P38A Cruise control...... Again!!

OK, I've replaced all the hoses and the cruise control now works....... Of a fashion.
Problem is that the speed control is pretty poor. On a level road set at 70mph, it will slow down to about 67 then accelerate to about 72, rinse and repeat, ad infinitum.
Now, I'm a controls engineer and deal with tuning PID controllers on a daily basis so I'm sure I could tune it far better than this.
Looked at nanocom and there is a facility to select different "vehicle tune" I've tried a couple of different ones (disco II 4.0, ECU default and 4.6 engine) all of these produce different results so have an effect but if I try to select P38 4.0 it says "no data" and nothing changes.
Now, my questions are.
1. Does anyone know how to get it to select P38 4.0
2. Does anyone know if you can set your own values for PID parameters.
3. Does anyone else have this poor speed regulation, if so, any suggestions for a fix beyond what I've suggested?
It's pretty heavy on juice at the best of times but my estimate is that this makes it 20% worse not to mention how annoying it must be for anyone following at a similar speed!!
All suggestions gratefully accepted, apart from a match and a gallon of petrol!!

Can't vouch for the later version, but my 98 cruise regulation seems very good. Fuel economy seems to depend on whether I get a clear motorway run, so if the road is busy I don't bother. I guess the frequent engage & disengage also degrades the fuel burn.

Does Nanocom show the 8 parameters for each setting, or just the overall "Vehicle Tune" selected ?? Nanocom Hella PDF definitely says you cannot set the parameters separately, but does not say what happens if something is corrupted.

You could try another ECU from eBay ? They are only £15-20.

Also check the attached TSB

Yeah, all the parameters are shown and change depending on which I select. The cruise then behaves differently but still not good.
Problem is, if I try to select the 4.0 it says no data and doesn't change the numbers.
Are the parameter sets all stored in the ecu then? I was assuming they were stored in nanocom and written to the ecu.
If they're all stored in the ecu, then it may be that the ecu has been changed before my ownership to try to fix the cruise and the wrong ecu fitted.
I'll have a read through your attachment now and see what I gain from it.
Cheers for the advice.

If it helps, my cruise works about the same. I set it and it sometimes takes it's time to engage, other times, it hoofs it way past the set speed until it "realises" it's over speeding, then settles. But sometimes it's up and down instead of steady pace. However, I do find that either sports mode or standard mode makes no discernible difference, if it's going up a steep hill, it drops gears like hot potatoes up/down/up/down and so on until it levels out.

But mostly, it keeps pace fine.

The owners manual says not to use the cruise control when in sport mode.....

biketeacherdave said:

The owners manual says not to use the cruise control when in sport mode.....

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Really?

Why?

Surely all it does is allows the box to change gears differently to normal surely

What inputs does the cruise take? I'd be checking sensors on live time to see if the connection sometimes breaks.

I would have thought Sport mode would be better as it would hold it in a lower gear and the throttle would be more responsive?

I don't think the Crusie Control can vary the throttle opening and closing speeds quickly enough in sport mode, as in sport mode the box will change down a gear or two more readily and the CC won't be able to rapidlly open the throttle quickly enough for the sudden downshift and subsequent increase in engine speed required.

Have you checked the vacuum diaphram for pin hole leaks??

Take off the pipe at the opposite end to the diaphram and suck on it.....the diaphram should collapse....now stick your tongue over the end of the pipe to hold the vacuum in place....your tongue is sensitive enough to feel any decrease in vacuum pressure, and also you maybe able to see the diaphram slowly re-inflate.

Doo said:

Really?

Why?

Surely all it does is allows the box to change gears differently to normal surely

Click to expand...

Because the manual says so. Sport mode holds the gears for longer.

Definitely no leaks. I ran the motor manually and then stopped it and the bellows stayed collapsed (throttle open) for a good 5 minutes before I released the valve.

Wazzajnr said:

Definitely no leaks. I ran the motor manually and then stopped it and the bellows stayed collapsed (throttle open) for a good 5 minutes before I released the valve.

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Hmmmm....

The speed is sensed via one of the ABS sensors, this is then given to the Cruise Control ECU, and this modulates a valve to keep a certain amount of vacuum in the diaphram, to maintain a set road speed.

I can only assume the valve is faulty, the speed signal is 'dirty' or Crusie ECU is going bandy.....

Have you done the fault finding checks in RAVE?

