Originally Posted by LATECH
If you wire the derale ground circuit to the OE relay coil ground as I described, it will prolong the life of the controller.
If you dont have the same setup as the OP just add a relay for the fan circuit, prefferably a 40 amp bosch, and use the derale controller relay to activate the bosch relay COIL, that way the derale control will last as it is only handling a small amount of current to run the Bosch relay which is handling the Load. And the bosch job is dispensable, or at least more so as it is quite a bit cheaper than the derale unit.
Now that's a reasonable idea. Just might have the controller relay trigger a pair of relays in parallel to help prolong the life of the secondary circuit and provide a backup. Might be a bit of overkill, but I hate it when you see the temp a little above normal and realize your fan won't come on.
Thanks for the suggestion.