L322 Tdv8 cruise control issue

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kermit_rr

kermit_rr

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Thought I'd finally try and fix this as I'm likely to start wanting more motorway/long distance miles
Right now cruise control is rubbish. 'resume' acceleration is glacial on a flat road and it won't hold the set speed on anything more than the slight hint of an incline. I have to use the accelerator to get back up to the set speed.
More noticeable at lower speeds or when the torque converter is not locked up, the accelerator is on/off every second or so. When the tc is not locked you see and feel the revs go up and down 6-800rpm, otherwise you can hear it load/unload and the mpg fluctuates 10-15mpg.. that's on a flat roof with no wind etc when it should just 'cruise'
There's no related fault codes, engine otherwise runs fine though i do get the puff of smoke when the second turbo kicks in for the first time
 
Cruise control is a function of the ECM, but it does sound like your TC may be on the way out. Does it behave normally with the transmission in 'Sport'?
 
When you say TC on the way out it does snatch gears sometimes and hangs in neutral when accelerating from overrun eg going down a steep hill.
Could it be related? There's been no change in TC behaviour, I've done ~35000 miles since i had it.. did a gearbox oil change at 105000 miles
 
I have said before, pushing the resume button when you are 20+MPH short of target is like "lets race to the next set of lights"
Only use resume when close to target, other than that find it to hold speed well.

J
 
Yes you have.. I'm guessing.. purely a guess.. there must be a sensor out of calibration somewhere but there's no error codes..
I also wonder.. could it be a ccf mismatch? I've found a few things now to suggest it's potentially not the correct one for the car (if that's even possible) or been badly poked about with
 
I've been trying to think logically, other than a poorly ecm.. what bits are involved? I'm guessing so might be wrong..
The only thing i can think of is the throttle.. it's electronic so there must be a position sensor. Could it need calibrating? If the car doesn't know the correct position then maybe it's hunting, trying to maintain speed?
Or could it be learned behaviour? Because i try hard to get better than 25mpg.. maybe my conservative driving is being taken into account? Seems like a bit of a stretch.

I assume i need to get some live data to see what's going on while in cruise mode.. any suggestions on what i should be collecting?
 
kermit_rr said:
I've been trying to think logically, other than a poorly ecm.. what bits are involved? I'm guessing so might be wrong..
The only thing i can think of is the throttle.. it's electronic so there must be a position sensor. Could it need calibrating? If the car doesn't know the correct position then maybe it's hunting, trying to maintain speed?
Or could it be learned behaviour? Because i try hard to get better than 25mpg.. maybe my conservative driving is being taken into account? Seems like a bit of a stretch.

I assume i need to get some live data to see what's going on while in cruise mode.. any suggestions on what i should be collecting?
Click to expand...
Mogadon for the stress.
 
Reading up on cruise in general.. if the throttle response is laggy then that could cause all sorts of issues, like the rev hunting and the inability to maintain speed on a hill.

Mine does lag.. ie stamp on the throttle and the revs don't change for about a second.. i thought that was normal but maybe i need to investigate that
 
I'm also thinking back to the whole dpf thing and the bodged sensors. If the cruise module relies on accurate data from them.. maybe it's going into some fail safe mode.. hmm
 
Graculus said:
Cruise control is a function of the ECM, but it does sound like your TC may be on the way out. Does it behave normally with the transmission in 'Sport'?
Click to expand...
after some more digging i've seen other cars with the same issue and its (the rpm fluctuation) been traced to TC clutch slippage
 
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