Citroen C5 Knick Sensor & Automatic Transmission

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Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

Hi Guys.

A while back I had the lovely "Anti Pollution System Failure" warning come up. After a local garage with a Lexia checked it, it appeared to be something called a knock sensor which triggered it. The garaged cleaned it up, reset the computer and bingo fault gone. I have driven the car for a month or two now and last night as I pulled off the motorway and stopped on the slip road, the warning has reappeared :(

I am hoping it is the same fault as I have no power loss and the car is not in limp mode. I was able to do 3500 revs and 45mph no problems last night after the warning appeared.

Two questions:

1) I understand this knock sensor is designed to look for abnormal vibration in the engine area. If it is knacked is it expensive to replace?

2) My car is an 4-speed automatic 2.0l petrol. When it changes up gear it is fine, but when it changes down it almost seems to drop the clutch. Now I have never driven automatics before - bar my wife's Micra which is a CVT so has no gears as it is pulley driven like the old Honda Melodies so it has completely smooth acceleration/deceleration. I am assuming that as the car changes gears it does so as fast as it can it, so does not do a smooth gear change and match revs to the clutch as you would in a manual. This is why modern car have double clutch systems to allow for a smoother change. So is it normal to expect a little jerking as it changes down gears due to the sudden higher revs in the lower gear? Also I can rarely do a smooth stop and it tends to be a touch... heavy which I assume is down to the brakes fighting the cars automatic transmission which wants to keep the car going forward.

Just want to see if it is possible the gearbox is causing the vibration in the way it changes gears and if that needs looking at when the car goes in next week to have the anti-pollution fault looked at again.

Any thoughts are really appreciated on this just to help me understand things.

Thanks
Max
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by CitroJim »

Max, the knock sensor is to detect detonation ('pinking') so that the engine ECU can retard the ignition timing just a tad to stop it. Generally all modern engines have them so that they can auto-adjust to any octane rating of petrol and at the same time run with the ignition advanced as far as possible for greatest efficiency.

The sensor itself is just a small piezo microphone and is very sensitive to being overtightened. Generally they are very reliable.

It won;t be having any effect whatsoever on your downchanges. The automatic should do a smooth downchange as there is no need to do any rev matching to engage a lower ratio. The smoothness will be entirely determined by how the various brake bands and clutches in side the 'box engage and dis-engage.

I'd suspect you have an AL4 and it is suffering from poor pressure regulation due to wear in the pressure regulation solenoids which seems a common problem on this 'box..

I have no direct experience but others on here have and I'm sure they'll soon advise.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

CitroJim wrote: I'd suspect you have an AL4 and it is suffering from poor pressure regulation due to wear in the pressure regulation solenoids which seems a common problem on this 'box...
Hi Jim.

Thanks for this, that helps out a lot. The only thing that worries me is that what you said above sounds like it could potentially be very expensive and I am not sure I want to ask the mechanic to check it at this time. Oddly the car seemed to shift smoother tonight (unless it was me subconsciously changing my driving style a little).

Will have to look into this a bit further I think.

Thanks for the info tho.

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Post by addo »

I believe the harsh kickdown is a design (software vs hardware) fault of the AL4 where the electronic instructions are not perfectly modulated to sync with the mechanically influenced processes. Add wear and it gets worse. It'd be really nice to have some open source code to play with for the transmission ECU.

The "knick sensor" sounds interesting. Does it tell when your date is going commando?
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

Thanks for spotting my little typo I put in on purpose (Ahem).... Oppsss :)

So I am assuming that short of a new box the software/hardware conflict cannot be resolved? Does it cause any serious damage or is it just a case if it is annoying at times when you feel it but otherwise there is no concern there?

Just wanting to know now.

Thanks
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by Mandrake »

I'm no expert on auto's Max but after the trials and tribulations I've been through with my 4HP20 (which is the slightly older bigger brother of the AL4) I would say that your abrupt downchanges which feel like it is slipping into "neutral" are definitely the gearbox and they may not be a good sign...it could be early signs of impending failure :(

I'd agree with Jim that the box is struggling to regulate the pressure accurately which will lead to rough changes as the clutch engagement overlap can't be controlled precisely. This can lead to it feeling like its in neutral part way through the change with it suddenly grabbing as the clutches engage late.

