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Gary, Chevy Technician
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Experience:  15 years GM master tech
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1990 chevy silverado 4x4 will not shift into 4 wheel drive.

Customer Question

1990 chevy silverado 4x4 will not shift intoXXXXX When selector is put into 4 wheel hi or low there is no grinding, but front wheels do not engage. is this system activated by electric or vaccum?
Submitted: 7 years ago.
Category: Chevy
Expert:  Gary replied 7 years ago.

electrical..the use an actuator in the front diff to physically lock it up..look under the front diff and tell me if #8 in this picture is black or its aluminum..then you need to remove that actuator then turn key on and shift inot 4WD and see if the actuator pushes out so when its in the front diff it will push and lock up the front diff for understand what we need to do?

Gary and 8 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
cant see the picture
Expert:  Gary replied 7 years ago.
click on view it located passenger front of diff right in front where the passenger side diff tube is bolted to the diff..if its aluminum there is a wire pig tail off on it @ 12 inches long to a connector.if its the black one the connector connects right into the black actuator
Customer: replied 7 years ago.
it is aluminum and it does not push out whenXXXXXis engaged. is there a way to test to see if it is that actuator that has failed or if its not getting the signal. i did check the fuse
Expert:  Gary replied 7 years ago.
ok ..what you have to do and this will fix it up is you need to follw this bulliton .they do not make that actuator any needs to be upgraded to this set up and wired according to the suprised that old original one lasted so long.if you do this it will fix it right up..and the parts are available from can not get them aftermarket..prit this out as soon as you get this..the pictures are times to expire.

Technical Service Bulletin
File In Section: 4 Drive Axle

Bulletin No.: 76-43-01A

Date: September, 1998

Slow or no Engagement of Front Axle when 4WD is selected (Replace Front Axle Actuator)

1988-97 Chevrolet and GMC K Models
excluding 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.

Parts Information

26060073Actuator Asm, Frt Drive Axle1
15313272Harness, Jumper (Z Splice)
(86-93 K1-3)

Parts are currently available from GMSPO.

Warranty Information

Some owners with actuators that are operating as designed (5-30 seconds engagement time) may choose to upgrade to the fast actuator at their own expense. The fast actuator is a new product enhancement and should not be used to replace actuators that are operating as designed.

For vehicles repaired under warranty, use:

  1. Remove the front differential carrier shield, if equipped.
  2. If necessary, clean the axle housing in the area around the existing actuator. Disconnect the actuator's electrical connection at the front axle housing.
  3. Remove the front axle actuator by unthreading the actuator from the axle housing.
  4. Install the spacer from the wiring kit into the axle tube with the flat side seated against the shift fork.
  5. Install the actuator motor and tighten until the shoulder of the actuator seats against the axle tube.
Important :

If binding occurs before the actuator is seated, remove the actuator and reseat the spacer by pushing on the spacer with a screwdriver. Then reinstall the actuator.

On K3 Models with the cast iron axle tube, there may be a casting burr which may contact the edge of the actuator. This should not create a concern and the actuator should seat against the axle tube. If this contact remains a concern, use a die grinder in order to remove the burr.

  1. Connect the service harness connector to the actuator and connect the 2-pin connector to the mating connector at the axle. For 1994 to current models, in the case of a complete axle replacement, refer to Figure 1.
  2. Route the wiring harness along the existing harness on the right side of the vehicle. Route CKT 241 (BRN) over the top and to the left of the transmission. Secure the harness using plastic ties.
  3. For 1995-97 interim vehicles, locate the 4-wire connector (C120) on the left side of the transmission. Splice the BRN wire of the actuator harness into cavity D of CKT 241 (BRN) on the engine side of connector C120. Follow the splicing procedure described in Section 8A (Electrical Diagnosis Repair Procedures) of the applicable Service Manual.
Important :

The new Fast Axle Actuator requires only 1 splice.

  • For 1993 and prior vehicles, splice into CKT 50 (BRN) as shown in Figures 2, 3, 4.

  • For 1994 vehicles, splice into CKT 50 (BRN) between connector C152 and P101 (Figures 5, 6).

  • For 1995-97 vehicles, splice into CKT 241 (BRN) between connectors C120 and C100 (Figure 7).
Notice :

Connections must be water tight in order to prevent water from entering the circuit and causing damage to the components.

  1. On K3 models, install the delay relay, included as part of the wiring harness kit, at the engine side of the bulkhead, adjacent to the existing relay. Mount the new relay with self-drilling fastener, P/N 22510339. Route and secure the relay wire harness behind the right oxygen sensor heat shield.
  2. Verify system operation. When shifting the transfer case, you should hear the actuator motor operate.
Important :

This relay incorporates a 5 second delay for the front axle to engage in order to allow the transfer case synchronizer to energize.

For 88-93 vehicles, if the axle has to be replaced, use jumper harness P/N 15313272. CKT 1695 (BLK/WHT) fits into cavity A of the fast actuator. The circuit includes the cable seal and the terminal pin. This circuit plugs into the 5-pin axle actuator connector.

  1. Do the following steps in order to install jumper harness P/N 15313272:
    1. Locate the axle actuator 5-pin connector on the service actuator wiring harness and remove the TPA.
    2. Remove and discard the cavity plug from cavity A.
    3. Install CKT 1695 lead (BLK/WHT) from the jumper harness that has the cable seal and terminal into axle actuator connector cavity A.
    4. Reinstall the TPA on the 5-pin connector.
    5. Install the service actuator harness 5-pin connector into the axle actuator.
    6. Connect jumper harness 2x2 connector into the existing vehicle 4WD mating connector.
    7. Strap the jumper harness and the service harness to the tab on the axle tube.