5th gear swap on 04 and cruise control still works

[mountain lion]

I installed a .658 5th gear today and got it out for a test drive. Everything shifted and performed perfectly. I was expecting cruise control to stop working, but even after a 25 minute drive in 5th, it stayed on. I do have a RC1 tune that was installed back in late 2011 and updated about 2 years ago with some fueling tweaks. I'm not sure if Jeff has the code built in to keep it working or if I'm just lucky and don't have the issue on my 04. I thought I read the tune had to be programed for the specific gear ratio, but maybe not?

In any case the job took longer than I expected, but for the most part went very smooth. After 150k highway miles in stock gearing, I'm wishing I did this years ago, the quiet ride is worth it alone.

 

[mountain lion]

Great...the power response isn't quite there like it used to be, but that is to be expected...70mph is ~2100rpm. It's smooth and quiet though, which for long commutes and drives is worth it (I'm usually on the interstate ~700 miles a week). Will be keeping an eye on mpg improvements, been getting 53 recently now that spring is here.

Again, should have done this sooner but put it off due to cruise control/having to be re-tuned and cost. Looks like RC offers a tool you can flash with from home now, so that makes it easier.

May ping Jeff if no one has any ideas why/how CC is still good, but I'm not complaining.

 

[ugenetoo]

Stupid questions are my expertise, so here goes:

Why would the cruise control not work with a ratio change?

 

[ugenetoo]

Does the ECU connect to some sort of sensor that reads actual road speed?

 

[ugenetoo]

So, if I understand rightly, the ecu receives a value that is sent by the final drive which the Speedo registers as an anolog number.
If so, when the cruise is set, it is set for whatever that value from the final drive is and really has no correlation to what actual vehicle speed is.
Perhaps I'm missing something, but CC operation shouldn't change, except the minimum speed that kicks out the CC when reached.

 

[ugenetoo]

The ecu receives signal in hertz from the impulse sensor.The ecu translates this analogue signal to digital and uses this to calculate fuel input and also for various other functions like door locks,abs, airbags, radio volume, speedo etc

The ecu programming has set parameters ie it should see X rpm at Y mph at Z load with a small deviation . If it goes outside this parameters the conditions for the ecu to activate CC are not met. No CC

When you set the CC, your eyes tell you you're setting it for, let's say 70 mph.
The ECU does not see that 70 MPH, but, only the RPMs of the final drive which correlates to that 70 MPH value.
The values change with the 5th gear ratio change, but the CC should still work nearly the same.
Unless there is a sensor somewhere in the system to measure the ground speed of the vehicle, changing the fifth gear ratio upstream of that impulse sensor should not change the operation of the CC.

 

[mountain lion]

I called Jeff from RocketChip today - he confirmed his tunes for the 04's disable the speed/rpm checks for CC and that's why it still works....the flash is not specific to any one ratio. So that clears that up It's been a few years since I had it flashed, so chances are the fix has been included for a while.

 

[ugenetoo]

Your not getting it. Its not the hardware, its the software. It should see 70mph at around 2500 +-50 rpm. Now it's seeing 70mph at 2100 rpm which puts it outside the SOFTWARE PARAMETERS.

But what determines the actual amount of ground covered by the vehicle?
IOW how does the vehicle know it's actually going 70 MPH?
My question is this:
Is there a sensor that reads how many feet per second (or some other unit of measurement) that the car ACTUALLY travels?
If not, CC shouldn't care what the actual speed is.
You set that speed as a variable amount that you use the speedo as a reference for.
If the speedo is off for tire size, that value isn't going to b e set in stone as the actual speed as well.

 

[ugenetoo]

Your over thinking the issue.

Car manufacture companies design a cheap, proven ,pretty accurate and Simple system. Why add $1000 system to a car that a $50 system does excatly same job for the sake of a small % out. They don't. Even GPS systems have tolerences of usually +- 1kmph. Unless you buy really expensive ones that are 99.99% acurate, whats the point?

Yes if you put bigger or smaller wheels ,the speedo will be out more, most people know that. That's where cluster and ecu coding comes in.you can change the coding to compensate for different wheel sizes.

I'm not advocating any such thing.
I'm only trying to understand why changing the 5th gear ratio would disable cruise control.
I suspect it doesn't and am asking some questions to confirm that suspicion.
You obviously don't have any answers.
Any one else?

 

[mountain lion]

He has answered the question. It's a software issue. Here, once again, from an ecu expert in these cars that tunes it out:
http://forums.tdiclub.com/showpost.php?p=1198884&postcount=421

Swapping new tires or wheels usually only changes speed error by several percent. Swapping the 5th gear changes the RPM drastically, in the .658 case, ~16%. The computer has code to monitor speed vs rpm. If the values fall out of bounds of that it disables cruise control.

 

[ugenetoo]

He has answered the question. It's a software issue. Here, once again, from an ecu expert in these cars that tunes it out:
http://forums.tdiclub.com/showpost.php?p=1198884&postcount=421

Swapping new tires or wheels usually only changes speed error by several percent. Swapping the 5th gear changes the RPM drastically, in the .658 case, ~16%. The computer has code to monitor speed vs rpm. If the values fall out of bounds of that it disables cruise control.

Aha!
That is how it's done.
Thanks.

 

[deejaaa]

wow, that was a hijack if i ever saw one!