Member Since: 18 Aug 2012
Location: Bolton
Posts: 673
Oscillating Revs
I've noticed a few posts about oscillating revs. Anyway yesterday on my 20 mile motorway commute I set the cruise control for about 2000rpm/70mph (2.7 TDV6 auto).
I then noticed the revs going up and down by about 200rpm. Normally the motorway is too busy for cruise control and when I sit at a steady 70mph with manual throttle input the revs are stable.
So a few times on the level I turned cc on and sure enough the rpm started to oscillate and yet not when I was controlling the throttle.
Odd I've never had a car do that before.
15th Mar 2014 9:30 am
AN
Member Since: 26 Jul 2005
Location: Hungary
Posts: 327
Mine doing it for the last 9 years (from new). Usualy it does it during the winter more offen, but can happen anytime. I suspect it is the TC clutch, but could not prove it yet. It does not do it permanently, just when it feels like it .
I was told that IID Tool could look at the TC clutch operation (On/Off) as live data, maybe if you have one you can check it out.LR D3 S Black, Auto TDV6
15th Mar 2014 10:33 am
Numpty
Member Since: 18 Aug 2012
Location: Bolton
Posts: 673
But as an engineer of 30 years why does it only do it with cruise control engaged?
15th Mar 2014 10:35 am
johnnychainsaw
Member Since: 24 Nov 2009
Location: West Sussex
Posts: 1222
Sounds like you need a gearbox flush
15th Mar 2014 10:37 am
AN
Member Since: 26 Jul 2005
Location: Hungary
Posts: 327
For me it does it with CC on and off. The flush did not help either.LR D3 S Black, Auto TDV6
15th Mar 2014 12:47 pm
Robbie
Member Since: 05 Feb 2006
Location: ¯\_(ツ)_/¯
Posts: 17932
Numpty wrote:
But as an engineer of 30 years why does it only do it with cruise control engaged?
The gearbox strategy changes to Cruise Mode when cc is engaged to prevent repetitive locking and unlocking of the TC, minimise the number of shifts and to allow for controlled slip/variable engagement of the TC clutch.
Land Rover - Turning Drivers into Mechanics Since 1948
I'm glad it isn't just me who has noticed this. I find it happens more when cold. Once up to temperature, it doesn't happen (4.4 V8 Petrol)
15th Mar 2014 3:33 pm
disco_steve
Member Since: 25 Aug 2009
Location: Are you sure this is the M5 ?
Posts: 1498
So. . . It it generally the TC that needs replacing, or is it a gearbox issue Never get mad - get even !!
06 D3 now gone to family member and still going strong at 265k
previous vehicles:
04 D2 TD5 - (clocked 189k from new)
52 X-Trail 2.2 SVE
various euro boxes !
15th Mar 2014 4:01 pm
Numpty
Member Since: 18 Aug 2012
Location: Bolton
Posts: 673
I don't believe this is necessarily a fault. But the whole point of a lock up clutch in a modern gearbox is to increase the transmission efficiency.
Having done control theory it looks to be like a little instability in the control loop which when I break the loop my turning cc off the oscillation goes.
I also don't like the D3 cc in that when I set it the car always drops back by 2 to 3 mph before regaining the lost speed. Never had any other car do that at the point of cc engagement.
15th Mar 2014 4:28 pm
AN
Member Since: 26 Jul 2005
Location: Hungary
Posts: 327
I think, if my idea is correct and it is the TC clutch. It is TC only issue, this is why the flush does not help.
It is minor annoyance, happens randomly and rarely, thus never bothered to change TC (which is not a minor surgery ).LR D3 S Black, Auto TDV6
15th Mar 2014 6:14 pm
ds23man
Member Since: 19 Jan 2013
Location: Ainum
Posts: 227
I don't believe this is necessarily a fault. But the whole point of a lock up clutch in a modern gearbox is to increase the transmission efficiency.
Having done control theory it looks to be like a little instability in the control loop which when I break the loop my turning cc off the oscillation goes.
I also don't like the D3 cc in that when I set it the car always drops back by 2 to 3 mph before regaining the lost speed. Never had any other car do that at the point of cc engagement.
Did it in the 3 Audi's I had. Doesn't do it if you back off the throttle and get the speed stable first - otherwise the CC has to play catch up - not a big deal - though when resuming I wish it wouldn't go hell for leather to get back to cruise as fast as possible, including a downshift sometimes. But that is the nature of a PID loop.Mark.
2006, D3 SE Auto - gone but not forgotten.
2014 BMW 530d M Sport Tourer.
1977 Triumph Spitfire 1500
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16th Mar 2014 11:20 am
Valhalla
Member Since: 23 Oct 2011
Location: Isle of Skye
Posts: 124
Numpty wrote:
I also don't like the D3 cc in that when I set it the car always drops back by 2 to 3 mph before regaining the lost speed. Never had any other car do that at the point of cc engagement.
The D3 cruise controller is calibrated to not "prime" the PI control loop with the "last-known-driver-input" torque value for a very good reason; these are big heavy cars that can have quite a lot of pedal input to get them up to a given motorway speed, so if they were to momentarily continue to accelerate at the same rate just after the CC speed was set, then the driver perception would be that they were closing the gap too much to the preceeeding car.
Example: You accelerate out from behind a truck in lane1, to join traffic in lane2, filling a gap nicely behind the car that has just overtaken you. In this case, you would not want the speed to increase beyond the intended speed, causing you to cancel the CC by having to lightly brake (or overide CC on the steering switch). That would be the consequence of pre-loading the last driver torque demand into the CC PI loop. Far better that the speed lags back slightly than overshoots.
Once engaged and under control, the CC should be authoritive - where there's no problem of trying to guess what the driver wants.
16th Mar 2014 10:30 pm
Numpty
Member Since: 18 Aug 2012
Location: Bolton
Posts: 673
That sounds like someone with inside knowledge ...do you work for Land Rover or one of the ECU/software suppliers? I worked with a guy at BAe Systems years ago that left to join Land Rover in the systems design/test area ...wonder whether he is still there as I'd love to get some techy answers from him.
16th Mar 2014 10:35 pm
Valhalla
Member Since: 23 Oct 2011
Location: Isle of Skye
Posts: 124
Well, I don't work for Landrover any more. I did have a fair bit to do with the diesel engine control systems and calibration though.....
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