Blazer Forum - Chevy Blazer Forums - 4wd questions answered

Ok. After many attempts of understanding the 4wd system. I think I can shed a little light on common 4wd questions no one has ever been able to answer for me. 2nd Gen Blazers.

How it works.
The dash button switch signals the TCCM (located under passenger side dash) The TCCM signals the transfer case motor to engage the transfer case. As the motor slides the gear into place it starts the vacuum switch (located on top on T case with 3 hoses going to it). That switch allows vacuum to go to the vacuum diaphram (located under battery) to apply vacuum which in turn pulls on the vacuum actuator cable. This cable locks the drive axle to the front axle. Thus allowing power to drive the front two wheels.

(for all of you lucky people that know this, good for you)

Many problems, few answers.
Transfer cases in general do not fail. If your transfer case is bad, it is most likely another issue (vacuum) that caused the problem. Keep looking.

When 4wd is "broken" there are a few things to check first.

The dash switch should blink in the diagnostic mode (on vehicle ignition) If the lights do not all light up when truck is started, most likely a Dash switch problem. If light ontinues to blink (in any drive) that drive is not engaged. Check owner manual on proper speeds and wether it needs to be in neutral or park.

You should hear a "click" under the passenger side dash. if no electric click sound. TCCM most likely bad. However, check your 4wd fuse first to see if it's blown. In order to have your TCCM/4wd system electronicaly checked. You need to have it diagnosed by a shop. You basic engine code readers will not work at pulling codes from TCCM.

You should hear the Transfer case motor attempting to engage the transfer case. (under your feet on the driver's side). If no sound. Most likely a bad motor. (also check 4wd fuse first) You can get this diagnosed when checking the TCCM. Replacing this is very straight forward.

Some things that should never happen.
The drive shaft should NOT turn when 2wd is engaged. Jack the front end off the floor so the tires are not touching. If you turn the wheel, the drive axle should not turn. If it turns in 2wd. Your front axle is not fully unengaged. Ther are no "locking hubs" The front axle always turns, the transfer case does not until allowed to do so.) You should never hear a "clunking" when 4wd is engaged and you are driving down a road. This is your Transfer case gears not aligning right with the transmission. This causes a lot of dammage.

Trouble shooting.Five places where the system can fail: Dash switch, TCCM, Vacuum actuator, transer case switch, and transfer case motor.

Transfer case switch. If in doubt, change it. It takes some flexibility and about 35 dollars. (usually the cause for the truck not to go into 4wd)

TCCM, dash switch, and transfer case motor already discussed.

Vacuum diaphram (located under battery) To chech this you need to remove the battery. The diaphram has a cable connected to it. This cable goes to the transfer case forks. This cable should be easily moved up to about 3/4" when in 2wd. If it is hard to move, or moves very little it is stuck and needs to either be released and lubed, or replaced. This is the main cause of the drive axle turning in 2wd. Also a significant loss of fuel economy.

I hope this is a little better (it helped me) at answering basic 4wd questions. Further tech and explinations in the respective generational help sections.