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Discussion Starter · #1 ·
'99 zr2, 4.3, np233(three button), auto trans, 4x4.
Pretty sure 4 low works but 4 hi isnt engaging or not engaging/ disengaging all the way. Just changed actuator, and change the vacuum switch at the TC 2 years ago with an ac delco switch. It seems like the motor is pushing more gears but the four wheel doesn't actually work in 4hi. What would cause this? Also, how far does one have to go in reverse, to unload the transmission when changing between 2 wheel, 4hi and 4lo? Thanks.

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4x4 Front Differential Operation Explained & Grinding from Front End

the 4x4 vacuum thread.

4x4 Info, manually test

Do you have any codes for the 4wd system?

The diagnostic trouble codes are displayed on the three transfer case shift select buttons. The shift select buttons are located on the instrument panel. When connector pin 13 on the data link cable is grounded (and the ignition switch has been OFF for at least five seconds prior to positioning the ignition switch to RUN), the shift select buttons will blink various times together in order to indicate a diagnostic trouble code from 1 to 4.

Position the ignition switch to OFF. Ensure the ignition switch is positioned to OFF for at least 6 seconds.

Connect pin 13 on the data link cable to a vehicle ground source. The data link connector is located in the cab under the instrument panel on the drivers side.

Position the ignition switch to RUN.

Note the shift select buttons for blinking codes. Refer to Diagnostic Trouble Codes.

If the shift select buttons all blink one time and stop, and do not continue to blink, no fault codes are stored in the TCCM.

The transfer case shift select buttons will blink in order to identify any stored DTC's. If only one code is stored in the TCCM memory, that code will blink repeatedly with a three second delay between blinking sequences. If more than one code is stored, the first code will blink once, then after a three seconds delay, the next code will blink. This sequence will continue until pin 13 is no longer grounded.

When reading the diagnostic trouble codes, the number of shift select buttons blinks will indicate the code number.
Diagnostic Trouble Code (DTC) 1
When the ignition switch is positioned to RUN the TCCM test to determine if RAM standby Power (maintained battery power) to the TCCM was lost since the ignition was last turned OFF. When power is interrupted on pin C6 of the TCCM, the TCCM stores a loss of RAM standby power.

Diagnostic Trouble Code (DTC) 2
During electronic shifting, the TCCM check the motor/encoder for normal operation. If the motor/encoder does not function correctly enough times, the TCCM stores a motor/encoder failure DTC 2.

Diagnostic Trouble Code (DTC) 3
The TCCM performs a test each time the electronic-shift motor is turned ON or OFF. If the motor does not function properly the TCCM stores a motor circuit failure DTC 3, and the shift select buttons blink in order to inform the driver of a problem.

Diagnostic Trouble Code (DTC) 4
Each time the ignition is turned ON, the TCCM tests the memory, the program, and the internal system in order to ensure that the TCCM is operating properly. If the TCCM detects a fault within the TCCM, the TCCM stores a RAM/ROM memory failure (DTC) 4.

When the TCCM is running a diagnostic, and a code 4 is displayed by the shift select buttons, replace the TCCM.
gmshades wrote:
Electronic 4x4

What is wrong, How to fix, How to install


You will probably know when your 4x4 has failed when you needed it in the mud pit. Mine, for instance, failed by not doing anything when I hit the 4HI button. However, it would go into 4LO.

To read the codes for the 4x4 with electric shift transfer case, install a jumper wire from terminals "J" and "A" on the ALDL (12 pin connector under the driver's side dash)

The lights on the 4x4 indicator (switch) will flash in a pattern, there are only 4 codes, one flash is code one, 2 flashes is a code 2, and so on...

Code "1" is "RAM standby power" and is considered normal, may be caused by clearing the codes.
Code "2" is "Encoder fault"
Code "3" is "TCCM motor circuit"
Code "4" is "RAM/ROM failure"

If you have the codes 2 or 3 check all the encoder motor connections at the transfer case, if OK it will need a new encoder motor assembly.
If you have code 4, replace the TCCM.

Make sure all connections are good and no corrosion, and then it's either an encoder or TCCM.


The TCCM is located behind the glove box, and is accessed from underneath the dash. To remove, the kick panel and black cover come off. Then a 1/4" screw holds the "brains" in the dash. The middle computer is the Truck's brain, the one on the backside is the TCCM, and on the front is the small ABS computer. Simply loosen the 2 10mm nuts on each side, and they will separate.


This is the most common problem, and was my problem.

Cheap way - To clear the codes and reset the computer, pull the 5 amp TCCM fuse, located at the bottom of the fuse box. Wait at least a half hour, and then replace the fuse. Turn the ignition key to run, wait for the 4x4 lights to blink, and then turn off. Repeat 5 times, and then start the truck.

