Question about relays and diodes

Thread Starter

John appleseed

Joined May 20, 2020
101
I have a 12vdc relay in my truck. The postive is fed directly from the battery and the negative is Coming from the bcm (switched negitive through body control module). My question is do i still need a snubber diode? Wouldnt the flyback voltage go back to the battery not the bcm considering the bcm is the relays negative? Is this harmful to the battery? Thanks
 

narkeleptk

Joined Mar 11, 2019
558
@Wolframore
Can't say I run across too many oem relays in auto that use the ones with the diodes built in. From what I see the protection is usually just handled by the driver ic's themselves. Not much need for any external diodes.
 

Thread Starter

John appleseed

Joined May 20, 2020
101
Can't say I run across too many oem relays in auto with the diodes already in them. From what I see the protection is usually just handled by the driver ic's itself. Not much need for any external diodes.
I was told its bad for the trucks bcm if the relay isnt flyback protected
 

narkeleptk

Joined Mar 11, 2019
558
Well I couldn't tell you if its bad or not but you can go check the oem relays in your truck now. See how many have diodes. My guess none. Almost always just a resistor.
 
Last edited:

shortbus

Joined Sep 30, 2009
10,045
Even though they don't show up on a factory wiring diagram, some relay wiring in GM vehiclhes do have diodes built into the wire harness. I know this because of working where the harnesses were made. These were wires controlling relays coming from ECU's And BCU's.
 

MrChips

Joined Oct 2, 2009
30,720
Whether or not the relay has a snubber diode it doesn't hurt to put another one across the relay coil.
Just make sure that you get the polarity orientation right.
Any 1N4001-1N4007 will do.
 
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