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schematic

simulate this circuit – Schematic created using CircuitLab

I am working on a project on my Jeep and I wanted to tap into the ground wire between the turn signal switch and the body control module and use that to activate a relay when the turn signal switch is engaged. I am worried about the voltage spike of the relay coil when it disengages. I need to make sure nothing makes it back to the body control module. I am using a Cole Hersee RC-400112-DN relay with diode. Since the switch is taking the ground away and leaving the negative side of the coil tied directly to the body control module will this cause any voltage to shoot towards the BCM? If so what can I do to protect the BCM?

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In the absence of hard information, just assume you need to put a reverse biased protection diode across your relay coil. Diodes are cheap, modules are expensive.

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  • \$\begingroup\$ I changed the diagram to illustrate the diode built into the relay. I really dont know what goes on in the BCM. I just know that once it gets its ground signal thru the wire then it outputs 12V thru another wire for the turn signals or flasher. Will the onboard diode in the relay protect the BCM even with a lack of ground once the switch is turned off and ground is lost? Im just scared of that direct link to the BCM once the ground goes away. \$\endgroup\$
    – Scott C.
    Jan 28 '17 at 2:30
  • \$\begingroup\$ @ScottC So with the relay coil and diode the voltage range seen on the IO pin on your module will be between 0V and 12.7V. If the module normally just has a switch to ground to sense the turn signal switch, then it must have a pull-up of some sort on the IO pin that creates a logic high when the switch is open. In that case, you can use a multimeter to figure out what voltage the pin is being pulled to. Just measure the pin value with the switch open and your coil unconnected. If you measure 12V then you know the pin normally tolerates 12V and you are probably OK connecting the relay coil. \$\endgroup\$
    – user4574
    Jan 28 '17 at 15:25
  • \$\begingroup\$ When the BCM senses a ground at any of the 3 wires I want to tap into it will then output 12V to the turn signal flasher depending on which of the 3 wires is grounded (RH turn, LH turn or 4-way). Is that what you mean? Im a little over my head. Sorry. But it should all be 12V Im dealing with. Dont think anything Im dealing with has higher voltage than that. I guess when vehicle is running voltage goes into low 14 volts with alternator charging. \$\endgroup\$
    – Scott C.
    Jan 28 '17 at 23:21
  • \$\begingroup\$ @ScottC. The module itself could be using any voltage it wants on the I/O pin that connects to SW1. Most electronics don't run directly on 12V anymore. The module almost certainly has converters inside of it that make either 5V or 3.3V that it uses internally. The IO pin may operate on 12V, but it is very possible that the IO pin only wants to see 3.3V or 5V. You can measure that by opening SW1 and probing the pin to see what voltage it naturally pulls up to. \$\endgroup\$
    – user4574
    Jan 29 '17 at 12:57
  • \$\begingroup\$ okay kinda like checking the ground side of the relay coil for voltage and showing 12V even tho the current isnt actually running thru or the relay engaging. Its the voltage potential basically right? I think I understand. I will check that. \$\endgroup\$
    – Scott C.
    Jan 30 '17 at 22:43
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What type of Jeep are you working on and what are you trying to accomplish?

If you're unsure, I don't see how adding an additional diode could hurt anything, and like someone said, they're cheap.

schematic

simulate this circuit – Schematic created using CircuitLab

Alternatively, it may be safer to trigger the relay from the turn signal circuit (the power running to the light), but that will of course make the relay click on and off with your signal). Hard to say without knowing what you're trying to do.

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