Decent price way - I just went and bought a rebuilt encoder motor from a company off of Ebay. Their company is Northern Auto Parts - Home The part cost me around $140 with a 1 year warranty and $38 core refund when I send the old one back.

Expensive way - buy a brand new one from GM for over $300.


Lucky way - Only thing that needs to be removed is the encoder motor itself. Tools needed are a 10mm hand wrench and a 8mm and 10mm socket wrench. The 10mm socket will work on the upper and lower bolts, while the regular wrench is needed for the middle bolt. (Tight squeeze). The 8mm socket is for the electric plug.
Then, just pull out, and wiggle it out of the truck. Install in reverse order.

Unlucky way - This means the encoder motor did not just wiggle out. Now the front drive shaft will have to be removed at the transfer case. Then, a yolk puller is needed in order to remove the yolk from the transfer case. Once this is done, go to the directions in lucky way.

Writeup by gmshades

Thanks hancockrw

Encoder Signal Check

The Transfer Case Control Module (TCCM) checks the encoder signal in order to verify that the transfer case is in either 2HI, 4HI, or 4LO.

Encoder Switch Monitor Check

While the transfer case shifts, the encoder within the electric-shift motor is monitored for the proper operating sequence. If during a shift, the encoder changes to an incorrect position, an error counter in the Transfer Case Control Module (TCCM) starts to count the number of times the encoder fails. If the encoder fails eight times, the TCCM produces a Diagnostic Trouble Code (DTC) of 2 indicating an Encoder Fault. When this happens, the TCCM outputs a signal in order to default the rail shift pattern in the encoder in order to allow for only shifts into 2HI and 4LO.
In order to protect against transient, random encoder faults caused by vibration, contamination, electrical noise, etc., the error counter reduces the count by one each time a good encoder value is detected. The encoder must fail 25 percent of the time for the TCCM to store a DTC of 2 and indicate a damaged encoder.

Motor and Relay Voltage Checks

Whenever the electric-shift motor is turned ON or OFF the motor and the electrical circuits are tested both in the de-energized and energized condition. If one or both voltage relays fail to detect the proper voltage after energizing or de-energized and energized condition. If one or both of the voltage relays fail to detect the proper voltage after energizing or de-energizing, the shift is aborted by the Transfer Case Control Module (TCCM) and a Diagnostic Trouble Codes (DTC) of 3 is stored in order to indicate a faulty TCCM motor circuit. If one or both relays fail to detect the proper voltage after de-energizing, both relays are turned ON by the TCCM, (even with the ignition switch positioned OFF) in order to prevent the motor from running. The TCCM then stores a DTC of 3 in order to indicate a faulty TCCM motor circuit. All of the status lamps blink three times, stop, and repeat the blinking continuously in order to alert the driver that the electronic shift system requires immediate repair. The battery. will drain when this condition occurs, regardless of the ignition switch position.

Motor/Encoder Circuit Operation

The Transfer Code Control Module (TCCM) operates the motor/encoder in one direction by energizing one relay while the second relay is de-energized. The TCCM operates the motor in the reverse direction by energizing the second relay while the first relay is de-energized. The encoder part of the motor/encoder sends shift positioning signals to the TCCM. The motor/encoder converts a shaft position, representing a mode of range selection, into electrical signals for use by the TCCM. By interpreting the four channels (P, A, B, C) of the encoder, the transfer case control module can detect what position the transfer case is in 2HI, 4HI, 4LO, or in transition between any of the two. The four hall effect sensors of the motor/encoder are used for channels (A, B, C, and P). These sensors provide a path to ground when a magnet passes over them. A rotating magnetic ring causes the hall effect sensors voltage to drop from 5 to 0 volts. The TCCM detects the voltage of all the channels and interprets the current transfer case shift position.

Hey BTW I found this info, I know your getting code 3... may worry u a little..

3 - TCCM motor circuit : Basically, every time the TCCM is powered up, it test the encoder motor. If something isn't right, it'll trip the code 3.
Possible causes
1. Open or shorted circuit between encoder motor and TCCM
2. Short/open connection in the motor itself
3. Malfunctioning motor relay
(this can cause the indicator lights to flash all the time (normal driving). If this happens, it must be fixed IMMEDIATELY. It can cause encoder motor failure even when the ignition is off)

· Been there Done it
16,775 Posts
If you have 4 lo and not 4 hi, you either have a weird transfer case shift motor that won't stop in one place, but will go past to another or a faulty TCCM. Those have been known to get wet and corroded connectors. Or just fail.
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