No 4x4 light on dash?

2003 ford f150 :

When i turn the shift on the fly switch on the dash i do not get a 4x4 light, no relay sounds or any resistance as if it shifted into 4x4. The 4x4 light does illuminate briefly when the engine is started so i know the light does operate. Where do I go from here to troubleshoot 4x4 system.

Since the 4x4 lights at least come on during the key on process then mostof the electrical system should be good but it wouldnt hurt to double check all the fuses in both the underhood and interior fuse panels and be sure they are ok.The F150 has a vacuum actuated shift fork mechanism in the right front axle tube off the front differential and the shift fork like to break. so thats one item to check but if the light isnt comin on at all when shifted in 4x4 you would need to see if the shift motor is turning the transfer case gearset to apply the front driveshaft. A diagnostic check for codes of the GEM/4X4 module would be key and you can check the motor operation as welll. Shift motors are common failures as well.

Can i myself scan  GEM/4x4 codes or it has to be done by a dealer . Also I have heard of common shift motor failures but would that be the cause of no light or relay sounds when switch is engaged.

A dealer or a shop with a dealer style scan tool capable of working with the GEM/4X4 module.Auto part shops only sell engine control module readers. The light wont come on until the gear range high or low is engaged and it moves the switch inside the transfer case. You can remove the motor and manually turn the shift motor linkage to each range and verify you have 4x4 engagement and condemn the motor, but its nice to be sure before buying a parts thats a few hundred bucks.

That will help.

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