Hello,
Not that we get tons of snow but occassionally I'll put my '98 Expy in 4HI when driving in snow and as long as I can remember it will sometimes switch back to AWD for a second and then return to 4HI, and I can hear a relay switching in the dash when this happens. I haven't been able to make it happen on purpose but it seems to happen more often the longer I drive (eg. the warmer the tranny is) - probably up to once/twice every couple of minutes with spurts that will switch back/forth like 4 times in 20 seconds. Obviously it isn't supposed to happen so I'm wondering what I could check.

I have done a full transmission fluid change (including torque conv) which made no difference. I have also checked the vacuum line and switch on the transfer case, for a different problem, which appears to be working correctly. (There is movement when switching from 2WD to 4WD).

This is a '98 which has the electronic selector in the cab with settings of: 2WD - AWD - 4HI - 4LO

I'm not sure if the transmission/x-fer case is actually changing or if it's just the indicator because it's a little hard to tell between AWD and 4WD driving normally in the snow, but the fact that I hear a relay switching is making me think it is.

Any ideas?

Thanks!