How do you reprogram the ECM to allow the fan to turn on at a lower temp.?

 

Mm.. that's easy. Replace the fan switch at the back of the passenger side cylinder head. You ECM has no say in the process.

Edit: For clarification, if your ECM did decide the fan on/off temps, it would cost about 300 bucks in chip burning equipment and software to change even that one little setting. Free definition files aren't available for our V6s (to my knowledge) .. Tunercat would be required.

 

The '90-'92 SD systems have the cooling fan controlled by the ECM. There is an EPROM inside the ECM that holds the code and calibration parameters. Can either erase & reprogram that EPROM (not advised) or get another EPROM or Flash ROM and reprogram that. It is just 4 parameters and the checksum that require changing.

If you have access to an EPROM programmer and a hex editor you can do it for almost no cost. There is an intro to PROM buring on the tech board.

RBob.

 

Well, now that you've put up what year your car is, there's no question it's on a thermal swich.. look into a hypertech switch (bleh) or read Willie's "Total Fan Control" tech article (yes).