As promised here is a slightly more detailed explanation of what is going on.xantos wrote:Just measured the electrovalves... When they are energized the car jumps up and then they are being held at 3V for a couple of seconds and back to 0V again. And then again after a minute.
The hardest thing to do is keeping my nerves calm...
What you need to understand is that the electrovalves require the full 12 volts to switch on, however after switching on they are held on at a lower voltage - an average voltage of about 3 volts, which is really a 12 volt peak pulse waveform at 1Khz with an on time duty cycle of about 25%.
The dilemma of Citroen's design is that permanently feeding 12 volts would dissipate 30 watts in the electrovalve coil which is too much for such a small coil to withstand permanently, and yet a lower voltage is not sufficient to switch the state of the electrovalve.
Their solution which is a bit of a kluge IMHO is to apply the full 12 volts for half a second to "slam" the electrovalve solidly from the off state to the on state, then apply an average of 3 volts to keep it held in the on state. The power dissipation from 3 volts is only about 1.9 watts, thus doesn't overheat the coil as 30 watts would.
The ECU constantly monitors the average current draw of each electrovalve and if it is too low it detects a fault and will shut off BOTH electrovalves for about one minute, then go through the same process again, where it applies 12 volts for half a second then back to the 3 volt average pulse width modulated signal.
When the back EMF diode is open circuit the ECU sees large voltage spikes during the pulse modulated signal period, and because this monitored voltage/current is out of spec it shuts down the outputs to protect the system.
Therefore when you have a faulty diode, when the suspension goes from hard to soft it will go soft for half a second during the full 12 volt phase and then almost immediately the power will be shut off putting the suspension back in hard for about a minute, it will then switch to soft for half a second and back to hard for a minute. That's the typical symptoms of a faulty diode.
What can also happen when the diode is faulty is that even if the ECU keeps supplying power the electrovalve will mechanically drop out back into the hard mode. This is because the current flowing through the back EMF diode during each turn off period in the pulse waveform actually helps to keep the solenoid energised, when the diode is open the average current flow through the solenoid coil is much lower and not strong enough to overcome the spring so it drops out.
If the diode is not completely open circuit but has an internal high resistance connection (I've had this happen to me) then it will sometimes pulsate rapidly between 12 volts -> 3 volts, zero volts -> 12 volts -> 3 volts -> zero volts in a cycle with each cycle lasting a couple of seconds.
One thing that you could try as a test only is to also shunt each diode with a 22 ohm 5 watt resistor. See if this stops the ECU from cycling the outputs on and off. If it does it shows that the load that the ECU is seeing is outside the normal specs, and that the ECU is probably ok.
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