Citroen XM Forum

This time on car and classic, and more reasonably priced.

http://www.carandclassic.co.uk/car/C247283

I should know that car but I can't put my finger on it.

Been through Roger's hands (?), and in my neck of the woods too...

I do recognise that KRA reg. Yes, Roger did have it about 2 years ago. I was thinking about buying it then. Was it not a forum member called Justin that bought it from Roger, then he had a low mileage P reg PRV Xm at the same time?

Occasional jerkiness going into 1st gear (a common feature of this box).

Cough bull s**t cough

D

He's quite right about the occasional jerkiness going into first gear. It's almost a sign of good health in a 4HP20....

CitroJim wrote:He's quite right about the occasional jerkiness going into first gear. It's almost a sign of good health in a 4HP20....

really? sounds awefull dodgy to me, isnt it a software issue?

D

Possibly a common feature, but not designed to be as such!

It's under certain circumstances only. Most often when braking hard to a stop, say coming up to traffic lights. You can definitely feel it changing from second down to first with a small jerk. When I say small I mean small. Just occasionally if you happen to accelerate again at the point it's just about to change down it really gets confused and for a split second you can sometimes even find a false neutral followed by a real clunk into 1st.

I've given this a lot of thought and reckon it's to do with hydraulic pressure switching. First and reverse in the 4HP20 use much higher pressures than the rest of the gears and there's a set of slide valves that regulate the line pressure in a manner not unlike how a Citroen hydraulic pressure regulator works; a bit of hesitation as the slide valves do their stuff for first and reverse is likely the cause. Just a theory and not at all proven. I suppose they think that with all the torque available in the low gears the clutches and brakes need to be very firmly operated; hence the near doubling of operating pressure to them under such circumstances. If the HP20 falls into limp mode (third emergency) then the high line pressure is in effect all the time and this causes both third and reverse to engage with a heck of a clunk, enough sometimes to break the engine mount torque reaction donuts. Maybe in the design they found a trade-off between smooth changes and line pressures and accepted that for first and reverse the high pressure was needed and lived with the fact this might make for a less than perfect change.

All 4HP20s do the occasional rough change if the auto-adaptive gets confused. It tries to predict the next change based on what has happened recently and if you decelerate say, when the ECU thought you might be going for an acceleration it will sit and scratch it's electronic head for a moment and make a slightly wrong decision which it will then hastily correct.

Remember the XM/Xantia HP20 auto-adaptive implementation is a very early one and I understand later ones are much better; more processing power and better algorithms I would assume. In 1997 it was pretty much state of the art.

The little AL4 in smaller Citroens has much the same issues in its early impementations. Driving an early auto-adaptive is a lot different to the HP14 and 18 hydromechanical 'boxes. The old hydromechanicals are in my experience much, much smoother on their changes generally.

Of course, in the context of this V6 under offer it would be educational to know the exact circumstances of the jerk and just how big it really is.

On a footnote, I knew of a 4HP20 that was dead-smooth everywhere and just could not be provoked into a rough change anywhere and under any circumstance. Two of us who know our HP20s concluded that this 'box was a wrong 'un and most likely suffering low hydraulic line pressure due to a partially choking filter or worn oil pump. A conclusion strengthened as it was considerably noisier than it should have been...

I stress this is all my ponderings of the characteristics of the HP20 and have no basis in proof.

Interesting though Jim and it does make sense, the hP18 seem to suffer a rough engagement into R as they age (just fixed mine) and as you say it seems to be pressure related, R uses nearly 200psi of pressure again nearly double that of other gears, to correct it i fitted new springs and seals to the pressure regulator valve and also new springs to the reverse selector accumulator and now all is well.
I take it the 20 has a electronicaly operated pressure regulator?

D