How do I fix my front wheel drive system on my 2004 ATV 400 Sportsman Late model? After extensive work and parts replacement my all wheel drive is not working.

I have battled with my 4wd, did many of the same parts changes you did. Put it in the garage and was about to tear it all apart, and I brushed against the red kill switch on the left handlebar, and the 4wd icon on the dash lit up, and everything worked! There is a little dead spot in the switch, and if it isn't clicked right in the middle, AWD doesn't work. Once I figured this out, it works great! Hope it's as simple as that for someone else!

Just a suggestion but you should consider soldering all your electrical connections to insure they are weather and vibration proof. Additionally, I added a small 3 amp circuit breaker, but a 1.5 to 2 Amp automotive fuse and fuse holder will do nicely.

Process:

1- Disassemble the 'Instrument/Light' cluster cover.

2- Remove the 'Radiator access' panel.

>>>Perform This Test BEFORE Proceeding and make SURE all power is OFF!<<<

3-On the drivers LEFT side of the radiator access area, locate two (2) connectors, each with SOLID GREY wires and BROWN wires with a WHITE Stripe on top and smaller diameter Solid Grey and Brown wires with a White stripe leading down.
--a) Disconnect both connectors by pulling the top and bottom sections away from each other. (DO NOT pull the WIRES, use the connector body to pull).
--b) Test EACH of the two (2) connectors with the SMALL DIAMETER wires leading down. (see Hub Coil Connectors pix)
--c) Using an OHM meter, measure the resistance across the two pins (one connector at a time) for each connector. Depending on the accuracy and sensitivity of the meter, each connector should read between 25 and 28 OHMS. NOTE: If EITHER of the two (2) Hub Coils reads ZERO (0) or less than 25 or Greater than 28 OHMS STOP!!! The Hub Coil itself OR the wires OR the connectors may be defective or in need of repair!
--d) Using an OHM meter, on EACH of the two connectors, measure the resistance between one pin of each connector and GROUND (any bare metal point on the ATV frame). NOTE: If EITHER of the two (2) connectors resistance to GROUND reads anything GREATER/ABOVE than ZERO ( 0 ) OHMS STOP!!! The Hub Coil itself OR the wires OR the connectors may be defective or in need of repair!

>>>If the test in 'c)' or 'd)' FAIL STOP, DO NOT proceed!! The Hub Coil itself OR the wires OR the connectors may be defective and MUST be repaired before ANYTHING else is done!<<<

4-(For my model) On the right handlebar, check the lamp in the AWD switch to make sure it is not burned out or missing. Replace lamp if necessary. (Not essential to the AWD operation, just nice to have to confirm voltage is being applied to the front wheel Hub Coils)

5-In the 'Radiator Access' area, locate the Red/White stripe wires providing (switched) full time + (battery plus) voltage (accessory power) via a ¼" female connector. "Accessory Leads" - in the wiring diagram.

6-Using a volt meter (or continuity light), CONFIRM that the terminal is OFF (NO voltage) when the ignition switch and the 'Engine STOP/START switch (on the left handlebar) are both OFF, and has + (positive) battery voltage, when the ignition switch and 'Engine STOP/START switch are both ON, with the engine NOT running.