Question about Cars & Trucks
Chevy truck 4x4 light comes on when ingaged but only rear wheels engage
Didnt specify year or model but problem is usually the transfer case actuator sticking there are four bolts holding actuator in place remove and lube actuator usually frees it up
Posted on Dec 10, 2012
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Posted on Jan 02, 2017
Sounds as if the transfer case is fine, but best to confirm. Jack up and support the front of the truck ( wheel chocks, jack stands, normal safety precautions ).
Turn the key from off to run ( do not start ) select 4WD ( L or H ). Try to turn the front drive shaft. If the shaft turns, the transfer case shift motor ( or linkage if Mechanical shift on the fly ) is the problem. With the 4WD lights illuminated, I would say the front shaft will not turn ( shift position switches are registering correctly ), and the problem is with the front axle locking in ( assumption, prior to testing ).
There is a vacuum shift diaphragm on the front axle. One of the vacuum solenoids is for 4Wd one is for 2WD. If the vac lines are cracked, the solenoid is not working correctly ( applying vacuum to the shift diaphragm ) or the shift diaphragm itself has an issue, the lever in the front differential will not shift.
Start by checking for vacuum on the shift diaphragm. The 2 vacuum lines are pink and blue. Pink is the 2WD operation, and the blue is the 4WD operation. Remove the vacuum lines 1 at a time, and install vacuum gauge. Start the engine, and shift into 4WD. The vacuum gauge should read and hold pressure. You can get the same test by watching the vacuum shift diaphragm and have some shift the truck into 4WD, but won't know if the lack of movement is from no vacuum or the shift diaphragm itself.
This is what is looks like on the front axle.
The vacuum shift solenoids look like this :
The vacuum shift diaphragm on the front axle, is located at #10 :
This is the vacuum line diagram
It could also be the vacuum reservoir or the vacuum lines to the shift solenoids ( shift solenoid has no vacuum to apply ).
There is the outside chance that the solenoids are not being signaled by the GEM, but that is a rare occasion. The GEM applies ground to the solenoid, so expect power to always be there, when testing to another ground if you get to that point ( still think it is a vacuum problem or the application of vacuum )
Hope this helps more than confuses.
Posted on Dec 20, 2009
the front axles have pinon gear box that the shaft from the transfer case goes to, it has a vacuum hose to it or a solenoid actuator check these for proper function this is how the power is turned on to the wheels.
Posted on Feb 13, 2010
By the way, if that happens again (Same thing happened to me) It was inside the front diff. That vacuum actuator that sits on the passenger side that has a vacuum hose going up to it has a built in cable that plugs into the front diff. There are little springs that go bad and there is no tension on the cable. It retracts but doesn't engage the axle.
It's 3 bolts, then pry it open just enough to get access to the spring that holds the cable together. ;) That's how I fixed my problem permanently
Posted on Dec 02, 2010
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