No 4 Hi or 4 Lo but auto 4 wd works

[327chevy]

This is the oddest thing I've run accross with the 4wd system in my 99 Silverado.To start with it has the Auto Trac transfercase (push button) and had been working fine but recently I had an occasion to need to lock it in for traction in the snow and 4 Hi would not engage the front wheels.I tried 4 Lo and the same thing,no front drive so I tried auto fwd and that worked.The lights on the button console are all working fine,push the button,the lights blink and it goes into the mode selcted.The shift motor on the t-case seems to be working,I can hear it so I'm assuming all's ok with that.I'm going to get it scanned hopfully soon so I can get an idea of what's wrong but will a 4wd problem show up on a Snap On scanner?

[Blue Bowtie]

If the scanner has the correct personality module it should read the manufacturer's codes. If it's a generic OBD-II, probably not.

I'd be looking at the vacuum control to the front axle lock actuator, the cable, and the actuator itself.

[Sublime69]

I had a problem where my 4x4 was doin weird shit. Would work some times and not another... I just went to the dealer and bought a new switch for about $50. problem solved.

[Blue Bowtie]

Nice Dart there.