[epapp]

Well, no, and that is JFPs point. I agree it would be hard to try and manipulate anything the DME has info/control over.

BUT, why not tap the data for coolant temp (or get crazy and add your own temp probe [easy]) and then simply make yourself look like the DME (activate the signal pin of the relay that the DME normally controls). I have tested this for the coolant fans and it works as I described.

You use your own control logic to activate the existing (well built) circuit/relay, and have the added benefit of the DME still controlling the fans if its (the stock temp) conditions are met.

Whether or not you need to compliment the DMEs ability to control the fans is still something I'm not convinced of. My engine compartment fan comes on in traffic but it IS doing its job, and I have indicators to tell me when it is on and off, just to monitor. It isnt something I want to necessarily control however.

The only thing I DO want to add is a control loop to kick the high speed fans on if the low speed fans have been continuously on for x amount of time. Pushing it on the freeway, the low speed fans will stay on for loooong periods of time, which isnt bad, but if its a really hot day and the low fans have been on for a while, just kick on the high speed fans and get the temp down (this already happens if you are WOT for some amount of time, but 5th gear 4-5k on a 105 degree day, temps will stabilize around 214 or so, just above the low speed ON condition, leaving them on for quite a bit of time)

I guess this would actually help as ~214 for long periods isn't desirable, but isn't horrific either.