No 4 Wheel Drive
BF Guru
Joined: Dec 2016
Posts: 5,601
From: Moscow, ID
The two primary culprits with the 4wd system are leaky vacuum lines and a bad transfer case control module (TCCM) or corroded connectors to the TCCM.
The leaky vacuum hoses (normally an easy fix) would result in the front axle not engaging due to lack of vacuum going to the actuator that pulls on the cable that engages the front axle gear. Being purely mechanical, this failure would not show up with any blinking lights or error codes. Because the the same vacuum system also controls the HVAC dampers, this failure often is accompanied by the air from the heater or AC being directed to the wrong place in the cabin.
The TCCM is located under the passenger side kick-panel. It is not very hard to access. The first thing to try is to listen for click coming from it. These clicks are reed relays being thrown that control the transfer case. When working correctly, you will only hear single clicks. When it is failing, you will hear a series of clicks as the TCCM tries to set the transfer case to 4wd. When it is totally failed you will hear nothing. Under normal operation, it is hard to hear these clicks when they occur and the system is working properly.
One of the reasons the TCCM may not be working is that that the electrical connector to it and its plug may be corroded. This requires a visual inspection of the connector and plug. A simple removal and reconnection may be sufficient. But if corrosion got into it, there is a problem with moisture and the pins should be cleaned and then protected with dielectric grease. The cleaning of the pins is not a bad idea even if if a simple removal and reconnection does work. The receptacles for the pins are harder to clean but a few removals and reinsertions should help.
If the connector is fine, then the TCCM may be to blame. This is what happened to me. A search of the GM parts retailers online can give you the proper part-number for your TCCM. Be mindful of the difference between the "3-button" and "4-button" (with Auto 4wd) transfer cases. A call to a GM dealer with your VIN will provide you with the most up-to-date part number. You might not be able to find a NOS TCCM at a GM dealer but you may find one at eBay. You may also be able to find one at a wrecking yard. Just be sure to get one from a Blazer with the same transfer case and one that has no corrosion on the interconnects.
There are other ways this system can fail so don't treat this advice as final. A good code reader can help with the diagnosis.
I'm sure others here will provide some other things you can check.
The leaky vacuum hoses (normally an easy fix) would result in the front axle not engaging due to lack of vacuum going to the actuator that pulls on the cable that engages the front axle gear. Being purely mechanical, this failure would not show up with any blinking lights or error codes. Because the the same vacuum system also controls the HVAC dampers, this failure often is accompanied by the air from the heater or AC being directed to the wrong place in the cabin.
The TCCM is located under the passenger side kick-panel. It is not very hard to access. The first thing to try is to listen for click coming from it. These clicks are reed relays being thrown that control the transfer case. When working correctly, you will only hear single clicks. When it is failing, you will hear a series of clicks as the TCCM tries to set the transfer case to 4wd. When it is totally failed you will hear nothing. Under normal operation, it is hard to hear these clicks when they occur and the system is working properly.
One of the reasons the TCCM may not be working is that that the electrical connector to it and its plug may be corroded. This requires a visual inspection of the connector and plug. A simple removal and reconnection may be sufficient. But if corrosion got into it, there is a problem with moisture and the pins should be cleaned and then protected with dielectric grease. The cleaning of the pins is not a bad idea even if if a simple removal and reconnection does work. The receptacles for the pins are harder to clean but a few removals and reinsertions should help.
If the connector is fine, then the TCCM may be to blame. This is what happened to me. A search of the GM parts retailers online can give you the proper part-number for your TCCM. Be mindful of the difference between the "3-button" and "4-button" (with Auto 4wd) transfer cases. A call to a GM dealer with your VIN will provide you with the most up-to-date part number. You might not be able to find a NOS TCCM at a GM dealer but you may find one at eBay. You may also be able to find one at a wrecking yard. Just be sure to get one from a Blazer with the same transfer case and one that has no corrosion on the interconnects.
There are other ways this system can fail so don't treat this advice as final. A good code reader can help with the diagnosis.
I'm sure others here will provide some other things you can check.
Last edited by christine_208; Dec 23, 2021 at 07:10 PM.
