failure rate of automotive 30a relay?

Printed From: the12volt.com
Forum Name: Relays
Forum Discription: Relay Diagrams, SPDT Relays, SPST Relays, DPDT Relays, Latching Relays, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=106066
Printed Date: May 15, 2025 at 5:13 AM

Topic: failure rate of automotive 30a relay?

Posted By: tekstyle
Subject: failure rate of automotive 30a relay?
Date Posted: July 11, 2008 at 9:30 PM

I installed an ignition cutoff and a fuel cutoff with 2 seperate relays for each. I have a third relay that is activated by my alarm's disarm output to activate the first 2 relays.

Because the coil will be energized while the car is disarmed (or while driving), it can be on continously for 30 minutes to even 10 hours (like on a road trip). From experience, these things get hot when you keep them closed for too long. Really don't want them failing on me while I am driving because it will be similar to running out of fuel or pulling the key out.

in all, there are 3 relays. I was wondering what is the failure rate of these relays during normal use?

Replies:

Posted By: i am an idiot
Date Posted: July 11, 2008 at 9:40 PM

If you are not pulling anywhere near the rated 30 amps through the relay, you should have no problems with them. They are designed for constant duty.

Posted By: tekstyle
Date Posted: July 11, 2008 at 11:49 PM

cool thanks a lot!

If it does fail, which part typically fails first? the coil, the bridge contacts between 87 to 30, or the return spring?

Posted By: i am an idiot
Date Posted: July 12, 2008 at 3:45 AM

I can not answer that question. I am sure that they do fail, but I have never seen it happen. Years ago I experienced a defective Potter and Brumfield relay. I do not remember the failure, but I have never seen a Bosch / now Tyco relay fail. Tyco now owns Potter and Brumfield too. I am sure they fail I have just never experienced one.

Posted By: KPierson
Date Posted: July 12, 2008 at 12:55 PM

Typically when a relay fails it is the contacts that go bad - this is especially true if you are switching the fuel pump under load (or any other big load). Switching the relay on and off while power is applied causes the contacts to arc - eventually wearing them out.

Relays are typically designed to withstand 1,000,000 switching cycles under full load. In a car, you will most likely never switch that many cycles. You can pull up some datasheets for 30A automotive relays on digikey.com if you know how to search correctly.

The best advice I can give you is to just keep an extra few relays in the glovebox. Use high quality sockets and if you ever have a problem starting the car you can try swapping out the relays. Better yet, you could make some jumper wires to install in the sockets to bypass the relays all together in the case of emergency. Of course, this backup plan could severely limit the overall security of the vehicle, but a sacrifice will have to be made somewhere.

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Kevin Pierson