1. Located the transfer case switch on the top of the transfer case, this switch will have two wires going to it. One wire will be Brown, this is Battery Voltage (B+) coming from the fuse block, the other wire is Blue and goes to the front actuator.
2. If there is B+ on the Brown wire, but no voltage on the Blue with the transfer case in 4x4, the switch will need to be replaced.
3. If there is B+ on both Brown and Blue, but no voltage on the Blue wire at the front actuator, check the wire between the transfer case switch and the front actuator for opens.
At the switch located on the top of the transfer case, check power on the Brown wire. If voltage is not present, check for high resistance or a open wire between the fuse block and transfer case switch.
Sorry for the delay!
Are there any 4wd lights flashing on the dash?
1. Disconnect the actuator in front, with it in 4WD, and see if it has power and ground on the Blue and Black wires.
2. If no power on the Blue wire, then check for power on the Blue wire backprobed at the switch on the transfer case.
3. If still no voltage, then check for power on the Brown wire at the switch on top of the transfer case.
4. If the Brown wire shows no voltage, verify if the 4WD fuse is blown.
Power is supplied on the Brown wire to the switch and when put in 4WD, the switch on the transfer case closes and puts power out on the Blue wire to the actuator in the front. The actuator is always grounded, so when it gets power on the Blue wire, it should activate to lock the front end.
Here is also a TSB for this problem.
Drivetrain - Slow Or No Engagement of Front Axle
File In Section: 4 Drive Axle
Bulletin No.: 76-43-01A
Date: September, 1998
Subject:Slow or no Engagement of Front Axle when 4WD is selected (Replace Front Axle Actuator)
Models:1988-97 Chevrolet and GMC K Modelsexcluding 1988-93 Models over 8500 # XXXXX
This bulletin is being revised to add information to the Service Procedure, add a new part number, and include schematics. Please discard Corporate Bulletin Number 76-43-01 (SectionXXXXXAxle).
Some owners may comment that the colder the outside temperature is, the longer it takes for the front axle to engage whenXXXXXis selected, or the front axle does not engage.
The front axle engagement actuator is a thermally activated component. Because of this characteristic, the time required for the actuator to completeXXXXXengagement is extended as the temperature drops.
Replace the front axle actuator and include the applicable wiring harness kit. Refer to the Parts Information in order to determine which wiring harness kit is needed.
1996 model trucks with a build date on or after January 2, 1996 are considered as "961". If the build date of the truck is unknown, verify the 3 character "axle code" on the front axle label in order to determine which wire harness to order.
This new actuator uses a motor to engage the front axle and is not affected by temperature. On 1988-93 models with a GVW rating over 8500$, the new actuator cannot be installed and the thermal actuator will remain the service part.
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