Dave of EEVBlog posted a blog video recently where he talks at great length about relays and the need for snubber / freewheeling diodes to protect the switching transistor.  We all will be familiar with the problem he describes. There are numerous posts from guys whose starter relay suddenly ceases to work and they eventually find out the ICU has been toasted.  The reason for the toasted ICU is the super high voltage caused by backEMF. across the starter relay.

https://www.eevblog.com/2021/07/31/eevblog-1409-the-dangers-of-inductor-back-emf/

It's a very technical discourse about the electronics of relay switching circuits so be warned.  This is for entertainment purposes.  Dave goes to great lengths to talk about the theory and then spends a great deal of time simulating the problem.

The take away is to be sure to use snubber circuits on the starter relay especially if you're replacing the OEM relay with a different aftermarket one.

Snubber circuits?  Never heard of those before today.

brickrider2 wrote:Snubber circuits?  Never heard of those before today.

Ignore at your peril. Just be careful if you ever need a replacement relay.

I vaguely remember snubber circuits from electrical theory classes bac in the late '70s. But can we get a "you need to add this" type tutorial post, for us that the theory of why hurts our brains?