Diagrams for four wheel drive system

SOURCE: four wheel won't engage

Theres a vacume dash pot under the battery box. It pulls a cable that engages the front diff. Put the vacume line and you'll have lights and four wheel drive. (it happened to my blazzer)

SOURCE: 1999 chev blazer 4x4 system

I have a 1999 chevy blazer that stuck in 4 wheel drive. How do you get it out.

SOURCE: four wheel drive wont engage

If you have push button 4-Wheel Drive, it is recommended that you use it every once in a while, in order to keep gunk, rust, and corrosion to build up on moving parts. The mostly likeley (though not definite) reason would be that you let the 4-wheel drive unused for so long that corrosion has built up on the parts and the actuator that engages the 4-Wheel Drive doesn't have the power to engage it past the corrosion. You'll need to take it in to be cleaned, or to have parts replaced. In the future, every month or so, engage 4-Wheel Drive and drive around in some mud, grass, sand, or any other loose ground for a bit, just to keep the parts moving and clean.

The service Four wheel drive light is to alert the operator the system is not being read properly by the ECU.

If the ECU loses contact with or position of the Transfer case actuator motor the service light will come on. Same thing goes for the front diff actuator.

So it may be a bad switch, sensor, or an actual problem in the 4WD linkage. This is not a Oil Change light that you can ignore a few extra weeks - it indicates a problem that should be taken care of ASAP to avoid costlier problems down the road.
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Harvey N Tawatao

• 3854 Answers
• Posted on Apr 08, 2009

SOURCE: 4 Wheel Drive Problem

Put the key in the run position and turn your 4wd on and off to listen for the electronic acuator to engage and disengage. If your can hear it then it's macanical and a Quick fix. On top of the transfercase on the driver's side, your going to find three vacuume tube connected to a cauume switch that may have gone bad. There common to go bad and there not the bad of a price at the Auto Zone. Check for any cracks on the vacuume line first then get your hands dirty. Good luck and keep an extra switch in your glove box.

SOURCE: 1997 Ford F-150 4x4: Electronic four wheel drive

Lift and support the front of the truck so the front wheels are off the ground.
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.

There are the two lines that operate the diaphragm on the axle. Facing the truck, the right solenoid (pink & black lines) is activated to disconnect the axle and go back into 2WD. The left solenoid (blue & white lines) lock the axle into 4WD

you'll have to get a vacuum source like a Mighty-Vac hand pump and apply vacuum to the pink and blue lines, one at a time. If one leaks down you'll have to check the line, including the diaphragm, to locate the leak. If vacuum was held, reconnect the bottom half of the line "connector" (black & white lines) to the solenoids and crank the truck. Switching from 2WD to 4WD and back verify each time that there is vacuum on the top nipple of the activated solenoid. If you're in 2WD the right solenoid (pink/black) should be sucking, in 4WD the left (blue/white) should be. If these operate as described then you axle shift fork may be bad. If you're not getting vacuum when you should be, replace the solenoid.

The Vacuum shift motor ( item #10 in the diagram )

Close up :
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Vacuum shift solenoids



The other source of the vacuum leak ( if the lines to the solenoid test good ) is the vacuum box behind the battery.