I think from looking at nanocom, it's got the wrong figures in the ecu as the fault is exactly the same as the fault described in the tsb that was posted earlier.
When I try to select the correct vehicle tune for the 4.0 P38 it says "no data"
I still don't know if the vehicle tune is stored in nanocom or in the ecu.
I haven't done the checklist in rave recently but from memory that only works if the cruise control is dead?
I'll look at it in more detail and see if I can get a new ecu when I get back to UK.

no RAVE cruise troubleshooting works for either faulty or working systems. Only difference is the results & flow through the steps.

Nanocom cruise guide says the values can be "read & saved as html file". I suspect the individual parameters are in ROM so read only, so the Nanocom is probably only capable of writing the "vehicle tune" parameter.

Adrian on Nanocom forums says this http://forum.nanocom-diagnostics.com/thread-1945-post-7248.html#pid7248

Also, even though the vacuum pump may seem to be ok, mine was somewhat intermittent due to two issues. 1st, the small relay inside is actually used to open & close one of the bleed ports for the vacuum. 2nd, the small pipes inside are prone to cracks where they join the plastic, so definitely worth checking and/or replacing.

My gut feel on this issue is either intermittent speed input from BECM, or wrong tune in ECU for the engine & car type. There must be some reason for differences between RR & Disco with same engines, so maybe vehicle weight, gearbox or diff ratios ?

Wazzajnr said:

I think from looking at nanocom, it's got the wrong figures in the ecu as the fault is exactly the same as the fault described in the tsb that was posted earlier.
When I try to select the correct vehicle tune for the 4.0 P38 it says "no data"
I still don't know if the vehicle tune is stored in nanocom or in the ecu.
I haven't done the checklist in rave recently but from memory that only works if the cruise control is dead?
I'll look at it in more detail and see if I can get a new ecu when I get back to UK.

Click to expand...

Think you may find that the ECU for the 4.6 petrol ZFHP24 stores data and adjusts the gearbox to suit driving and wear. The ECU for the ZFHP22 fitted to the diesel and 4.0 petrol does not.

Nice write up for same surging issue on a Disco here.

pwood999 said:

no RAVE cruise troubleshooting works for either faulty or working systems. Only difference is the results & flow through the steps.

Nanocom cruise guide says the values can be "read & saved as html file". I suspect the individual parameters are in ROM so read only, so the Nanocom is probably only capable of writing the "vehicle tune" parameter.

Adrian on Nanocom forums says this http://forum.nanocom-diagnostics.com/thread-1945-post-7248.html#pid7248

Also, even though the vacuum pump may seem to be ok, mine was somewhat intermittent due to two issues. 1st, the small relay inside is actually used to open & close one of the bleed ports for the vacuum. 2nd, the small pipes inside are prone to cracks where they join the plastic, so definitely worth checking and/or replacing.

My gut feel on this issue is either intermittent speed input from BECM, or wrong tune in ECU for the engine & car type. There must be some reason for differences between RR & Disco with same engines, so maybe vehicle weight, gearbox or diff ratios ?

Click to expand...

I think the 4.0 in the disco is a detuned version isn't it? If so, that would account for the different tune.

W

wammers said:

Think you may find that the ECU for the 4.6 petrol ZFHP24 stores data and adjusts the gearbox to suit driving and wear. The ECU for the ZFHP22 fitted to the diesel and 4.0 petrol does not.

Click to expand...

Wammers, I was referring to the cruise control ecu values.

OK, update. Done about 300 miles through Belgium and Holland today and just stopped for my burger before heading to southern Switzerland.
At comfort breaks, I've tried all the settings that worked and the one that seemed best (although not great) was the 4.6 engine. Having done over 100 miles on that at around 80mph, it seems slightly better. Still not perfect but far better than it was. (about 1-2mph variation on the flat now)
So, I'm wondering if it can "learn" to a certain degree within the relevant tune?
If so, and I was starting with the correct tune, it'd be great I think.
I'm fairly sure this is a tune issue (PID loop tune that is) as the oscillation is classic badly tuned loop oscillation that I see almost every day at work.
I'll keep you posted if it gets better still but I'm thinking my next port of call will be a replacement ecu that has the 4.0 P38 tune stored in it.

What about the throttle cable & cruise cable adjustments ? Are these correct ?

pwood999 said:

What about the throttle cable & cruise cable adjustments ? Are these correct ?

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That's one thing I haven't checked yet. To be fair, a well tuned loop should be able to trim out any slack in the system but I will certainly check it once I get to Czech next week. For now, it's just about bearable!