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Do I Need Blocking Diodes?


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xcalibr1 
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Posted: January 11, 2017 at 9:46 PM / IP Logged Link to Post Post Reply Quote xcalibr1
I want to tap into the ground inputs on a body control module on a 2002 Jeep Grand Cherokee for the turn signals and hazard lights. I want those grounds to ground the coils of relays to cut power to DRLs anytime the turn signals or flashers are on. My question is do I need to use blocking diodes to protect from any surges going into the BCM when the coils are de-energized even though I am on the ground side of a circuit? Im using 4 relays with the maximum of 2 relays being used at any given time on a ground signal. Either side turn signal will engage one relay per side and the flashers will engage 2 relays at the same time. If I need diodes then what exactly would I need and which direction do they get installed? I have installed many remote starts in my vehicles but never did anything that used diodes so I am clueless as to their use and sizing.
the12volt 
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Posted: January 11, 2017 at 10:02 PM / IP Logged Link to Post Post Reply Quote the12volt
Use diodes across the coils of the relays with the cathode side of the diode (side with stripe) on the relay coil terminal you supply 12V + and the anode side of the diode to the other coil terminal of the relay where you supply ground from the BCM.
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xcalibr1 
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Posted: January 12, 2017 at 6:34 AM / IP Logged Link to Post Post Reply Quote xcalibr1
Im sorry for being stupid here. As I said I have never wired anything quite like this. But the ground is not coming from the BCM. When you engage the turn signal lever or flasher switch you are completing a circuit to ground for the BCM on any of the 3 wires. The BCM is not supplying the ground. Does that change anything? I also need to know what diodes to use. I can post a pic of wiring schematic if that helps. Thanks.
the12volt 
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Posted: January 12, 2017 at 6:44 AM / IP Logged Link to Post Post Reply Quote the12volt
Whether the ground signals you are using to trigger the relays are coming from the BCM or going to the BCM, the diode connections will be the same.
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xcalibr1 
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Posted: January 12, 2017 at 7:24 PM / IP Logged Link to Post Post Reply Quote xcalibr1
Do I Need Blocking Diodes? -- posted image.
I found these relays that already have diodes in. Makes it easier for me. Should work right? Would it be overkill to still put diodes directly into the 3 wires between the point where I tap in and the BCM itself?
the12volt 
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Posted: January 12, 2017 at 7:42 PM / IP Logged Link to Post Post Reply Quote the12volt
Putting the diodes inline would be of no benefit at all. They will only suppress a spike when placed across the coil.
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xcalibr1 
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Posted: January 12, 2017 at 7:51 PM / IP Logged Link to Post Post Reply Quote xcalibr1
Okay. That shows what I know. I was only thinking of it as a way of making sure positive voltage could in no way ever reach the BCM.

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