Hello all! I have a 96 blazer with manual 4wd and the lights are on the lower center console. When I switch into 4wd the light with the front half of the drive train lights up. But when I drive down the road it will intermittently turn off and on. If I'm cruising it will turn off until I push in the clutch and then it comes back on. Any ideas?

 

Most likely a loss of vacuum to the front axle actuator under the battery tray. Check the following for vacuum leaks: vacuum lines to and from the vacuum switch on the transfer case, the front axle actuator, vacuum reservoir, vacuum check valve, vacuum source line to the check valve. Best to use a hand held vacuum pump with a gage on it.

 

I did a quick check of the vacuum system with just my finger. it is a strong vacuum at the ball and check valve. the only other place i could get to was by the battery where there is a T connection where it T's off to a bigger hose with a switch on it. that switch must be the feedback that the diff is locked in. Now the vacuum is very weak coming into that T line. so it must be at the vacuum switch which i cant easily get to. I will either have to pull the distributor or jack it up and try to get those lines up to the top so i can check there. I might as well buy a new switch anyways since i will be under there. Now can the switch cause this? how does it work?

 

The switch has two positions, the brass valve inside is spring loaded and it slides back and forth when linkage inside the transfer case pushes on it.



The valve is sealed with O rings. Depending on which O ring leaks, vacuum to the actuator could be weak. When you accelerate, vacuum drops and it could drop low enough to disengage the axle actuator. Let off the gas, vacuum increases, axle engages again.

The vacuum switch is on the driver side of the transfer case. You can see the switch just above the shift lever in this image:



There are 3 vacuum lines at the switch. The lines are in a keyed rubber connector that plugs into the switch so it can only be connected one way: One line has manifold vacuum when the engine is running. One goes to the vacuum actuator under the battery. The 3rd line goes up to vacuum Tee. One side of the Tee goes to the steel vent tube just above the switch. The other side of the Tee goes to a filter that "should be" zip tied near the top of the transmission dipstick tube with a small filter in the end.

Edit: If you disconnect the connector at the vacuum switch with the engine at idle, one line should have manifold vacuum. Shut the engine off and it should hold vacuum. One line should be unrestricted and not hold vacuum, (vent line). The third line, if you apply vacuum to it, the acutator under the battery should pull the cable to the front axle, and hold vacuum indefinitely.

 

That is a great post! I cant thank you enough. I jumped under the beast and found that one of the vacuum hoses had been rubbing on the transmission and rubbed a little tiny cut through one of the vacuum lines. another one of the hoses had a bulge in it right at a T connection i shorted that one and put a temporary splice in the one with the cut. Thank you again for the help!!! Fixed. I hope this helps others who may have the same problem. I will be getting all new hoses and connectors for next weekends project.

 

Good job! Get some new line on there and you'll be good to go. Thanks for posting back.