MOSFETs as relays

I've got some 60A-90A assorted MOSFETs that I'm thinking of using to replace (or add in as) some of the relays under the hood of the XJ. Has anyone tried this? With 12v circuits, it should be easy to do with just a resistor (depending on configuration). These have VERY low resistance 'on', so dissipation with most loads, like fan, compressor, etc, should not be a problem.

How did they perform? Any reason why not? Anything I should take into account in this type of environment?

Conceivably, I could even put them into little boxes like the relays, but they should last forever, rather than physical contacts...

That should work if you know what you're doing. But--HOLY CRAP!--where'd you get such high-amperage ones?

My '99 Neon R/T has a MOSFET as an OEM Solid State Fan Relay. They cost about a $100 to replace, but at the same time, they should never wear out.

The high cost is probably why you never seen them implemented as Relays from the Factory. I don't see why it wouldn't work, if you retro-fitted some MOSFETS you had on hand. As long as you did it properly, I'm guessing a balancing bridge wouldn't be that hard, but I'm inexperienced enough I would probably screw it up.

The reason why a Cheapo Neon R/T would have a MOSFET as a Relay. The Fan is no longer a simple on/off affair, the Engine Computer varies speed of the electric fan according to need, NOT simply turn it on and off. It does this thru Pulse Width Modulation (PWM), the most effective way of controlling electric motor speed. The Engine Computer just outputs a Square Wave to the Electric Fan and changes the square wave as necessary. A simple Mechanical Relay could never keep with the Square Wave, so they use a MOSFET to simply amplify the square wave with the current needed. To prevent the explosion of Dealer Techs Heads and reduce them totally screwing up the troubleshooting, they just call the MOSFET a Solid State Relay.

OOOOhh, I hadn't thought of PWM too! That would be GREAT for running the electric fan at reduced speed to lower the noise, yet have it on all the time! And for the AC fan too - continuously variable! All I'd need is a 555 timer and a pot...

I went to Digi-key and found some with great specs for 12v applications (like 60Vds max) and high amps for less than a couple bucks apiece. Got them to play with around the Jeep, like controlling accessories I put in, but then I got to thinking, 'why not replace some of the native stuff too?'

A simple switcher with one to bump up the voltage to the power windows and the locks on the 89 isn't out of the question either...