4hp20 lock up question

citrov6 · Post by **citrov6** » 17 Nov 2010, 11:55

I was thinking today more than usual what this car would launch like without the lock up in place, if the electronic control could be disconnected temporarily. Don't know if IIRC that there is one connection that recieves a varied pulse for this.

Post by **CitroJim** » 17 Nov 2010, 12:54

Not quite sure what you mean here Citro..

The Torque Convertor lockup function only operates on second gear and above so would have no effect on a take off in bottom gear.

There's no launch control either, you just feed power in and take off. Job done.

There's nothing that can disable (or enable) the lockup function. It's all ECU controlled.

Hope that is OK or have I missed the point here?

spider · Post by **spider** » 17 Nov 2010, 13:46

I'm confused.

If you could manually enable lockup from a standstill, it would just immediately stall dead or lurch off (similar effect to just violently dropping a clutch on a manual transmission)

I did not know lockup functioned outside of fourth gear though, that's useful to know. I assume it is the same effect really (to reduce losses / power) and thereby increase efficiency / fuel consumption. My last auto (non PSA) only had a lockup on fourth.

Post by **CitroJim** » 17 Nov 2010, 14:10

Yep, these days it's all quite clever Andy. The HP20 has three lockup modes. Open (i.e no lockup), closed (full lockup where the pump and turbine are locked together) and piloted where a limited slip is allowed. This is to avoid the jerks and clunks you would have if full lockup was applied straight-off in the lower gears.

I find the HP20 is very enthusiastic to lock up. As soon as you are in second or above and not overly braking or accellerating, it locks up very rapidly.

This helps its fuel economy enormously.

It's pretty keen to drop and gets the message as soon as you give it a bit of welly..

These auto adaptive 'boxes are a bit good...

citrov6 · Post by **citrov6** » 17 Nov 2010, 19:46

no no point really just had too much time today to think i think! I just notice the car does not get moving until 20mph just wondering if that could be improved but looks like not.

The way it seemed to me was that the torque converter is spun up by about 2.5krpm and in first if you want to move in traffic it locks up around 1100rpm, and then if you floor it in first it pulses it till 2500rpm then locks up. might be wrong thats just what it felt like.

Not important just was curious thank you

XMV6 sadist · Post by **XMV6 sadist** » 17 Nov 2010, 21:57

The 4HP20 does lock up fairly readily and for it's time it was a reasonable box although boxes have come on immensely. I have a 7G Merc as well as the XM and the difference between the boxes is huge. An XM with something like a later 6spd c5/c6 box would be very interesting although I have heard they can also be hesitant on take off.

citrov6 · Post by **citrov6** » 17 Nov 2010, 22:15

6 speed would be lovely, actually even a 5 speed would be lovely. Otherwise for daily use it does amazingly well considering 1st gear does 50mph

XMV6 sadist · Post by **XMV6 sadist** » 17 Nov 2010, 22:33

Yep no real complaints about the 4HP20 (apart from having to change them every 70K!) but it's does change as smoothly as a newer box.

Given the OP question I don't think that the 6sp C5/C6 box will give better step off than the 4HP20.

citrov6 · Post by **citrov6** » 17 Nov 2010, 23:19

thats a shame, it seemed like such a good idea. 4hp20 is amazingly smooth, i think it loses a lot of torque with that long gearing so we don't get that jolt the 6 speeds give in first.

would you believe one of my 4hp20's is on 150k most of that on country roads and commuting to birmingham

lexi · Post by **lexi** » 17 Nov 2010, 23:32

Only adavantage to playing with lock up is on a solenoid switched one . Ie older type box that locks up in fourth overdrive only. If you pick up the ECU control for it with a switch ,you can lock up in second and third.

As mentioned if you don't turn off the switch you have made as you come to a standstill ,you will stall it..

None of this applies to our boxes of course.

For quick standing starts I sit in 3rd with sport button on.

Post by **xantia_v6** » 18 Nov 2010, 07:38

citrov6 wrote: 4hp20 is amazingly smooth, i think it loses a lot of torque with that long gearing so we don't get that jolt the 6 speeds give in first.

A "good" 4HP20 does not behave as you describe. On my Xantia, the takeup is quite firm, any unusual rise in RPM is due to wheel-spin rather than torque converter slip. Note that the whole purpose of a torque converter is to convert a speed differential into a torque multiplication, so locking it up will decrease your rate of acceleration at a given engine RPM (and probably also move the engine to a part of the torque curve where it is reduced anyway).

The Xantia V6 which Chris now has behaves more like your description, with the revs sometimes intiailly rising, and then being pulled back down by the transmission. I believe it to be a transmission fault, perhaps related to low oil pressure (it is also very smooth).

citrov6 wrote:

would you believe one of my 4hp20's is on 150k most of that on country roads and commuting to birmingham

Not so unusual. Mine died at 151,000 due to oil starvation (no change in behaviour even after it had spun an output bearing and the torque converter nose bearing), I think that Jim's was in the high 150s, but with a completely different failure mode.

Chris570 · Post by **Chris570** » 18 Nov 2010, 07:48

xantia_v6 wrote:
citrov6 wrote: 4hp20 is amazingly smooth, i think it loses a lot of torque with that long gearing so we don't get that jolt the 6 speeds give in first.
A "good" 4HP20 does not behave as you describe. On my Xantia, the takeup is quite firm, any unusual rise in RPM is due to wheel-spin rather than torque converter slip. Note that the whole purpose of a torque converter is to convert a speed differential into a torque multiplication, so locking it up will decrease your rate of acceleration at a given engine RPM (and probably also move the engine to a part of the torque curve where it is reduced anyway).

The Xantia V6 which Chris now has behaves more like your description, with the revs sometimes intiailly rising, and then being pulled back down by the transmission. I believe it to be a transmission fault, perhaps related to low oil pressure (it is also very smooth).
.

Mine has changed in the past few hundred miles from being powered with cream to being a lot more solid in the way it changes gears if that makes sense, the TC will still slip in doddery git mode but not much. the gear changes are more noticeable now than they were. i'm watching this one with a close eye

Post by **CitroJim** » 18 Nov 2010, 07:57

Yes,

I agree. The HP20 should take off like a scalded cat and with urgency. If you can't break the front wheel grip and produce copious amounts of wheelspin in 1st with a bit of welly then things are not quite right..

The description of a HP20 as "firm" is pretty much spot on... You should never feel any slippage anywhere.

Mine died at 159K from a broken valve block spring (amongst other things)

Post by **xantia_v6** » 18 Nov 2010, 08:17

Chris570 wrote: Mine has changed in the past few hundred miles from being powered with cream to being a lot more solid in the way it changes gears if that makes sense, the TC will still slip in doddery git mode but not much. the gear changes are more noticeable now than they were. i'm watching this one with a close eye

From my experience of that car, it will gradually get smoother again. The auto-adaptive logic is struggling to find a set of parameters that it is happy with.

The "firm" (if not "clunky") phase is rather like the behaviour for the first few miles after a Lexia reset of the transmission parameters.

Stempy · Post by **Stempy** » 18 Nov 2010, 08:20

At what point does it tell the engine ecu to back off the ignition timing?