# Service 4WD message 2001 chevy 2500HD



## ss502gmc (Sep 12, 2008)

I cant figure out why this message is on and my 4x4 doesnt work, I scanned the code and it says there is a short circuit to the transfer case control module or that the module is bad. I have traced every wire i could find and cleaned all grounds but still wont work. the module is over $400 that i dont want to spend if im not 100% sure its the problem because i cant return it. Every
once in a while the message will go out and 4x4 will work again. Ive tried everything ive researched online but no luck! any help would be appreciated seeing how its supposed to snow this weekend. Thanks in advance....


----------



## 2006Sierra1500 (Dec 28, 2011)

Actually it's supposed to snow tonight. Did you check all your connections as well?


----------



## skostur79 (Oct 20, 2012)

try unplugging and plugging the tccm

if no go youll need to see if a gm scan tool with commutcate
with tccm

the module will need programmed at dealer any how 

sorry


----------



## shovelracer (Sep 3, 2004)

any other electrical glitches? another option is the dashboard switch or in my case a bad firewall ground.


----------



## ss502gmc (Sep 12, 2008)

I've had no other electrical stuff except my airbag light is on for an unknown reason and happened around the same time as the fwd message but could not find anything common with the 2 problems. Where is the firewall ground? The one under the hood or another one? The switch is good also.


----------



## skostur79 (Oct 20, 2012)

are you getting 2wd ,4wd light when key turned on?

ground is on drivers frame horn / body mount under driverseat (out side where body mounts 2 10mm bolt and wires)

need to check powers and ground at tccm with a load 

some test lights dont draw much current and will not show opens


----------



## ss502gmc (Sep 12, 2008)

Yes, I checked the grounds underneath the driver side door and cleaned them. The lights light up on the switch when I turn key on and the 2wd light stays on as it should. I have a dig meter if I knew what to check and how. I'm in my truck now about to pull dash apart to get at the tccm.


----------



## skostur79 (Oct 20, 2012)

Transfer Case Shift Control Switch Inoperative 
Circuit Description

The transfer case shift control switch circuit consists of 3 normally open switches. The transfer case shift control module supplies a regulated 8 volts DC to the switch through the 8 volt-reference circuit. The current travels through a 9.09 K Ω resistor located inside the switch. Then switch returns current to the transfer case shift control module through the 4WD switch signal circuit.

The transfer case shift control module constantly monitors this signal voltage to determine the condition of the shift control switch circuit. The transfer case shift control module will set this DTC If any of the following conditions exist:

• None of the buttons are pressed, and the transfer case shift control module detects a voltage level outside the possible range (approximately −0.5 to +1.0 volts) for more than 5 minutes 

• A button is held down, or a button is stuck in the down position for a period more than 5 minutes 

Each switch will complete a circuit through that switch's own specific resistor, when depressed. The transfer case shift control module continuously monitors the switch input to determine if the following button selections are made by the driver:

• 4HI 

• 2HI 

• 4LO 

Neutral may be obtained if the following conditions are met:

• The engine is running. 

• The automatic transmission is in neutral, or the clutch pedal is depressed on a manual transmission application 

• The vehicle speed is below 5 km/h (3 mph). 

• The transfer case is in the 2HI mode. 

Once these conditions have been met, press and hold both the 2HI and 4LO buttons for 10 seconds in order to shift the transfer case into neutral. The red neutral indicator lamp will illuminate.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

2 This steps tests the functionality of the transfer case shift control switch buttons.


3 This step tests the modules ability to operate the transfer case modes.




Step



Action



Yes



No




Schematic Reference: Transfer Case Control Schematics 



1


Was the Transfer Case Diagnostic System Check performed?


Go to Step 2


Go to Diagnostic System Check - Transfer Case 



2 


Important: NEUTRAL range will only be obtained by pressing and holding both the 2 HI and the 4 LO buttons simultaneously for 10 seconds. Refer to the Circuit Description. 

1. Install a scan tool. 

2. Start the engine. 

3. Place the transmission into neutral, or depress the clutch pedal on manual transmissions. 

4. Set the park brake. 

5. Observe Commanded Mode Indicator states on the scan tool while depressing each of the shift control switch buttons. 


Does the state change with the press of any of the shift control switch buttons?


Go to Step 4


Go to Step 3



3 


With a scan tool, command through all Commanded Mode Indicator states.

Does the Mode Indicator state change?


Go to Step 4


Go to Step 5



4


Replace the shift control switch. Refer to Transfer Case Shift Control Switch Replacement .

Did you complete the replacement?


Go to Step 6


—



5


Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement .

Did you complete the replacement?


Go to Step 6


—



6



1. Use the scan tool in order to clear any DTCs . 

2. Operate the vehicle within the conditions for allowing the symptom to re-occur. 


Does the symptom re-occur?


Go to Step 2


System OK


----------



## skostur79 (Oct 20, 2012)

all you can try is unplugging and reconnecting at tccm
it is comunicating because you have bulb check 
if it was dead or not powered you would have no lights at the switch

unless you want to try a switch , i suggest getting codes pulled


----------



## ss502gmc (Sep 12, 2008)

The only code I have is a C0308 which is the A or B Circuit for the tccm.


----------



## skostur79 (Oct 20, 2012)

should of posted that in first post ,,

--------------------------------------------------------------------------------

DTC C0308 
Circuit Description

The transfer case motor is a bi-directional, permanent magnet, D.C. motor. When energized, (through Motor Control A or Motor Control B, the ground is provided by the opposing motor control circuit and then grounded through the transfer case shift control module ground circuit), the motor, through a series of gears, rotates a shaft which moves the mode and range forks to shift the transfer case between 4H, 2H, N, and 4L ranges.

