This page is to help you diagnose and repair the Auto 4WD system found on most General Motors (Chevrolet/GMC) GTM800 (1999-2006) platform SUVs and some pickup trucks. Repairs to the components of the 4WD system can cost hundreds or thousands of dollars, and many people have had multiple repeat visits to the dealer without getting a fix for the root problem.
When my 2003 Suburban's 4WD system failed, I was 4 hours from home with my travel trailer! The front axle was halfway engaged, so it produced a gut-wrenching grinding noise during turns for the entire drive home. However, I fixed it myself-and I bet you can, too!
Please note the site is still under construction!
When my 2003 Suburban's 4WD system failed, I was 4 hours from home with my travel trailer! The front axle was halfway engaged, so it produced a gut-wrenching grinding noise during turns for the entire drive home. However, I fixed it myself-and I bet you can, too!
Please note the site is still under construction!
Step 1: Check the free stuff!
These are the things that everyone skips, but you may find a free or cheap fix for the problem. There are two fuseboxes that have 4wd fuses. At the driver's door side of the dashboard, there is a fuse labeled "4WD" and under the hood is the "TREC" fuse.
These are the things that everyone skips, but you may find a free or cheap fix for the problem. There are two fuseboxes that have 4wd fuses. At the driver's door side of the dashboard, there is a fuse labeled "4WD" and under the hood is the "TREC" fuse.
|
|
Step 1.5: Pull the error codes
Getting the error codes can be a huge help in pinpointing the problem, but they are not perfect.
The good news: The Transfer Case Control Module stores codes related to any failure it sees.
The bad news: Most code readers cannot retrieve these codes. They are not OBD "Powertrain" codes, but custom GM "Body" or "Chassis" error codes.
Maybe you have a friend with a GM Tech 2 diagnostic computer or you can find a friendly dealership or independent garage that can pull these codes for you for a fair labor charge so that you will know what to change first.
Note: Some people have had good luck pulling code using an ELM327 OBDII module and "terminal" mode on a code scanning app. See here for details from a Chevy Trailblazer owner.
Common TCCM fault codes:
BO790-transfer case nuetral indicator circuit malfunction
CO550-ECU malfunction
CO379-front axle control circuit malfunction
CO374-general system malfunction
CO359-discrete output circuit malfunction
CO306-motor A/B circuit malfunction
CO300-rear propshaft speed sensor malfunction
U1301-class 2 communication circuit low volts
B2725-mode switch circuit malfunction
U1000-class 2 data link malfunction
If you don't want to spend the money for a scan, read on. If you got a scan and now have an idea of what to replace, go to the appropriate section.
Getting the error codes can be a huge help in pinpointing the problem, but they are not perfect.
The good news: The Transfer Case Control Module stores codes related to any failure it sees.
The bad news: Most code readers cannot retrieve these codes. They are not OBD "Powertrain" codes, but custom GM "Body" or "Chassis" error codes.
Maybe you have a friend with a GM Tech 2 diagnostic computer or you can find a friendly dealership or independent garage that can pull these codes for you for a fair labor charge so that you will know what to change first.
Note: Some people have had good luck pulling code using an ELM327 OBDII module and "terminal" mode on a code scanning app. See here for details from a Chevy Trailblazer owner.
Common TCCM fault codes:
BO790-transfer case nuetral indicator circuit malfunction
CO550-ECU malfunction
CO379-front axle control circuit malfunction
CO374-general system malfunction
CO359-discrete output circuit malfunction
CO306-motor A/B circuit malfunction
CO300-rear propshaft speed sensor malfunction
U1301-class 2 communication circuit low volts
B2725-mode switch circuit malfunction
U1000-class 2 data link malfunction
If you don't want to spend the money for a scan, read on. If you got a scan and now have an idea of what to replace, go to the appropriate section.
Step 2: What does work?
Do you have a yellow status light on the dash switches? Which mode is lit? The solid lights on the 4WD buttons are telling you where the transfer case encoder motor thinks it is, and blinks on the mode that it is trying to go to after you push the button. If it fails to move to the new position, it goes back to a solid light on the original position and illuminates the "SERVICE 4WD" light.
