Fuzzy illogical gearbox

Stempy · Post by **Stempy** » 11 Jun 2004, 04:56

After testing my V6 Xant on the Cit Elit analyser and still finding nothing obviously wrong and after some advice from this forum it occured to me that the autobox could be at the root of the 'misfire' trouble I've been having, so I've done some experimenting and this is what I've found.
First thing I tried was to remove the fuse that supplies the gearbox ecu. In this condition the gearbox remains locked in one gear, 3rd I believe, and in this state the engine ran smoothly from cold to hot at all revs. This led me to the conclusion that the problem is somehow caused by the gearbox or its management system. I believe that the gearbox ecu and engine management ecu exchange information, but I don't know how much influence the gearbox ecu has over the power output of the engine or whether it can ask the engine ecu to retard the ignition or such like. I've heard the phrase ' torque modulation' mentioned with regard to the gearbox/engine ecu interaction.
Having experimented further with the gear select lever and trying various settings in both sport and snow mode I seem to have narrowed it down to being a problem in 1st and 2nd gears and the automatic selection between the two. 1st gear can also come in with a real bang sometimes when pulling away after braking to a stop. The problem is at its worst when the engine is neither fully hot or fully cold and seems to occur mostly below 2000rpm with the engine under load, it also shows up after a long motorway run when returning to urban type driving. Interestingly, in snow mode there doesn't seem to be a problem, and this dispenses with first gear, using second to pull away.
I really can't determine whether the gearbox ecu is also making the engine stutter or whether it's just the gearbox hydraulics are having a spasm.
If anyone can come up with a bright idea of what might cause this then I would be eternally grateful
[:(]

Richard Gallagher · Post by **Richard Gallagher** » 11 Jun 2004, 12:57

Stempy I think you're looking along the right lines because modern auto boxes do have some influence over the power output of the engine by way of reducing it in order to smooth out the gear changes.
Does the transmission fluid need changing, is there a filter in the gearbox which is now clogged and is there a temperature sensor on the gearbox are the thoughts that come to my mind on what you have said above.

Stempy · Post by **Stempy** » 11 Jun 2004, 20:58

The boxes are supposed to be sealed for life, but it's got a drain plug, level plug and filler hole! I believe there is a gauze type filter but not sure about a temperature sensor though I assume there must be one as the programmes change according to oil temp. It also counts the temperature cycles for oil wear but this can only be read by the diagnostic gear.

JohnM52 · Post by **JohnM52** » 12 Jun 2004, 03:03

Stempy,
I think you said that your car has about 100k miles up? I recently changed the gearbox oil on my 4hp20 at 83000. The oil was filthy. Although I had not noticed any problems previously, the changes are now much smoother and crisper and the smoothness now confirms that there was a slight jerkiness before.
I know the rules said 'sealed for life'. Sorry, but where auto boxes are concerned there ain't no such thing.
Go buy 3 bottles (6 litres) of part no C9736 22 from Citroen. This is labelled as oil for 4hp20. Do not accept anything else!
Drain down the oil into a clean container. Be prepared for up to 5 litres. The advice I was given is that 4 litres is the normal amount out. Measure the amount from the box and replace with the same quantity. No filter involved - I'm not sure if there is one behind the sump, but it is not easily accessible and the sump is vertical up the front of the gearbox. A long tube attached to a filler will ease the refill. The amount measured out of my box was 4.75 litres.
I would recommend you start with this...if there is any tendency to slip between 1 and 2 then it could be the gearbox ecu is cutting back the power to lessen the slip/overrev.
PS about 30GBP for the 3 bottles of oil.
Hope this helps

Stempy · Post by **Stempy** » 12 Jun 2004, 03:16

Thanks. Mine has done only 56k miles so should have loads of life left in it. PR Net doesn't recognise the part number for the oil, guess i'll try a dealer directly. It's certainly worth a try.

xantiav6 · Post by **xantiav6** » 13 Jun 2004, 01:28

I don't know if you noticed, but the transmission has a dipstick (you have to remove the air filter housing to get to it).
I think you check the level with the transmission warm and the engine idling.

JohnM52 · Post by **JohnM52** » 13 Jun 2004, 05:20

I think both Stempy and I picked up on the fact there is a dipstick (i used that hole to refill the box) but replacing a measured quantity saves time on the change - a check can be done later once the box is running at the required temp (58 -68C, not at full running temp).
Thanks

Stempy · Post by **Stempy** » 13 Jun 2004, 15:14

Yes, I'm aware of the dipstick, will attempt to source some oil next week and if I can get some I'll change it as soon as I can. In the mean time it would be interesting to know exactly how the two ecu's interact and how the engines power is controlled.