citrojim wrote:
I reckon on your MK2 you should be able to fool the ECU by putting a short circuit in place of the switch. Although that begs the question of how the HA ECU knows when to pop the suspension into hard on heavy braking. I guess it does it by calculating the rate of change of speed from the VSS
Yes, I do wonder about that.
One of the rules even in the MK1 Xantia ECU calculates deceleration based on the change in measured road speed, and if the deceleration is more than a certain amount it will trigger a switch to hard mode - presumably this is a backup in case of failure of the brake pressure switch.
If they've decided to do away with the brake pressure switch in the MK2 and rely just on the deceleration measurement, I'm disappointed - thats just plain stupid - it takes time to measure deceleration as it is not instantaneous - so the suspension will already have dived BEFORE it switches to hard mode.
With the brake pressure switch the reaction to hitting the brakes is so fast that it switches to hard mode BEFORE deceleration and thus prevents the car from diving in the first place.
The problem with the failsafe mode is that if the switch remains open circuit the ECU is too dumb to figure this out properly - and you get that situation described where it keeps switching into hard mode most of the time above 25Km/hr - and it takes a lot of driving for it to figure out there is a fault and go back to soft mode, and then when you slow down and drive off again it has to figure it all out again.
Although I haven't tried it, I think a temporary workaround of replacing the switch with a short circuit (instead of open circuit) should work - a short corresponds to no braking, so it will stay in soft mode most of the time as it should, and you have the backup rules that measure deceleration for a switch to hard mode.
(It also uses rapid deceleration and no change in the brake pressure switch as a sign that the brake pressure switch is faulty, so it will then start ignoring it anyway)
You could maybe fit a switch. It, I believe, sits in a "K" junction in the front brake feed line under and behined the LHM reservior.
I'm curious now. I was always pretty sure my MK2 Activa had the brake pressure switch. I'll have a closer look when I can but it's not easy to spot.
It's not easy to spot at all, but you could measure its function - pull the plug on the ECU and measure from the appropriate pin to ground while you press and release the brakes (with pressure available of course) - thats how I tested my switch when it was faulty.
I see no reason why the switch couldn't be retrofitted, and if the post about the V6 is anything to go by it may actually be fitted, just not connected in the loom anyway.
Regards,
Simon
. I've only plugged it in with the car stationary but opening a door activated the electrovalves which it wasn't doing before. I'll carry out further testing over the weekend and report back.
) to pick up a Lexia. I'm pleased to say that all went well with the ECU working 'as new', mind you when I plugged the Lexia in and tested the Hydractive ECU it had just about every fault you could think of recorded. I'll scan it again again over the next few weeks and see what happens. Hmm, Lexia I'm in love 
what a wonderful tool 
