Scrufffy

I'm glad to someone else has blazed this path! I'm very new to all of this and the learning curve has been steep. That 1k pull-up resistor is interesting. I've never seen it used for PWM on anything else, but I bet it makes for a much cleaner signal coming from the ECU... How big of flyback are you using? I know when it's going to a solenoid, it's supposed to be rated for the amperage that it takes to operate the solenoid. But that's not really feasible with pumps drawing 20+ amps and fans drawing 40+ amps...

Any time you're running an inductive load it's a good idea to use a flyback diode. It'll even help normal relays last longer by saving the contacts from extra arc burn. I'm wondering if anybody has tried these Crydom SSR's? They have a huge current capacity, accept PWM, they isolate the ECU from any noise from the load with a optocoupler, and you can pick up a heat sync for cheap on amazon. I ordered two 60a (overkill, but they were unused old stock for 40% off) ones off eBay and am going to test them with a fuel pump and cooling fan with PWM from the ECU. But I will definitely be using some big diodes to protect them. They even have a wiring diagram showing how to wire the diode for an inductive load. I'm still waiting on delivery but I'll try and report back when I see how they work.

Great writeup! I really wish I could see the pictures though