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Questions and Answers

Chevy Blazer 4WD Shifting Problem

Q. I have a 1994 Chevy T10 Blazer 4x4 with the 2.8 liter V-6 Vortex engine with an automatic transmission. When I go to engage the 4hi or 4lo, the lights on the switch flash and I can here the transfer case engage and then immediately disengage. Is this a vacuum problem? I found the relay switch box under the dash and disconnected it then reconnected it.

Chevy Blazer 4WD Shifting Problem

Sometimes this works, sometimes it doesn't. The other day someone told me the vacuum actuator is located under the battery. As soon as I removed the battery bracket, battery still connected, The switch worked and the 4h/4l worked, I tightened up the battery bracket and all of a sudden it stopped working again.

I repeated the above steps and everything is working properly for now! Any ideas as to what might be causing this?

Thank You...

A. I'll try to explain how this system works for you. When you push a button, the signal goes to the Transfer Case Control Module (TCCM), a small metal box under the dash. In turn, it sends a command to the encoder motor which is mounted on the side of transfer case.

On a working unit, the transfer case will shift to the selected range. This will cause an internal movement of the linkage which pushes up on the ball of the vacuum switch. The vacuum switch has 3 hoses to it. One is the supply from manifold vacuum, one is the feed that goes to the actuator under the battery. The other is to drain off the vacuum when it's taken out of 4x4.

There should be DTC's stored if you can't get it to shift. These are read by jumping 2 wires in the ALDL plug under dash. All codes will have you check for 5 volts at the encoder motor plug on the 4 terminals. I have had this trouble in the past and fixed a few by running a new ground wire to the encoder terminal that is ground. I don't have a schematic handy so I can't help you there.

Unplug the TCCM, try tapping on the encoder motor and re-install the plug after it's been unplugged for at least 2 minutes. Cycle the ignition on and off 5 times, then on. Now try to make a shift.

I have seen ruptured servos under the battery. Battery acid leaking down and eating into the servo cable that runs to the front axle making it too stiff to move the cable. If the switch light changes from 2 to 4h, the TCCM and encoder should be okay and the problem is most likely in the vacuum switch or servo area.

Additional Information provided courtesy of AllDATA

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© 2005 Vincent T. Ciulla

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