This DTC detects a short to ground in the motor control A or motor control B circuit.

Conditions for Setting the DTC

• The system will test the motor circuits: 

- The system checks for unwanted voltage. 

- The system supplies voltage on one circuit and reads the voltage back on the other. 

• If the system detects a problem with the circuits, the DTC is logged. 

• The transfer case shift control module senses a low voltage return in Motor Control A or Motor Control B when a high voltage is expected. 

Action Taken When the DTC Sets

• All shifting will be disabled. 

• The SERVICE 4WD indicator lamp will be commanded on for the remainder of the current ignition cycle. 

Conditions for Clearing the DTC

• The transfer case shift control module will clear the DTC if the condition for setting the DTC no longer exists. 

• A history DTC will clear after 100 consecutive ignition cycles without a fault present. 

• History DTCs can be cleared using a scan tool. 

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

2 Listen for an audible motor noise when the transfer case encoder motor operates. Command both the ON and OFF states. Repeat the commands as necessary.


3 This step tests for a short to ground in the motor control A circuit.


4 This step tests for a short to ground in the motor control B circuit.


5 This step tests for a higher than normal resistance in the motor control A and B circuits through the module.


6 This step tests for a short to ground in the transfer case encoder motor windings.


7 This step tests for continuity across the motor circuit. Resistance readings vary depending on the location of the brush contact inside the motor assembly.


8 This step tests motor control A circuit for a short to ground.


9 This step tests motor control B circuit for a short to ground.


10 This step tests motor control A and B circuits for an open or high resistance.




Step



Action



Value(s)



Yes



No




Schematic Reference: Transfer Case Control Schematics 



1


Was the Transfer Case Diagnostic System Check performed?


—


Go to Step 2


Go to Diagnostic System Check - Transfer Case 



2 



1. Start the vehicle. 

2. Put the vehicle in NEUTRAL. 

3. Using the transfer case control switches, attempt to shift through all modes and ranges. 


Does the transfer case encoder motor turn ON and OFF?


—


Go to Testing for Intermittent Conditions and Poor Connections in Wiring Systems


Go to Step 3



3 



1. Turn OFF the ignition. 

2. Disconnect the four wire connector at the transfer case. 

3. Connect a DMM between the motor control A circuit harness connector and ground at the transfer case. 


Is the resistance reading less than the specified value?


10 K Ω


Go to Step 8


Go to Step 4



4 


Connect a DMM between the motor control B circuit harness connector and ground at the transfer case.

Is the resistance reading less than the specified value?


10 K Ω


Go to Step 9


Go to Step 5



5 


Connect a DMM between the motor control A and B circuit harness connector at the transfer case.

Is the resistance reading greater than the specified value?


2 Ω


Go to Step 6


Go to Step 10



6 


Test terminals A and B at the transfer case side of the harness for a short to ground.

Was a problem found?


—


Go to Step 11


Go to Step 7



7 


Test the resistance across the motor circuit.

Were the resistance readings within the specified range?


0.5–35 Ω


Go to Step 12


Go to Step 11



8 



1. Disconnect the transfer case shift control module. 

2. Test the motor control A circuit for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. 


Did you find and correct the condition?


—


Go to Step 13


Go to Step 12



9 



1. Disconnect the transfer case shift control module. 

2. Test the motor control B circuit harness connector for a short to ground. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. 


Did you find and correct the condition?


—


Go to Step 13


Go to Step 12



10 



1. Disconnect the transfer case shift control module. 

2. Test the motor control A and B circuits for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems. 


Did you find and correct the condition?


—


Go to Step 13


Go to Step 12



11


Replace the encoder motor. Refer to Transfer Case Motor/Encoder Replacement .

Did you complete the replacement?


—


Go to Step 13


—



12


Replace the transfer case shift control module. Refer to Transfer Case Shift Control Module Replacement .

Did you complete the replacement?


—


Go to Step 13


—



13



1. Use the scan tool in order to clear the DTCs. 

2. Operate the vehicle within the Conditions for Setting the DTC as specified in the supporting text. 


Does the DTC reset?


—


Go to Step 2


System


----------



## shovelracer (Sep 3, 2004)

ss502gmc;1547967 said:


> I've had no other electrical stuff except my airbag light is on for an unknown reason and happened around the same time as the fwd message but could not find anything common with the 2 problems. Where is the firewall ground? The one under the hood or another one? The switch is good also.


There is a major ground just next to the brake MC IF this one goes it plays all sorts of trouble with the dash switches, radio, windows, etc.


----------



## ss502gmc (Sep 12, 2008)

The wiring checks out ok with no shorts, so it's either the encoder or the tccm. I have a spare encoder that didn't do anything so I'm thinking it's the module. Does anyone now how many ohms I should get checking the encoder motor?


----------



## hatefulmechanic (Dec 27, 2012)

I suggest using a TechII to do the diag, a DVOM is better than a test light for sure, but a Tech II or comparable scanner will give you the plate position data.

From experience, the module is very common to fail, typically most common issue on these is the switch assy, module, then encoder. The people who don't use their 4wd typically have encoder failures due to not being moved and seizing, the ones who use their 4wd typically have module failures due to the switching relay in the module burning out. 

There are several people online who rebuild the modules, and I have had good luck with them.


----------

