How is it possible to test an automotive relay to determine that the flyback diode is not shorted or open?


2 Answers 2


You apparently have a relay with integrated diode accross the coil in the same unit. If this is really the case, you should say so explicitly. The term "automotive relay" by itself doesn't specify this. It only means a relay intended for automobile use.

The diode will limit the voltage drop in one direction. Use a 12 V supply (can be the 12 V from your car) with about 200 Ω in series. Briefly connect this to the relay coil in each direction and measure the voltage accross the coil. If you get over 1 V in one orientation and under 1 V in the other, then the diode is working. If you get less than 1 V both ways, use a smaller resistor and repeat until you do get over 1 V in one direction.

Keep the power dissipation of the resistor in mind. 200 Ω directly accross 12 V would dissipate 12V2 / 200Ω = 720 mW. A 1/2 W resistor can take that for a short time, but don't keep it connected for more than a second or so to get the voltmeter reading. Also don't hold the resistor directly with your hand. It will get hot enough to hurt quickly. The best would be to use a resistor rated for the power, or multiple smaller resistors to effectively get one higher power resistor.


Your question requires a bit more clarification.

As far as I understand,

you can check the flyback diode by disconnecting one terminal of the diode from circuit and can test it as with resistance meter to check if it is not shorted on both side as a simple diode.

To test a relay you have to check its contact changing with continuity testing.

Please do some research before you ask. such kind of problem are well answered plenty of times over different web pages of internet. One relay testing link is here. If it is not what you looking for, then please describe a more in you question.


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