The dash light on my 2003 F 150 will not illuminate when engaged in four wheel drive.  But the light will function during vehicle start up.  Since the light does work would checking back from the 4x4 switch at the transfer case be a logical way to go?  Thanks,  ed9750

IMHO: Yes, but with all the computer interfaces that relate to lights and switches, it may be a dirty or corroded connection in-line.

For reference read the second post in this thread.

Thanks for information especially the reference thread.  Will check for a dirty connection at the transfer case.

Could also be the actual switch at the transfer case. It might be bad or for some other reason, not closing the circuit when the transfer case is placed in 4 wheel drive. If you can access the switch, you should be able to check it and the operation of it with an ohm meter.

I am not familiar with the F150 since I own a Dodge and a Chevy. The Dodge has the switch at the transfer case and the Chevy has it at the front differential. With Ford I don't know where it would be, but it might not be at the transfer case.

It could be the motor on the transfer case and Ford has tons of problems with these.You will not go out cheap on this,the Ford 4wd systems are the most expensive to fix than a GM and Dodge.

Well the reference thread that Gerry D mentioned had some interesting information about the transfer case switch. When 4x4 is engaged a working switch will ground the circuit so the dash lites; the gray wire for 4x4 H and the light blue/stripe wire for 4x4 low. So to test to see if the switch is bad I put a ground on the gray wire(couldn't get the connector off the switch so just fed in a wire for grounding thru the connector's wire end) and the dash light lit! Did the same with the light blue/stripe wire and the 4x4 low dash light lit also. So I'm assuming that the switch is bad since both dash lights worked when the function of the switch(grounding)was done. I still have to double check for corrosion by removing the switch's connector. It has two " hold" clips and is hard to pop off, but I'm guessing it will be clean so the switch is bad. Thanks for everyone's help.

Excellent job of diagnostics. You are on the home stretch now.   In addition to a bad switch, it could also be something internal which is supposed to actuate the switch but isn't doing its job.

Way to go, Gerry! Great support for our HCOA family.

Well I just would like everyone to know that I finally found some free time and replaced the switch at the transfer case we talked about. It was a slight pain to remove as there isn't much room; you all know I'm sure the routine: shimmy a box end wrench up around the nut, make a quarter or less turn of the nut, remove and flip the wrench over and make another quarter or less turn, etc. There almost appeared to be an intentional depression in the transfer case to allow you to do this. Anyway once the new switch was installed and connected the 4x4 and 4x4 Low dash lights worked when engaged! Odd too, that the Ford Service manual didn't mention checking power at the switch. Thanks again. Ed