1991 K2500 pickup, 350tbi,4l80 trans, 4wd not
On the full size, the front actuator is electric, not vacuum.
When in 4wd mode, a switch on top of the transfer case conducts 12
volts out to the actuator. Wire #50 (brown) is the 12v feed, and it
conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.
Check voltage at the connector for the front actuator (at the
front differential - it looks like a large bullet that threads into the
front diff.) If it has 12 volts, the actuator is the problem. If it
does not have 12 volts, check the transfer case switch and the power
feed to the transfer case.
I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated
by voltage. When it heats, the pin in the end swells and pushes a shift
fork in the front differential. The shift fork engages a spline on the
passenger side of the differential, and boom! you have 4WD.
GM offered an upgrade to this actuator that is a motor instead of a
heated device. However, it requires a small harness addition, a spacer,
and a new actuator. It is about a $150 to $200 option.
luck. The actuator is usually the problem 90% of time but recheck all
of the wire harness just to maker sure there is no short before
replacing any parts.
88-UP C/K ACTUATOR UPGRADE
VEHICLE SERVICED: 1994 CHEVROLET FULL SIZE
PURPOSE: IMPROVE AXLE CONNECT ENGAGEMENT TIME
SYMPTOMS: NO FRONT DRIVE AXLE ENGAGEMENT WHEN SHIFTED
Obtain the following GM part numbers:
(1) 26060073 Actuator
(1) 88959465 Harness/Spacer Kit
on Feb 02, 2012