My first thought was a leaky hydractive electrovalve and I thought of a cunning way to test this theory; disconnect the working electrovalve and connect a spare bare electrovalve in place of it. The Hydractive ECU will then be happy in that it "sees" an electrovalve but the working one will remain in hard mode.
This I did, front first, and no change in the tick rate. OK, no worries, it's the rear one then, I'll just swap over to prove it. No change in tick rate
Odd. Then to prove further, I then used two "test" electrovalves and hey presto, a satisfyingly long tick rate.
So, is that TWO leaky electrovalves then? I thought on and thought that it could not be as whicever one was substituted, the tick rate stayed substantially the same at about 20s. Surely both could not be duff and leaking by the same amount.
I thought on and wondered if anything else might cause this behaviour. I could not think of anything positive but swapped the antisink sphere. As expected, no difference.
So what could cause a short tick interval just in slow mode bar the electrovales? Nothing really, it had to be the electrovalves leaking.
The electrovalves are switched into soft mode by the application of a 500mS pulse of 12V and thereafter, to hold them operated, a 1KHz squarewave is applied to them as holding current. This prevents them overheating and burning out. It also explains why they hum.
Mine hummed OK and gave a good ON click but the OFF click was a bit muted and somehow, the hum did not sound quite right.
I dismantled a duff electrovalve and determined that the leakage return is positively closed when operated to the soft mode. It is open in hard mode but the valve is held very firmly closed by a spring. Therefore, under normal operation, the valve should only ever squirt a bit of leakage on changing mode as the solenoid moves the needle valve. But if the valve was not being held postively open by a duff ECU signal, it could leak constantly.
I then got my oscilloscope out and looked at the waveform across an electrovalve. I saw this:
My understanding is that this waveform should be a 50:50 ratio squarewave. This one clearly is not and much of the time, the valve is off. This one is more like 25:75.
I then hung the 'scope on my Activa and the waveform is indeed much closer to 50:50.
So, the ECU would appear to be giving a bad signal to the valves. They're only just holding on and potentially allowing leakage. Strangely, it has no real effect on ride or so I can detect.
Going on your groundbreaking work in finding duff electrolytic capacitors in your Hydractive ECU Stempy, I'll now be taking a close look at the electrolytics in mine to see if that's the cause of the distorted waveform.
I'll report back.
Presumably then, unless the coils have shorted turns the problem must be with the ECU. Having several XMs doesn't half make fault finding easier. 
