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2002 F350 4x4, what is this fitting for (see red arrow)? It is located on the top, front of the steering knuckle and has a rubber cover, which I've removed for the picture.
Is that the fitting for the ESOF vacuum line (to engage/disengage the hubs)?
Do your hubs work when you flip the switch in the cab (if equipped)? If not, that's probably why. It's common when the hubs stop working to just cap off the lines and work the hubs manually, switching between auto/lock when needed.
its the vacuum fitting for the ESOF system. if you have the transfer case shift lever on the floor you don't have / have no need for the vacuum system, which is why it's capped off.
its the esof vacuum line fitting, it doesnt matter if you have it or not ALL Fords have the nipple in the knuckle. If it were me id remove the nipple and install a stainless 1/4" plug. My dads 2000 rubber cap rotted off and for years water/salt/ etc got into the nipple and rusted up everything really good.
Hmmm. that is kind of surprising that there would be a difference. I would expect them to be the same?
Maybe the 550 front axles are different? I didn't think they were, other than the wheel hubs and the brake rotors. Maybe the absence of the fitting is another difference?
Back in 2000, ESOF was not even an option on the F-550 (nor F-450 chassis cab). ALL F-550's were manual transfer case only... even the Lariats (mine is a Lariat).