matt?) the better the sensor will work, in general.
Also, like ABS sensors, beware filings or metal swarf on the magnetic sensor tip - plays havoc.

There is no adjustment to the sensor position Chris and the starter ring has a series of square holes as the attached illustrations show. I note your point about the sensor tip that I did check before refitting but I think the reduction in length reducing the reading effect may be the issue here! With regards, Matthew T.

No adjustment does simplify things. I think condition of sensor tip is important, so if it's been 'reduced in length' (polite term, Matt) that can't be good.
Looking at those square holes, they don't run all the way round, do they? Are there 'plain' sections (about 3 missing holes) at N,S,E&W?
That would give an distinctive waveform.

As you say, obvious to replace the suspect sensor. Hope it goes well.