2008 VW T5 Seikel modified - Distance Impulse Number

This is a bit rare car - the Seikel modification changes the tyre size from standard 215/65 R16 to 235/70 R16, and the gear box is modified with different gear ratios.

When I got the car the cruise control was not working at all due to broken switch on the left stalk. I fixed that obvious problem, but the CCS behaves strange - it does not work on the 6th gear at all, it disengages itself soon on 4th gear, works fine on 5th and 3rd...

After a lot of search and investigation I found official statement from Seikel that this modification may cause CCS to stop working, but I can't believe this is "unfixable".
I found the reason for that may be the engine unit and the instrument unit calculate differently the gear shift and this discrepancy causes the CCS to disengage.
At the time of the modification the instrument cluster was modified by Seikel, but I am not sure what the modification was about and the CCS was working I think, it is hard to tell what happen in the meantime it stopped, but there were other incorrect coding values so somebody was obviously playing with it.

There is a lot of topics about speedo being off. This car has also a +5-10% on the needle speedo, this is usual, but thru that I came to that magic "Distance Impulse Number", there is a very interesting topic at https://forums.tdiclub.com/showthread.php?t=316833 about how this could be calculated from tyre diameter and ABS pulses... and it could influence how the instrument cluster calculates the speed/gear.

Now to the question - I used the HEX-CAN and I found this Distance Impulse Number thru VCDS in the 17 instrument cluster, Advanced measurements to be 22188 but the description file for the 7Hx-920--xxx-17.lbl does not offer any coding change to influence this impulse number.
It offers only:

On the T5 GP (2010+) everything is different, there is a gear shift indicator on the MFI, there is a sensor on the gear box etc. How the CCS knows the gear on the T5 <2008 its hard to tell.

On this T5 GP the 7Mx-920-xxx-17.lbl states:

So the last number is telling something about this impulse number and the K-value, that may influence it, but even there only two values are documented.

I can not believe it is not possible to change this on 2008 model as even older cars have a way to modify this with different tiye size. Is there any possibility, it is just not documented and is the same for T5 and T5 GP? Any thoughts on different way to change this.... or fix the CCS?

IMO, the problem is not the tire size. Different tries size is what distance impulse number could compensate for (but only for the instrument cluster) and shouldn't kill the cruise control, because all the RPMs would still match what the ECU expects.

IMO, the problem here is the different gear ratios in the transmission. This is well known to kill cruise control because it causes the vehicle's measured speed (via the tires) not to match the what the ECU expects it to be in a particular gear. Plenty of people have changed the top gear in their transmission for one reason or another and found that cruise control no longer works in that gear. Anyway, the only known fix is for someone (a competent tuner) to adjust some values in the ECU software to match the actual transmission ratios.

-Uwe-

You are right - I came to that too, I've contacted two companies doing a tuning for VW diesels (one of them really is doing tuning for race teams) and they told me, they can remap the ECU in the gearbox - until I told them this is a manual 2008 - in this gearbox there is no electronic to work with, only a DSG has ECU for those models. I've also contacted Seikel, they responded quickly but were not very helpful. I know Seikel did some modification to the cluster, but they were not changing anything in the engine ECU (to my knowledge). This is a reason why I am doing a research on my own, trying to find out what could be the problem. As I understand you think the instrument cluster is not involved in CCS - it is only a matter of wheels vs. engine ECU... hm, Thanks anyway.

covex said:

I've contacted two companies doing a tuning for VW diesels (one of them really is doing tuning for race teams) and they told me, they can remap the ECU in the gearbox - until I told them this is a manual 2008 - in this gearbox there is no electronic to work with, only a DSG has ECU for those models.

Click to expand...

Right, it is the the software in the Engine Control Module that needs to be modified. I'm confident this can be done, but you'll have to find the right tuner.

-Uwe-