PCM Diagnostics & Tuning - Throttle Position / butterfly location / sensor voltage
10-16-2005, 10:28 PM
I did the TB coolant bypass today and I was looking to see if the butterfly position in the TB bore at WOT is fully open and it doesn't appear to be opening anything close to 90 degrees. I can see that it's not a linkage issue based on the contact mark on the throttle body, and it appears that I need to grind the stop to allow more travel. Per my Predator, it show TP to be 100% when I put the pedal to the floor. I did a search and found some old threads from 2002 talking about measuring TP voltage (stay below 4.7 volts at WOT) and then I saw something a while back about keeping TP below 87 degrees. Obviously I can't read degrees, so what is the right way to ensure the butterfly is fully open? Also, how significant is it? I haven't pulled mine from the car and tried to measure the actual travel, but I bet it is only opening 85 degrees at best. Any advise...?
10-17-2005, 12:18 AM
Just keep it below 4.7V and you'll be fine.
10-17-2005, 12:18 AM
use a multimeter to measure voltage output and grind the stop,keep output under 4.7...VCM recognizes 100% throttle opening after 4.4 volts (i think I cant remember the exact number)
10-19-2005, 02:49 PM
Last night I checked the TP voltage as it was and it read 4.2 volts, however when I checked the throttle position on my Predator, it said 100% - how can this be, as I thought full throttle was defined as a voltage between 4.4 and 4.7 volts? Regardless, I proceeded to grind the throttle body stop until I read 4.6 volts output on the TP sensor. I took off almost an 1/8 inch of material that translated into almost a 1/4 inch of added movement of the butterfly, making it very close to fully open. My question is this, does the PCM use the voltage directly or the throttle position percentage value to determine the timing and fuel value? If it uses the voltage, the increase in power should be more significant. However if it uses the percentage value, I was already at 100% throttle according to the Predator and any benefit realized would be a result of getting the butterfly closer to fully open. Can anyone tell me which is the case? If it uses voltage to determine timing and fuel values what is the difference between the values at 4.2 and 4.6 volts, and roughly how much of an increase in power would this be? If the benefits are just a result of the improved air flow, then I would guess the benefit to be worth a couple of hp at best...? Anyone have any data or experience to say which is the case?
10-19-2005, 04:44 PM
Anything over a certain voltage is declared to be
100%, and WOT is anything over 94%. It's got to
accommodate the physical & electrical manufacturing
slop. And because that linkage is not known I prefer
to grind the bump-stop by ruler (stop at dead center)
rather than some electrical artifact.
Besides which it's a whole lot handier on the bench.
10-19-2005, 05:31 PM
Thanks for the information. Along with the voltage, I was also keeping an eye on butterfly position to make sure I didn't go over center. I haven't had a chance to run it hard today, but based on the fact that I was already getting 100% throttle, I think this qualifies as "difficult to measure, but fundumentally the right thing to do"...?
...also, anything over a certain voltage (can't remember what it is right now, but thinking at 4.7V) will set a code. Remember, the idea is to get the butterfly at a 90* angle, not to reach a certain voltage. For example, when I did the bump stop mod 4 years ago my WOT voltage was 4.36V, after the mod 4.60V (as measured with a voltmeter off the pot wires).
10-20-2005, 05:25 AM
So to summarize, the goal is to grind the throttle stop until the butterfly is fully open, taking caution not to exceed 4.7 volts on the throttle position sensor to avoid a setting a code. Obviously I was going about it a little backwards when I did mine, focusing more on voltage rather butterfly position, but in the end it worked out okay as it's just shy of fully open and I measured 4.6 volts. Thanks for all the info....
10-20-2005, 06:19 AM
The PCM goes by both percentage and tps voltage, in an A4 if both do not correlate to one another the shift points will be off, As far as any gains in HP. related directly to the Blade being at a full 90% vs say 80-85% you would think a larger air flow opening would cause an increase, this is simply not so, At least on a couple of cars I tuned on the DYno. After a couple of pulls we noticed that the T/B was at around 80% or so. The customer ending up grinding off the bump stop so it would be parrallel at 90%.....Results, No gains in horsepower or Torque....
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