2ZR Turbo Yaris

[06YarisRS]

Originally Posted by CrankyOldMan

I'll have to pull up the full EWD before I can solidly comment on the switched DC power, but that sounds reasonable.

Short answer, yes the triangle is ground.

Long answer is more complicated. You don't have to read this stuff, just some general theory and why there are different symbols.

The term "ground" is rather loose and fluid and causes no end of confusion--even to myself. The "E" in all of those connections probably refers to "earth", i.e. the planet, which is the standard zero reference for most voltage measurements. But in the case of your car, it's too insulated by the tires to be able to use that, so instead we use the negative terminal of the battery as zero. Connections direct to the Earth normally use the three shrinking lines. A local ground reference or "common" connection normally uses the downward triangle to differentiate it and in some cases may not be connected to the earth ground. There's also a "chassis" ground symbol that looks like a 3-tined rake. They are frequently used arbitrarily or will have different meanings in different international standards, leading to lots of confusion if you don't understand what the original intent was or what standard (if any!) it was drawn to.

If you get really technical, everything that conducts electricity has some resistance--even gold plated contacts and super pure copper wires. Good old Ohm figured out the relationship between resistance, voltage and current and his Law tells us that there is a voltage difference between any two points in a circuit that have resistance between them and a current flowing from one to the other. Generally speaking, anything connected directly to the battery with zero resistance is also at zero volts. Ideally you would be able to directly connect all of the ground wires to the battery terminal in a "star" configuration to minimize any voltage offsets. You can see in some wiring diagrams that multiple wires converge at a connection to ground, which is normally on the chassis. The wires and chassis all have some (very small) resistance in them, which creates a (very small) voltage difference between the ends of the wires or between the attachment point of the local ground and the nearest big wire on the negative terminal. That small bit of offset usually doesn't impact anything because it's effectively zero. If you have to take a very accurate measurement on that circuit, you should measure from the local ground point, not the battery. In the case of electrical signals or data communication, you need that reference point to be consistent and electrically "quiet". If you use a ground node near your injectors or the voltage regulator on your alternator you can inject lots of unwanted spikes or noise into your ground reference, messing up the relative voltage between that point and the supply/battery.

So the trick here is that you want as many of the ground references for the AEM and ECU to be as close as possible to minimize noise or offsets. In other words, choose wisely where you connect those wires.

Case in point: my horn doesn't work. The horn switch is fine, the relay is fine, the fuse isn't blown, and there's 12V at the positive terminal when the horn is pressed. It doesn't work because there is too much resistance or poor continuity between the horn's internal ground connection and the chassis. There's corrosion on the bolt threads between the horn and the battery as a result of taking the front bumper apart a few times and living in a state that salts their roads. My next step in fixing it will be to run a new ground wire from the nearest ground point to the horn itself and bypass the bad connections in between. If that doesn't work then I will run it to the battery directly. There's no risk of it causing a dangerous short though because the battery is on the other side of the relay contacts and there's a 10A fuse in that circuit as well. Some cars use the opposite method and put the horn on the battery side of the circuit and connect it to ground with the relay. That means the horn input is always "hot" and has the potential for a short to ground before the contacts, resulting in a horn that won't stop honking unless you pull the fuse or relay.

Wow! Thanks Sam! If I'm understanding correctly, then grounding the "signal ground" from the F/IC at the battery would be ideal. However, would that long a run (probably about 3.5') pick up interference and cause faulty voltage readings? I could run the "signal ground" to the battery. I have tons of wire and conduit. Thoughts?

I have the Scion xD engine control diagrams if you don't. I'd be happy to send that to you if you wanted to let me know if my switched power is actually switched.

Thanks again!