It could be wear or it could be really dirty oil or a bit of both. Auto's are extremely sensitive to the state of the oil in the box and both the AL4 and 4HP20 are "sealed for life" using the same unique Mobil/Esso LT71141, which means by the time they get to 100,000 miles or so with never having had the oil changed the oil is disgusting, totally black and contaminated. (essentially "sealed for life" is a bit of a con, "sealed until warranty runs out" is closer to the truth... :roll: )

First thing to do on an auto box that is showing erratic changing behaviour is an oil change. Because you only get 1/3 to 1/2 of the oil out each time you drain an auto gearbox you might have to do more than one change of oil if the oil is really bad, normally you would leave at least 100 miles driving between each change if you're doing multiple rapid changes to allow time for the new oil to homogenise with the old. You might not notice an improvement in symptoms until the 2nd or 3rd change, so if the oil is really dirty and you don't notice much difference after one change, don't give up.

I had problems with rough shifting and a very worrying "groaning" or grinding noise under load at low RPM that was getting to the point that I thought the box was not long for this world, but 4 oil changes over a month or so have pulled it back from the brink and it has been working fairly normally since with nice smooth changes.

Have a look at page 21 of my massive thread below for pictures of the oil that came out after each change and you can see just how incredibly black and dirty the oil was (probably burnt in fact) after only 100,000 miles:

http://www.frenchcarforum.co.uk/forum/v ... &start=300" onclick="window.open(this.href);return false;

One oil change showed no improvement, 2 changes showed a small improvement in symptoms, after the 3rd and 4th change of oil the symptoms were all gone and have remained in remission for for the three months since then.

If you're changing the oil on an AL4 there is a special procedure to be followed (which Jim has a copy of, if not I should be able to dig it up) and you'll need access to a Lexia to reset the oil wear counter after each change. (You don't actually reset it to zero, you subtract a certain value off for each oil change so the gearbox can keep track of how dirty the oil is) You must also use Mobil/Esso LT71141 and nothing else.

I would also recommend doing an "auto adaptive reset" after you're finished with the oil changes which is also done with the Lexia. This resets the ECU's parameters controlling how the clutches are controlled, forcing it to "re-learn" the characteristics of the gearbox, which will temporarily result in harsh shifts while it re-learns (maybe for 20 miles or so) but after a while it should result in smoother gear changes.

Changing the oil really is the only "user serviceable" thing that you can do to fix an automatic gearbox so its worth trying, its also best done as a preventative measure at the first hint of any symptoms, or even without symptoms if the car is new to you, has done a good mileage, and the oil condition is unknown.

If you leave it until the symptoms get really bad it's probably too late as automatic gearbox failures tend to occur through a domino effect where one thing leads to another in a vicious circle until it drops dead. For example the oil might be dirty with metallic particles floating through it from wear, these particles may lodge in a control valve that prevents the main rail pressure being regulated properly or may clog the filter which also causes a loss of pressure, the loss of pressure then leads to slipping clutches which overheat under load and shed lining material into the oil, this lining material then further clogs the filter or interferes with the control valves etc causing the situation to rapidly spiral out of control until something breaks. Catch it early and it might be perfectly alright after a few oil changes.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

The car has done 150.000 miles - I have no idea how well or badly the car was looked after before me. Probably not overly well recently at least looking at the dents and minor faults with it when I got it in December (2 dodgy door locks, a piece of plastic sill trim missing and it really needs new carpets as they are beyond valeting).

The car does not feel like it is going into neutral at all when changing gear. It just seems a touch... jerky/heavy when shifting down at times - shifting up feels okay. bit like a new driver who doesn't quite know how to feather a clutch bite and power to make a smooth gear change. That said it is not much worse than a 2008 Octavia Taxi I was in about 6 months ago which was also jerky. The gears shift okay and are fine, I have plenty of power etc and (now I am mostly used to it) the car is not doing any crazy down shifts due to me putting my foot to the floor to hard. There is no issues in selecting gears either in full auto or sequential shift modes. In essense I assume it was me driving an automatic with gears (rather than a CVT with no gears) and the car only having one clutch it was just a touch harsher than I expected. As I said in my original post I know part of the reason VW came up with the DSG (Double Clutch) system was to give better power and smoother gear changes which is why they fed it into their Audi and Porsche brands as well.

