As I understand it, most electrical circuits in cars have:

  • a primary side (control) and secondary side (active?), with a relay being the nexus between each side.
  • signal wires going to the relay are either from a user-controlled switch (e.g. power window switch) or a sensor (e.g. coolant temp sensor)

Bi-directional scanners can control electrical components, so they must be wired up to the secondary side in some way. However, I haven't been able to find information on how they do this. Does anyone here know how they are wired into electrical circuits to be able to control components?

  • This is too broad a question in my opinion. Most control circuits don't use relays, for example the fuel injectors are not controlled by relays. Can you be more specific about what you are trying to do? After reading your comment under paulster's answer, can you reword your question to correct it?
    – HandyHowie
    Sep 12 at 8:36
  • every component will have its own method. Note that with CAN (Controller Area Network) bus components, sometimes the component "talks" back to the controller like a local internet.
    – Tiger Guy
    Sep 12 at 13:57
  • @HandyHowie Yea, as I understand more. I think you're right. I think I was applying that archetypal circuit example too broadly. Specifically, I have a sliding car door that won't open. I'm trying to understand what the best way to diagnose that problem would be and in so doing I started learning about various ways, one of them being to use a bi-directional scanner. Hence, I was trying to understand how scanners control components better. I'll reword the question once I get a better grip on it. Thanks.
    – TLex
    Sep 12 at 23:12
  • @TigerGuy Ahh, that sounds pretty sophisticated and cool. That throws a wrench (no pun intended) into my general understanding of automotive circuitry. In the end, it really depends on the circuit, it would seem.
    – TLex
    Sep 12 at 23:14

1 Answer 1


If by a "scanner" you mean something on the lines of OBD2 or what have you, the answer is, they don't control the electrical components directly. They tell the ECU (or other controller) to "do something" and the ECU (or other controller) does the something.

The scanner doesn't breach over the line from input to power on a relay. Think of a relay as having two sides. The input side, which is where the switch activates it, and the power side, which is the part of the relay which provides power to the high draw piece of equipment.

  • Yea, sorry, I was a bit loose with my language. I understand that the scanner doesn't directly control electrical components, but rather tells the ECU to "do something." I guess what I really meant to ask is how does the ECU control electrical component? Is the ECU somehow connected to the input side of the relay for each component? If so, how is it connected?
    – TLex
    Sep 12 at 3:22
  • @TLex - Either directly if low power (ie: Injectors, throttle body, sensor) or through a relay if higher power (ie: starter). There is a ton of wiring inside a vehicle. Here is a good example of a wiring harness with the ECU (bottom right) Sep 12 at 10:33
  • I see. So, then pretty much all signals coming from switches or sensors first go to the ECU? And then the ECU activates the power side of the circuit either directly or by sending an "on" signal through a relay?
    – TLex
    Sep 12 at 23:07
  • Not exactly. The signals to go whatever controller is in charge. There are multiple. Whatever controller is in charge will send out the signal for something to operate, whether that is directly or through a relay. Mind you, the big difference between whether something is controlled directly or through a relay is the amount of juice it takes to operate it. The controllers can only handle so much power, then it has to tell something else (the relay) to handle the load. Sep 12 at 23:17

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