Does the "SERVICE 4WD" message come up as soon as you start the truck, or does it only activate after you try to select a mode other than 2 Hi?
Do you have a yellow status light on the dash switches? Which mode is lit? The solid lights on the 4WD buttons are telling you where the transfer case encoder motor thinks it is, and blinks on the mode that it is trying to go to after you push the button. If it fails to move to the new position, it goes back to a solid light on the original position and illuminates the "SERVICE 4WD" light.
Does the "SERVICE 4WD" message come up as soon as you start the truck, or does it only activate after you try to select a mode other than 2 Hi?
- If the "SERVICE 4WD" message is lit as soon as you start the vehicle, that probably indicates a constant wiring or part fault and the Transfer Case Control Module is noticing that fault as soon as it is energized when the truck is turned on. Check wiring continuity (a Digital Multimeter is best, but you can do some of it with a test light), verify grounds, and verify power. This step is very important and could save you a lot of money and time! See the wiring diagrams page for more information.
- If the "SERVICE 4WD" message only comes up after you attempt to actuate 4WD, it probably indicates that the problem only occurs during actuation. I had this condition when the ground wire for the front axle actuator was damaged. It could be the front axle actuator, or the "encoder motor" on the transfer case that is responsible for moving the transfer case to the correct position. Everything is normal in 2WD, but when a 4WD mode is commanded, the error occurs. See below for tips on how to test individual parts.
Check the dashboard buttons: These buttons tell the Transfer Case Control Module what mode you want. If the button module is damaged, it can command modes you don't want, or fail to command the mode you want. If nothing happens when you push a button (no changes to the lights, and no noises from under the truck), the buttons may have failed.
Check every function for response: 2HI, Auto4WD, 4HI, 4LO, Neutral.
Pull the dashboard apart (it is just held with clips) and pull out the buttons. Disconnect both connectors.
Using the DMM, measure the resistance between the innermost pins on both connectors. The resistance should be steady (no changes while the button is pressed). Note that some measurements are in "Ohms" and some are "Kilo-Ohms" (1000 Ohms).
No buttons pushed: 8.63-9.54 K Ohms
Auto 4WD: 61.7-68.1 Ohms
2 HI: 1.50-1.53 K Ohms
4 HI: 656-670 Ohms
4 LO: 2.32-2.37 K ohms
Neutral (2Hi/4Lo): 1.015-1.035 K Ohms
If your resistance measurements fall outside these ranges, or jump around a lot, try spraying some contact cleaner or compressed air into the switches to see if it improves the problem. Some people have had success with squeezing the button module. If the readings are wrong, it may be time to replace the buttons. These are available here.
Check every function for response: 2HI, Auto4WD, 4HI, 4LO, Neutral.
Pull the dashboard apart (it is just held with clips) and pull out the buttons. Disconnect both connectors.
Using the DMM, measure the resistance between the innermost pins on both connectors. The resistance should be steady (no changes while the button is pressed). Note that some measurements are in "Ohms" and some are "Kilo-Ohms" (1000 Ohms).
No buttons pushed: 8.63-9.54 K Ohms
Auto 4WD: 61.7-68.1 Ohms
2 HI: 1.50-1.53 K Ohms
4 HI: 656-670 Ohms
4 LO: 2.32-2.37 K ohms
Neutral (2Hi/4Lo): 1.015-1.035 K Ohms
If your resistance measurements fall outside these ranges, or jump around a lot, try spraying some contact cleaner or compressed air into the switches to see if it improves the problem. Some people have had success with squeezing the button module. If the readings are wrong, it may be time to replace the buttons. These are available here.
Encoder motor and transfer case position encoder:
If the transfer case won't go to the proper position, or behaves erratically, it could be the encoder inside the encoder motor. If it does not move at all, it could be the motor itself. The entire motor can be replaced, or just the less expensive encoder inside can be changed.