If it the gear box needs an oil change, for me then it is basically a garage job then, as there is no one in my area with a Lexia (nearest person on the map is over 40 miles away so it is totally impractical) so I could not do the reset even if I could change the oil myself. And in fairness I don't have access to ramps, axle stands etc anyhow so I could not do it either way - even if I had the first clue how to - which I don't. This is something I obviously need to pick up with the garage when it goes in to have the anti-pollution fault checked. I suspect there is a good chance it was changed once if I am lucky.

Just looked at what little service history the car had... 31st August a Holdofrds Autocetre had the car. The docs show things like (but not limited to "replacement radiator due to leaking", "Gear box leaking oil". I have not seen any signs of an oil leak or water under the car that would make me think either was an issue. That said I just spoke to the Halfords Autocentre and they did not fix it - just advised the owner of it. So I cannot tell if this has been fixed/bodged/left at all as I have nothing there after this to work on to show what has been done or not done with it in the 4 months between this and my obtaining it.

I hope this is not cause for concern as I lot my old Primera after 18 months and £3000 of repairs when part of the engine exited the casing (and I am in debt because of that which is not good). This car cost me £800 and if it costs more than £300 to fix It isn't going to be worth it. The last lexia check/fix cost me £70 and I just spent £180 on tyres so my limit is almost reached on this car...

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Post by addo »

Normally, wear in the auto trans isn't significant in any one area - it doesn't take much to colour the oil up. A new valve body and complete flush through with cleaning solvent would probably see it back to more or less as delivered new - but even then shifts were not perfect. You can work with the trans by forcing it to kick down slightly before it otherwise institutes a harsh downshift.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

I have just emailed the garage to ask if they can give me a price to check (and possibly replace) the oil when it is in on Thursday. I have just said it's cause the car has done 150,000 miles - not giving them the rest of the reason so they cannot look for a problem that may not be there. This way it sounds like me just thinking wisely to have it done as a precaution / good maintenance.

If that seems to ease the problems then it is worth a go, and in fairness not knowing when/if it was done last and the mileage on the car - it seem s like a wise move either way.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by myglaren »

Can you be certain that they will use the correct spec oil for the change or are they likely to chuck Dexron in and keep their fingers crossed?
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

No idea what Dexron is. While not the cheapest mechanic in my area, he does do a very good job and I would trust them to put the right stuff in. When I have gone to them with queries on other cars I have owned I always got good answers and service and they have previously shown me boxes/bottles when asked to see what they have done. So they do not hide anything in that sense. My brother has used them for 7 years now with Fords and Citroens (he has a Picasso now) and he trusts them explicitly.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by myglaren »

Just be sure to make them aware that only Mobil (formerly ESSO) LT71141 as mentioned above is the only one suitable.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by MaxPrimal »

Going to ask the stupid question now. I know manufactures recommend certain products/oils but what difference does it make if they don't put in Mobil and choose (for a random example) Castrol instead?
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by Smarty2 »

I have had the old chestnut with the anti pollution light coming on and off since I bought my C5 2 litre SX auto, initially it was quite an expensive investigation, (Citroen) had no idea, if it aint on their computer then they are stuffed, so took it to a very well respected independent who had all the latest software from Citroen, first off was the ignition coil pack, then when that was replaced it came up with a faulty sensor, and when that was replaced it then diagnosed a cold start air injection thinga me jig! I drew the line at that, and have just left it as it makes no difference to anything, Citroen did say it was my gas system that was messing it up until I pointed out the fault was already there before I had the gas system installed! The service light went off for 2 weeks and only yesterday came back on again, so i wouldn't be too concerned it always passes an MOT with the anti pollution fault present.
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Re: Citroen C5 Knick Sensor & Automatic Transmission

Post by Smarty2 »

MaxPrimal wrote:Going to ask the stupid question now. I know manufactures recommend certain products/oils but what difference does it make if they don't put in Mobil and choose (for a random example) Castrol instead?
It "has" to be that oil, there is no alternative since it is specially made for the AL4. so i was told. My garage always had this oil and never used their normal ATF dexron.
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