问题:2002s-104轮驱动键按四轮驱动键时高低闪过一次并返回二轮驱动

when i push 4 wheel drive button either hi or low it blinks once and goes back to 2 wheel drive I have replaced bad cable from the vacuum switch under battery box to transfer case the vacuum switch seems to work and have tried to hold the vacuum switch back by hand with truck running and it did not make a difference also i replaced the little switch on the transfer case that has 3 vacuum lines coming into it and on other end has something like a ball bearing sticking out, this is the one that is tucked up under the axle or driveshaft also I have replaced the dashboard switch what do i do next, there is a hose that comes up from under the engine and it is connected to something that looks like a vacuum pump but i am not sure what it is and it comes up to the top of the engine and i can not find out where it hooks into, it is about 1/2 inside dia hose it is kind of like a heater hose and the end of this hose is open is it a breather hose, even my mech.无法解析

我的车有15万里
我的车自动传输