Steering Wheel Sensor

Unread post by **DHallworth** » 28 Jan 2010, 12:54

I ran a quick scan on my Activa last night with the Lexia just to see if anything was detected.

Found 3 faults, one was the central locking, and one was the airbags but both of them were apparently created by a low voltage from the battery which figures as the battery in it is a knackered one that will just start it to move it around the shed.

The third fault was an intermittent signal from the steering wheel sensor.

Does anyone know where this is located or how you can replace it?

Thanks

David.

Unread post by **CitroJim** » 28 Jan 2010, 13:55

I'd not worry about it David.

All Activas (and in fact all Hydractive cars) flag this intermittent error on a regular basis.

It's almost a dead cert that any Hydractive ECU you care to scan will give this error.

One of those quirks that make it a Citroen!!!

Unread post by **DHallworth** » 28 Jan 2010, 14:36

Ah well......

Was just thinking of doing it so that I knew it was 100% Jim.

Might not bother now if I know it's not going to do any harm.

David.

Unread post by **CitroJim** » 29 Jan 2010, 07:53

David,

Just for the record, the sensor is on the steering column and can be seen if you take the wheel and cowlings off. It connects via a big grey connector. It's a photo-interrupter (i.e. a slotted disc with a photocell one side and an LED the other).

It picks up drive via two big red O rings, friction basically.

I sometimes think it might get a bit confused if the steering is moved without the car actually moving. One thing the Hydractive ECU does is regularly calculate the "straight-ahead" position using information from this sensor, the forward road speed, time and the yaw sensor. If anything upsets this process then I believe this is the cause of the error but have absolutey no evidence to prove it...

xmexclusive · Unread post by **xmexclusive** » 29 Jan 2010, 10:14

It is rather old sensor technology.
A good few years ago we had these on the end of the axle of recording vehicles to measure linear distance moved very accurately. Part of a pre GPS know where you are system. There were odd read errors once on the move but these were insignificant compared to the havoc caused in the controlling software by fractional roll back of the measuring wheel on brake release. This generated one or more spurious pulses of wrong direction movement. At each change of sensor direction you always had an accuracy uncertainty of at least 2 pulses. Because of this I suspect that the steering sensor errors are fundamental to the system design. The sensor watches for light to dark and dark to light transitions. It cannot tell whether the first two edges it sees are both sides of a single slot (continuous one direction movement) or the same slot edge seen twice because direction reverses between the readings.

John

Unread post by **DHallworth** » 29 Jan 2010, 11:35

Ah well,

I'll just clear the fault and leave it to it's own devices then.

Thanks for the input guys.

David.

Sl4yer · Unread post by **Sl4yer** » 29 Jan 2010, 16:34

CitroJim wrote: ...and the yaw sensor.

Yaw sensor Jim? I thought it used the steering wheel angle and vehicle speed to calculate potential yaw.

James

Unread post by **CitroJim** » 29 Jan 2010, 19:33

John,

That's a beautifully elegant explanation

Old technology indeed. 20 years ago, back in my HF radio days I used to work on Racal receivers that used a slotted disc behind the tuning knob. It had two photocells so that it could detect if the knob was turning clockwise or anticlockwise. Technology being old, the photocells were lit by small filament bulbs. If one went out it would cause the receiver to tune one way but not the other. Used to confuse the operators a treat

Going back even further, another piece of kit I worked on, dating from the 60s, needed a stream of pulses and these were generated by a motor driven slotted wheel. a photocell and a bulb. The wheel had lots of various holes and slots so a whole family of different pulses could be generated.

Sl4yer wrote:
CitroJim wrote: ...and the yaw sensor.
Yaw sensor Jim? I thought it used the steering wheel angle and vehicle speed to calculate potential yaw.

James

Indeed James. There is a yaw sensor right on the end of the front anti-roll bar. It effectively measures the twist in the bar...

It's almost impossible to see, being well hidden up in the front subframe. You might see the linkage to it if you look closely at the RH side of the ARB. Never known them give a problem.

addo · Unread post by **addo** » 29 Jan 2010, 19:36

I would have described that as a pitch sensor!

Alfa yaw sensors for the (Bosch) VDC computer use a motorised gyroscope.

Unread post by **CitroJim** » 29 Jan 2010, 19:44

addo wrote:I would have described that as a pitch sensor!

Alfa yaw sensors for the (Bosch) VDC computer use a motorised gyroscope.

Yes, that might be more accurate Adam. I once called it a yaw sensor and it's stuck in my brain. Nowhere else is it called that

The "book" calls it a Body Height Sensor which is also inaccurate as it could only measure the front height.

The fact it measures on the end suggests it helps measure roll, pitch and yaw maybe...

Sl4yer · Unread post by **Sl4yer** » 29 Jan 2010, 23:32

CitroJim wrote:The "book" calls it a Body Height Sensor which is also inaccurate as it could only measure the front height.

The fact it measures on the end suggests it helps measure roll, pitch and yaw maybe...

Ah, that one! Yes, seems to register variation in pitch mainly (maybe -8 to zero when dropping from high to low, and the front goes first). Can't see it having any real impact on the Activa system. The Citroen Guide doesn't mention what it does on the regular Hydractive either though. Can only imagine it switches to Firm if it exceeds a tolerance.

James

addo · Unread post by **addo** » 30 Jan 2010, 08:42

It's on Hydractive as well, so I reckon it forms part of the hardness "deployment" criteria - rate of change being the test.