I hooked the meter up to the ECU to adjust my TPS. (91 N/A). When the car is not running (but is fully warmed up) and the key is on, I get a reading of 1.05v. Then, when I crank the car and let it idle, I get a reading of 0.88v. When the car is not running, the voltage maxes out at 4.74 without any drop offs. Should I set the TPS with the car running or not? Or, is there something else wrong because I don't get the same readings whether the car is running or not?

 

I always adjust them with the car warmed up (make sure it's in the normal warm idle at ~500-750), and running.

Use the closest ECU harness ground if you can, you'll get the most accurate reading that way.

With a S5 you should be at exactly 0.8v, and you'll be good to go. While you're down there you should look into re-grounding the ECU as well. I ran 4 new wires off the harness grounds and ran them to a good chassis grounding point.

 

Ground them separately, or all bundled together like they are now?

 

All the new grounds are wired to the kick panel bolts above the ECU. I really don't know if it makes much of a difference, since I did the rest of my grounds at the same time.

 

I ran a 12 gauge wire directly from the starter (did other grounding work, 4ga. to the starter) directly from there to inside the car to the ECU. The 3 or 4 ECU grounds were spliced and soldered in, then another 14 ga. wire from there to the chassis at the ECU. Also ran a ground to the pressure sensor (as well as the normals... rear housing ground, alternator, etc etc).

As long as you took a nice drive for 15-20 mins or so... come in (have the carpet etc already pealed back) and check it right after you shut it off, should be OK. If you're getting back fires/hesitation/weird idle... then just play with it a bit.

 

Guys, if the S5's are like the S4's, the ECU actually only has 3 physical ground wires (and they all terminate at the rear rotor housing ground, to boot). That 4th one is the isolated "ground" circuit for the 5vref sensors, and does not actually ground to the chassis. The ECU routes it internally to one (or more) of the other 3 pins. If you've grounded this 4th wire at the ECU to the chassis, and nothing seems amiss, just be aware that your puny sensor voltage inputs may be compromised a bit, because the "gang ground" circuit potential has been altered...

Chuck- that's weird, your TPS voltage should not change depending on whether the car is running or not. The throttle linkage should be in the exact same position either way, therefore the TPS output should be the same. The ECU's 5vref output is also the same whether batt power or alternator power is being used (at least on S4's)...If in doubt, use the idle (running) voltage for your benchmark.

Poor red neck- I see what you are trying to do by grounding the ECU to the big ground's potential near the starter, but you're forgetting that when running, the alternator is pushing the power through the engine housings, not the battery through that ground, because of the voltage potential difference between the two. If this sounds confusing, just use the quantum theory for electrical flow- pretend that all off your wiring is swapped, grounds are now power, and vice versa (in reality, this is how it works anyway). Your alt is "pushing" 14v or so, which is "picked up" by the rear rotor housing ground for power to the ECU. Anybody that's rigged their grounds at the ECU instead of that rear rotor housing is actually grabbing their power source once it's flowed through the housings and the bonding jumpers to the chassis, a loss of perhaps .1 to .3 volts compared to that housing ground...

Everybody got that, lol?...I know I lose some of you with all of the techno-speak