DBW pedal position correlation with throttle body

[Jim_PA]

My throttle blade is open 100% at half of the gas pedal travel, and extending beyond about 50% of the gas pedal puts me in limp mode.

I have been all over the place trying to come up with a fix for this, and one way is to keep changing OS's until I find one that works with my pedal and TAC, but I really don't want to do that.

Has anyone figured out a way to re-map (or change) the gas pedal correlation with the TAC/PCM?

[Taz]

Hello Jim,

There are some year to year, model to model, compatibility "rules" with APP (gas pedal), TAC (throttle actuator control), TB (throttle body), and PCM OS.

The following information would be helpful:

what OS are you using
what APP are you using
what TAC are you using
what TB are you using
Once this information is known, the issue may be a simple component mismatch.


Regards,
Taz

[Jim_PA]

Ok, here is what I gathered so far..

OS is 12587603
APP is the fixed metal pedal with a long throw (also tried a 98 corvette pedal with the truck APP sensor and it gives the same result)
TAC is 12573059
TB is the 2003-2005 truck style (the engine and wiring harness is out of a 2004 Silverado 2500)

[Taz]

Hello Jim,

OK ... looks like the TB / TAC / OS are all 2004 vintage truck parts - these should be compatible. Your APP sounds like a 1997 - 2004 Corvette piece - which should work.

Is the PCM from the donor vehicle also, or did you flash the tune into a different PCM ?

When you have a chance, please post your tune - will check for any obvious issues.


Regards,
Taz

[Jim_PA]

Hello Jim,

OK ... looks like the TB / TAC / OS are all 2004 vintage truck parts - these should be compatible. Your APP sounds like a 1997 - 2004 Corvette piece - which should work.

Is the PCM from the donor vehicle also, or did you flash the tune into a different PCM ?

When you have a chance, please post your tune - will check for any obvious issues.


Regards,
Taz
The Corvette pedal sensor did not work. I'm not sure if the sensor was faulty or ... ? I had to swap the sensor off my truck pedal onto the Corvette pedal, but it still gave me 100% WOT at half the pedal travel.

Unfortunately, All I know is that the complete engine/wiring harness came from a 2004 Silverado 2500. The computer, TAC and pedal I have no idea where they came from, but I tried to follow GM PN's to make sure I got things that would work together.


Tune is attached! -- Thanks for looking :)

[Taz]

Hello Jim,

There are many variables that could be contributing to your current issue - the most probable are the wiring between the APP and the TAC, and the tune file itself.

To recap, you are using a 2004 truck:

wiring harness
TAC
TB
OS
These should be mutually compatible. I always attempt to minimize variables when troubleshooting a problem. Here is what I would suggest as a course of action:

full flash the PCM with a 2004 truck CAL & OS - do not move on to a COS until the DBW issue is resolved
a full flash will write the ETC programming - important if you are using an unknown PCM
confirm the wiring between the TB and TAC
confirm the wiring between the TAC and APP
repin the APP connector as necessary for the specific APP (pedal) being used - truck and Corvette APP pinouts are different
Once the DBW issue is resolved - move onto a COS.

I have attached a PDF file based on 2004 Truck vs Corvette components that should be of assistance.


Regards,
Taz

[Jim_PA]

Oh man. That PDF Is perfect -- THANKS!

I am going to try to use the proper wiring to the corvette APP sensor and check the results. This is the easiest thing I can do at the moment! --

I had the problem before I applied the COS, I've just been dealing with the issue for the last year. I moved the whole LSx setup to a new swap car, and am trying one last time to get this sorted out before adapting the GM pedal to my BMW pedal.

Appreciate it very much! --

[turbo_bu]

Probably a dumb question, but I'll give it a try. As has been repeated many times before, try and keep you PCM / APP / TAC / TB and OS all the same to try and make sure it all works like it should. Now, the question that keeps bugging me, is somewhere in this loop "should" be the key to keeping it all working. Which one of these thing(s) is the key ?????

I have read that on some TB's they have a 0-5 V signal, a 0 - 2.5 V signal, a 5 - 0 V signal, etc... I am not sure if these are accurate or not, but to me, as long as you have the correct wires with the correct signals going into the system (TB, TAC, APP), then that part of the chain should be good. And is maybe not as critical as the rest.

A side thought of this logic comes in that when someone gets a 1/2 throttle input and a 100% WOT TB, it is most likely due to a 0-5 signal going into a 0-2.5 volt input. Don't know, but just a theory.

The next key would be the governor parameters (guessing some form of closed loop PID setup ... maybe only PI??? ... or maybe PD???) Would these only be kept in the TAC, or would they reside in the PCM too? My guess is that this is a little more complicated than just keeping the TB and TAC together .... and making sure the APP has the correct wires connected to the system.

If a person were swapping stuff around, or got dealt a mis-match of parts, would they then have to manually hack the PCM code to put in a different set of DBW PID parameters (guessing with a Tech II to put in a cal for the DBW portion or manually do it in Hex code)? I know these are just random thoughts, but I wanted to throw something out there that might help us to better understand which part ... or are all the parts that critical. Or are there work arounds that have been successfully done.

[Jim_PA]

It's possible. The engine came out of a 2004 2500 silverado, while the computer came from a 2003 3500 GMC Sierra.

I am good for now. I made the pedal stop at 100% throttle opening and when I hooked it into the other pedal, I made sure I had just about a full range of motion so it doesn't seem so sensitive.

