Four by four vacuum line off

Tiny
BRIAN WINEMILLER
  • MEMBER
  • 1995 CHEVROLET S-10
  • 4.3L
  • V6
  • 4WD
  • AUTOMATIC
  • 144,000 MILES
I have three line that go from the transfer case up to top of engine, but I do not know where the third one goes.
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Wednesday, December 21st, 2016 AT 12:22 PM

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Tiny
STEVE W.
  • EXPERT
The lovely T-Case vacuum switch.
Two lines are vacuum, the third is connected with the T-case vent.

To determine which is what when you cannot see the switch. Connect a chunk of vacuum line to any port and blow into it. Move the line until you find the port you cannot blow through. This is the port that goes to the engine vacuum. Connect it up.
Start the engine and shift the case into four wheel drive. Now you should find one port of the two left has vacuum. That port goes to the axle actuator. The last port is the vent.

The valve works :
In two wheel drive, the incoming vacuum is blocked front axle actuator is disengaged. The actuator port and vent are connected so it cannot move on it's own and the case vent functions.

Shifting into four wheel drive the valve moves and sends vacuum to the actuator, it also blocks the vent port so there is no vacuum applied to the T-Case seals.

Shifting out of four wheel drive the valve moves down, it blocks off the vacuum feed first, then opens the actuator port to the vent line, That lets air into the actuator and it moves and unlocks the front axle.

If you can see the switch. The picture shows which lines go where based on the alignment pin on the switch.
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Wednesday, December 21st, 2016 AT 5:41 PM

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