Faulty Speedo Causes Hard Ride?

[regdorpten]

This may be one for the hydraulics section as much as the electrical but here goes:

If you have a faulty speed sensor (ie the speedo stops working about 5 mins into each journey) does the suspension switch to hard mode by default?

I'm about to change the centre spheres because the ride's pretty go-karty, but it's occurred to me that if the ECU's not getting a speed reading it may have locked the car in hard mode as a safety precaution....

IN which case I should tackle the sensor before the spheres....

[Ciaran]

Generally, if the signal from any sensor goes tits up, it will default to hard. Try unplugging the likes of the accelerator pedal pot. and see what I mean.

Don't see why the speed sensor would be any different.

Ciarán

[regdorpten]

It makes sense doesn't it? The ECU takes a whole load of different bits of information relating to speed, steering, braking, etc and makes a judgement on whether to put the car in hard mode or not. Take away one of those inputs, and it'll likely panic and put the car in hard mode.

Now, second question: will a faulty ABS sensor prompt it to do the same thing (I've got one of thsoe too)?

[DerekW]

Failure of an ABS sensor won't default suspension to hard.

Nor, from my experience, will failure of the speed sensor. My green estate had both not working when I bought it and it rode beautifully.

Derek

[regdorpten]

That's reassuring, but also counter-intuitive. If the ECU has no idea whether the car is crawling along at 3pmh or racing at 120mph, would common sense not cause it to default to hard mode?

[Peter.N.]

Correct me if I'm wrong, but to my knowledge the ride control is not speed sensitive. Why would you want the suspension hard because you were going fast? If you suddenly change direction or brake - yes, that's what it does, but if you are cruising along the motorway you want the suspension nice and supple, or at least I do.

Peter.N.

[regdorpten]

True.

So it would guage how hard you're breaking by hydraulic pressure rather than speed calculations?

[archman1]

In my 1993 S1 I was always getting stuck in sports mode and I traced this back to a faulty speed sensor. Being the cheapskate that I am I found that if you disconnected the speed sensor then the whole car was always in 'soft' mode except when using the accelerator pedal.
The speed sensor connector was a white plastic round cylinder a coupleof inches long located alongside the accellerator cable roughly about level with the bulkhead & LHM tank. Of course being a purely mechanical system this disconnects the trip computer but the speedo works as normal.

It may be that a 1994 S2 uses the same system.


In the S1 XM in which you can hear the sports mode engage via the radio 'interference' - Why can't you hear this interference in an S2?

Cheers
Steve

[Peter.N.]

Hi Steve

All mine have been mechanically injected diesels so have no engine management ECU. I thought the Mk1s had a cable drive to the speedo, so where do they take the signal from?

Peter.

[Peter.N.]

Sorry! I've just read your post properly.

[archman1]

Hi Peter - I am hiding from the christmas mayhem - I can hear my name being called even now!

Found this reference on the tinternet ... www.citroen-ds-id.com/xm/Hydractive1_fault_finding.doc

I will be doing the odd 1000 miles in the next week so my fingers are crossed there will be no trouble!!

Saw a C6 in Coventry - 08 registration - however it was going towards the Citroen garage there, so it was probably only the Citroen manager driving it. The M42 heading towards the NEC must be the bumpiest bit of motorway! Good for XM testing!

[regdorpten]

Thanks for that. Mine's deffo an S2 though.

[Peter.N.]

Thanks Steve, that proves I don't know what I'm talking about I suppose the speed signal must be taken fron the gearbox sensor at some point. The problem is, I only understand the things I have had trouble with, which is most of the car but doesn't extend to the speed reference signal

Useful info that, I will have to print it out.

Peter.

[DerekW]

The Hydractive 2 system switches to hard at highish speed (around 80mph) so if the speed sensor is inoperative there's no confusion, the ECU sees it as below the changeover speed so it remains soft.

Incidentally, the steering sensor doesn't only detect steering wheel angle, it also detects rate of change of angle. Both of these are then compared to a look-up table plotted against speed, so that's another parameter that will be ignored by the ECU if there is no speed signal.

In fact the speed input plays a part in several of the controlling parameters so the suspension will be staying soft sometimes when it should have changed to hard. This is a good reason for getting your speedometer working.

With the H2 system you cannot, unfortunately, put a tell-tale light in the car to show when you're in soft. As has been documented elsewhere the hydractive valves get a short burst of system voltage to move them, this voltage then drops to a small holding voltage which would be insufficient to light the filament..

Derek

[DerekW]

Hi all,

When I wrote that the system changes to hard around 80mph I was wrong, but at that speed it requires very small inputs from steering wheel or body movement sensors to switch to hard.

For anyone interested, the five parameters are: Steering wheel angle, steering wheel speed (rate of change), body vertical movement, rate of accelerator depression, braking.

Brake application switches suspension to hard when the pressure in the brake circuit is above 3 bars and the vehicle speed is above 15 mph.

The remaining sensors send their information to look-up tables where they are each compared to vehicle speed before deciding whether to switch to hard. There are separate tables for each sensor depending upon whether "Normal" or "Sport" is selected.

So without speed input your suspension is permanently soft.

Derek