Tricking the throttle

79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
I have been playing around with the throttle while using cruise on my trips back and forth to work, 70 miles each way. If I leave my foot on the accelerator, 25% according to Torque, my mileage increases significantly according to Torque, which has been pretty accurate for the last 8 months. What I am contributing this to is that there are a LOT of hills on my travels, therefore I am able to gain more speed going down hill, building up speed for the uphill journey. I will be doing this for this entire tank to see if it really does help my mileage.

Now for my question. Is there a way that I can trick my CPU to thinking the throttle is at 25%? I would ONLY do this by supplying power from the CC somehow. My thought is to use a relay that would be powered by the CC, so when ever I have my CC set and turned on (actually working) it would power a relay where I would switch the power that is acting like the throttle. I would have a switch inline to manually deactivate it as well. The relay would kick out when the CC kicked out, when I deactivate it, hit the brakes, or push in the clutch. This would keep the engine from revving up on its own.

I am assuming the throttle is a rheostat of some sort, so I think I would be able to use a resister or something in order to get the correct signal. I am just not sure where I would plug this reduced power source into.

Am I out of my mind in thinking this is doable? I am sure there is the possibility of doing harm so what is it, besides smoking my CPU. Please do not flame or anything like that. I am not sure if I am even going to do this yet. If you have good reasons it should not be done, please state why. My life is all about the learning adventure, but if someone knows it will not work and can tell me why, then I do not need to find out for myself.
 

Lug_Nut

TDIClub Enthusiast, Pre-Forum Veteran Member
Joined
Jun 20, 1998
Location
Sterling, Massachusetts. USA
TDI
none, 2014 Chevy Volt, 1988 Bolens DGT1700H
What you want is a locking throttle that will hold the pedal at a 25% potentiometer value at all times. The 25% is not from the ECU, but from the pedal. The cruise is in parallel to the pedal so leave the cruise as is, and switched off.
The cruise varies the command equivalent of the accelerator pedal to maintain a set road speed. What you are describing is holding a set command and letting the resulting road speed vary with terrain.
This is simple enough to accomplish by muscle and mental power.

and it does work. The constant load technique is the one I have found most effective for fuel economy in the TDI. An advantage in operator control (versus your proposed modified electronic cruise control) is that you can anticipate. If the next climb is a bit too steep and top gear speed will drop too much, you can go to 30 or 35% for more momentum before reaching the climb.
 

79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
I know the signal does not come from the ECU, it goes to the ECU from the throttle. When the cruise control is working there is no signal from the throttle, it is getting signal from the CC. All I want to do it trick the ECU into thinking while the cruise is working that there is also 25% coming from the throttle. I also understand that I can control the throttle with my foot, which I have been doing for the last three days. I just want to see if there is a way I can do this electrically. I do not want this to control the CC, but rather the CC to have control over this, hence why I would like to run a relay controlled by the CC.

Lug, I know you are a very trusted person on these forums. I am not trying to argue with you. You seem pretty hard set on keeping this mechanical with foot control, but are you saying I should not do this electrically? Or just stating I would get better results mechanically?
 

Jettascuba

Veteran Member
Joined
May 27, 2009
Location
South Africa
TDI
2002 VW Jetta
Use a seperate potentiometer in parallel with the throttle's. Use same value pot as the throttle. Get a pot with an on off switch so that that you can switch the throttle in or out. You also need some microswitches to cancel your setup when touching clutch, brake or throttle. Now you'll have to make sure that it is also off when using the cc.
 

Lug_Nut

TDIClub Enthusiast, Pre-Forum Veteran Member
Joined
Jun 20, 1998
Location
Sterling, Massachusetts. USA
TDI
none, 2014 Chevy Volt, 1988 Bolens DGT1700H
Why do you want to have 25% from the pedal when the cruise is operational? The ECU will operate based on whichever 'signal' is greater.
Going up-hill when your foot position isn't enough to maintain speed the cruise will add fuel to maintain speed.
Going down when the car is speeding up the cruise will back off and the fueling will be based on the right foot position.
 

79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
Lug, that is basically what I am after. When I am going downhill the 25% was enough to give me the extra speed, without over doing it, to get me most of the way up the next hill without the cruise kicking in too much. Yes it does still kick in, but it is not running a lot on the hills.

The point of this idea is that I can run the CC at 60MPH and still be able to have the needed extra push going down hills to gain the speed required to go up the next hill. All of this will be without the need of touching the throttle. During normal operation on the flats I think my car required about 30-35% throttle. This was with the CC set at 60 and then me pushing the throttle until I started to accelerate more. This all depends on the amount of head or tail wind I have too.
 

Jettascuba

Veteran Member
Joined
May 27, 2009
Location
South Africa
TDI
2002 VW Jetta
He wants to keep fuelling constant irrespective of load. Thus trade off gain in speed on downhills with loss of speed on uphills.
 

Lug_Nut

TDIClub Enthusiast, Pre-Forum Veteran Member
Joined
Jun 20, 1998
Location
Sterling, Massachusetts. USA
TDI
none, 2014 Chevy Volt, 1988 Bolens DGT1700H
No he doesn't. He wants 25% accelerator position on downhills even when the cruise goes to 0% because of road speed greater than the cruise set speed.
However, he also wants the cruise to be able to go to 100% if needed to maintain speed on the next uphill.
That's not constant fueling.

The question then becomes does maintaining 25% fuel on the downhill (and increasing road speed and air resistance) in order to delay the cruise activation at above 25% on the following up hill use more, or does it use less, fuel than using 0% fuel on the downhill (with less speed increase) and the earlier resumption of greater than 25% fueling on the next uphill?

My trials have shown me that maintaining the accelerator position and allowing the speed to vary is most economical.
Use 35% on the flats, keep 35% and allow speed to drop on the climb, keep 35% on the downhill and let the speed rise. Really steep hills will call for more or less to permit maintaining top gear or to maintain a safe descent speed, but as constant a pedal position as possible has proven to be the technique to use for maximum mpg in the TDIs.
 

79TA7.6

Veteran Member
Joined
May 8, 2006
Location
Live: Wilbur/Creston; Work: Moses Lake Washington
TDI
2003 Jetta GLS 5 Speed
Lug is 100% correct. I am not after constant fueling, but more of a mixture. I know from past experiences that if I rely purely on my foot that I will end up speeding a great deal. I am not sure if 25% is the ideal position, but that is where I would like to start. Ideally I would have a potentiometer inline to be able to vary this input so under different situations I could refine it.

What I guess I need to know is what is the range of the throttle so I can match it with my potentiometer, where I would get my cruise signal, and where to tap into for the output of my potentiometer.
 
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