[turbo_bu]

Just to confirm, you are using the truck gas pedal (APP) with the truck TAC and TB and it works now? Didn't know if you tried your Corvette APP too (with the wires hooked up correctly :) ) and got it to work as well.

[Jim_PA]

Oh yeah, I tried the corvette APP on the corvette pedal with the wires going where they were supposed to go, and it didn't work.

So, I put the truck APP back on the corvette pedal, cut the end of the pedal off, mounted it to the floor, and I have the BMW gas pedal actuating the Corvette pedal...

I have yet to have a problem over-extending the pedal like I did with the truck pedal when I gave it full travel. Maybe the corvette pedal does have less overall travel.

Still, the TB is open 100% at maybe closer to 3/4 of pedal travel. Maybe this is normal.

Can't wait until a part needs to be replaced, I'll have no idea what to order somehing for :)

[turbo_bu]

Thanks for the follow up. Guess that throws my theory of the APP could be from any vehicle as long as the wires were hooked up correctly out the window ...

[swingtan]

Have you tried logging both APP and ETCTP in scantool? You may find that the pedal sensor is reporting "WOT" at half travel, hence the TB being set to wide open by the ECM.

Simon.

[Jim_PA]

That seems to be the case, but figuring out what is not matched properly has stumped many people.

[swingtan]

The ETC system is fairly simple in operation.

The APP sensor provides a 0-5V signal to indicate the throttle position. Generally it's "around" 0V at closed throttle and "around" 5V at WOT. the WOT voltage could be anything from 4V to 5V depending on the application. The ECM will take this voltage and convert it to an actual pedal position, then command an associated throttle blade opening. the TPS then is used to measure the actual throttle opening and is again a 0-5V signal, as for the APP sensor, low voltage is closed and high voltage is open. If you read the wrong PID's in Scantool, you will get an actual TPS reading that only goes from 20% to about 85%, because the older GM.TP PID actually reports the TPS sensor voltage percentage rather than the actual opening %.

So here's what I'd do...


Measure the actual Applied Pedal Position sensor voltage to see what output voltage is sent from the pedal. To do this,

Measure the 0% pedal height from the floor. Let's use 5 inches as an example.
Depending on how accurate you want to get, divide the height into the number of test points you want. 5 should be enough for a test, so we will test for pedal changes of 1 inch.
Check the APP sensor voltage at 0% pedal and note it down.
press the pedal down 1 inch and again note the voltage of the sensor.
Take further readings at 1 inch increments.

Use the above data to see what the sensor is telling the ECM the actual pedal position is.
If you have logged both APP and ETCTPS, look at the correlation between the two and you should be able to work out where the "pedal" position needs to be to achieve 100% on the throttle blade.
You now have two choices...

Alter the physical travel of the pedal so that 100% throttle blade open occurs at 100% pedal movement.
Alter the sensor OP voltage so that the sensor voltage for 100% throttle blade opening occurs at the current 100% pedal movement point.



If the vehicle is being used for circuit work, then the 0% throttle position is critical to allow for effective "heel - toe" work during cornering. This should be kept in mind if altering the actual pedal travel.

Simon.

[Jim_PA]

Well, as a follow-up.. I've been driving the car for about a month now, and I'm worried about my clutch life. If I can concentrate heavily on extremely lightly feathering my gas pedal, all goes well. But if I'm trying to get up and go in a hurry, My gas pedal foot tends to bump the throttle a bit more heavily.. Not good for the clutch!!!

So, as a hopeful work-around, I observed TP% while doing what I consider "bumping" the throttle while pulling out, and TP% goes up to usually about 50% in what I consider normal pedal actuation. Seems a bit aggressive to me :)

So, I started messing with B2308 "ETC pedal response"

Stock values from 12.50% to 50% pedal go like this, along with my 50% reduction:

pedal % - Throttle % (stock) - Throttle % (with my modifications)
12.5 - .381 - .190
15.63 - .586 - .293
18.75 - .947 - .474
21.88 - 1.377 - .688
25.00 - 1.895 - .947
28.13 - 2.539 - 1.270
31.25 - 3.389 - 1.694
34.38 - 5.703 - 2.852
37.50 - 7.5 - 3.750
40.63 - 9.6 - 4.8
43.75 - 12.002 - 6.001
46.88 - 14.502 - 7.251
50.00 - 16.904 - 8.452

After multiplying the values up to 50% by 1/2, I added a sharp curve from 50% up to 100%.

I realize that by doing this, I've lost some pedal granularity above 50% throttle movement, but in reality, it's pretty rare I'm looking for pedal control/granularity above half throttle anyway.

In the driveway, the pedal feels MUCH more reasonable now, and I might have taken away too much, but I'll have to reserve that judgement until I do some driving.

I have my fingers crossed that the modifications don't trigger a limp mode or fry my PCM (as warned about in the editor...) but I'll have my laptop and a backup file on hand in the event that it does.

[Jim_PA]

Oh my! What a difference in overall light throttle driveability. Any pre-mature clutch wear from this point on is all in driver error. Feels like a totally different car now.

Wish I would have done this sooner.

[swingtan]

Sounds like you have got it nailed. Good work.

[joecar]

Jim, thanks for the follow up :cheers:

[Jim_PA]

I generally prefer to solve the problem rather than circumvent it, but with this one, I just need to keep telling myself it's OK that I can't figure it out :D