I am needing the vacuum specs. for a 1998 chevy blazer. 4WD won't engage when you push button but if you run a vacuum line directly to the actuator it engages. The vacuum line running from the vacuum switch has very little vacuum but has new switch. How much vacuum should be on the line coming from the switch?

 

Does the a/c controls work(floor,vent, defrost)? I mean change when selected.
I would check where the vacuum is supplied on the intake. That connection dry rots.

 

Vacuum level at the actuator when the 4wd is engaged should be close to the vacuum level present at the intake manifold vacuum source.

 

ArRay68 & swartlkk - I have verified all the controls work. I did find a bad vacuum cable line on the intake but still have very low vacuum at the 4wd actuator. thx

 

see if you can locate the check valve from the canister to the 4wd solenoid. that sounds like it will be isolated to the line that is from the canister to the solenoid. My 2 cents have been applied.

 

i just brain stormed. You may split the control vacuum line to connect to the solenoid for the 4wd. WYE off it is what I am trying to say.

 

there is a vac switching valve that is on top of the transfer case, it has a ball and spring in it and when 4wd is on it sends the vac to the actuator. there is a vac hose that runs from the engine to the transfer case and then couples with 2 other hoses into a connector that plugs into the switching valve. check the line going from the motor to the t case and make sure there is no holes or cuts if thats ok take the switching valve out and see if its stuck they are under $20 i just had to replace mine