The transfer case encoder motor moves the transfer case selector automatically. In Auto 4WD, it continually moves the input shaft of the transfer case to vary the amount of torque that goes to the front driveshaft. In 4Hi, 4Lo, or Neutral, it just moves to the proper position and locks in place. The 4Hi position is the same position as 4Auto with a 50/50 torque split front to rear. The encoder motor keeps track of the proper position using the encoder feedback. The encoder is just a variable resistor. It can get worn and give bad readings to the TCCM.
Here is a nice write-up on replacing the encoder (sensor wheel) inside the encoder motor.
If the transfer case won't go to the proper position, or behaves erratically, it could be the encoder inside the encoder motor. If it does not move at all, it could be the motor itself. The entire motor can be replaced, or just the less expensive encoder inside can be changed.
The transfer case encoder motor moves the transfer case selector automatically. In Auto 4WD, it continually moves the input shaft of the transfer case to vary the amount of torque that goes to the front driveshaft. In 4Hi, 4Lo, or Neutral, it just moves to the proper position and locks in place. The 4Hi position is the same position as 4Auto with a 50/50 torque split front to rear. The encoder motor keeps track of the proper position using the encoder feedback. The encoder is just a variable resistor. It can get worn and give bad readings to the TCCM.
Here is a nice write-up on replacing the encoder (sensor wheel) inside the encoder motor.
Front Axle Actuator: This part engages the front axle. It is similar to locking the hubs in the old days, but it happens inside the axle housing. The actuator has 12V, ground, command signal in, and position signal out at its connector. Use a DMM or test light to check if the actuator is getting a signal from the TCCM. If not, the motor may be bad. You can bench test these with a set of alligator clips and a 12V source.
New actuators are available in Dorman's low cost and genuine GM versions.
See connector and wiring diagram information here. If the wires are broken, the actuator won't operate and the actuator won't move. If it does not move to the proper position, the buttons on the dash will blink and the "Service 4WD" indicator will be lit because the TCCM never sees a complete 4WD engagement.
The ground wire to my front axle actuator was the culprit on my truck. The wire was almost broken, but I could still get about 9 volts when I measured between the +12V and ground wires on the connector. However, when I checked the resistance of the wires between the connector under the brake master cylinder and the TCCM, I could see that the resistance was over 20 Ohms. It should be less than 1. So even though the wire could pass some current, it was not enough to supply enough power to the motor. The actuator was stuck halfway out, which halfway engaged the front axle, causing the noise I heard.
New actuators are available in Dorman's low cost and genuine GM versions.
See connector and wiring diagram information here. If the wires are broken, the actuator won't operate and the actuator won't move. If it does not move to the proper position, the buttons on the dash will blink and the "Service 4WD" indicator will be lit because the TCCM never sees a complete 4WD engagement.
The ground wire to my front axle actuator was the culprit on my truck. The wire was almost broken, but I could still get about 9 volts when I measured between the +12V and ground wires on the connector. However, when I checked the resistance of the wires between the connector under the brake master cylinder and the TCCM, I could see that the resistance was over 20 Ohms. It should be less than 1. So even though the wire could pass some current, it was not enough to supply enough power to the motor. The actuator was stuck halfway out, which halfway engaged the front axle, causing the noise I heard.
Transfer Case Control Module: This is the system's brain. It is located under the dashboard above the parking brake pedal, behind a large metal bracket. There are three connectors for various sensors and outputs. These computers are programmed to match your vehicle, but Dorman sells a computer that does not require programming to your vehicle. If shopping for a junkyard part, be sure to get a computer that is from a vehicle with the same 4WD option as yours (check the glove box for "NP8" and/or check for the 4 button 4WD switches). If buying from an auto parts store, these computers may not be returnable, so save this expensive part for last!
Transfer Case: Here is a good summary of the function of the NP248/NVG248 transfer case. If the internal parts of your transfer case are damaged, you may rebuild it or buy a used unit. If you are taking the transfer case apart, you may want to put in a "pump rub kit" to prevent a leak and costly repair later.