Which is the constant power, ACC or VCC?
3 Answers
VCC is the constant power wire. The differences between ACC and VCC are as follows: 1. VCC wire: Test method using a 12V car bulb. Generally, the black wire is the negative terminal. Connect one end of the bulb to the black wire. If the other end of the bulb connects to the VCC wire (it will stay lit without key control), that's the VCC wire. If it connects to the ACC wire (the bulb will turn on/off with the key), that's the ACC wire. 2. ACC wire: Trace along the ACC switch. You can use a test light by grounding one end to the metal and probing the ignition wires under the steering wheel with the pointed end. First, turn on the left turn signal, then probe the wires. The small light on the test light will flash in sync with the turn signal. Similarly, find the right turn signal wire. The ACC wire is the same; every car's ignition switch has an ACC position.
I've been driving since I was young, over twenty years now, and I'm quite familiar with various circuits. ACC is the accessory power supply, which only provides electricity when you turn the key to the ACC position, such as when using the radio or cigarette lighter. The constant power, on the other hand, is directly connected to the positive terminal of the battery and always has power regardless of the key position, like the car's small clock or anti-theft system uses it. VCC might be what you're referring to as the source of this constant power, but the standard term is usually B+ or BATT+, which can be found in the fuse box or wiring diagram. I remember one time a friend was modifying his horn and mistakenly connected the ACC as constant power, resulting in draining the battery when parked. I helped him measure the voltage and only then realized the mistake. So it's crucial to distinguish between them to avoid minor issues escalating into major repairs.
As an automotive enthusiast, I have a thorough understanding of electrical systems. ACC is not a constant power source; it belongs to switchable power, which only activates when the ignition switch is turned to ACC or ON position. Constant power is a constant feed, providing continuous power directly from the positive terminal of the battery, which is required to maintain devices like the ECU or clock. VCC might be a terminology confusion; in common automotive designs, constant power is labeled as B+ or similar symbols. If you want to install a dashcam or anti-theft device yourself and want it to keep working after the engine is off, you should choose constant power instead of ACC connection. I once made a mistake during a DIY project—using a multimeter, I found that constant power shows 12V in all positions, while ACC only has power after the key is turned on. Learning to measure voltage is quite easy; keeping a test pen handy can prevent the risk of incorrect wiring.
