Question about 1999 GMC Sierra
I cannot get my truck in 4 wheel drive, high or low. The transfer case will not engage my 4x4 from the switch. I have a 2002 F350 diesel. Anyone know of this problem and how to fix it?
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Posted on Jan 02, 2017
The vacuum shift solenoids have power to one side of them at all times.
The GEM applies ground to them, to activate them, after the shift on the fly position confirm from the trans case.
The power to both should be on the White w/ Light Blue stripe wire.
Ground to them ( from the GEM ) is
2WD : Gray w/ Black stripe wire
4WD : Red.
The only TSB I know of is for the 04-06 MY trucks, for the IWE solenoids ( the kind without the water shield ).
Did you jack up the front end ( and support correctly ) turn the key to the run position ( do not start ), and shift into 4WD, and check the front drive shaft ?
If the shaft still turns, the problem is in the transfer case, if not you are looking in the correct place.
The GEM will not signal a vacuum solenoid until the transfer case has shifted into 4L or 4H ( 4 switches that are the return signal to the GEM ). Could be a problem with the connector on the transfer case ( think it is a 8 or 10 pin connector ) to the contact plate position switches in the trans case.
There is a manual front ale cable, but I would think you would want to confirm why the GEM is not signalling the shift solenoids, prior to replacing them
Posilock make a kit for the F-150
Posted on Dec 12, 2009
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
I have recently had this problem with a customer. It seems the 4x4 low will only engage in neutral with the brake applied. And also check fuse. Then select drive and you will feel it lock in. Don't "goose" the fuel when attempting this.
Posted on Jan 14, 2009
On the full size, the front actuator is electric, not vacuum.
When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.
Check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.
I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.
GM offered an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.
If I were you, I would buy a new heated actuator. They are available at most auto parts stores, and are about $90. They are also on eBay for about $55 plus shipping. These are very easy to change - just thread out the old one, and thread in a new one.
Good luck. The actuator is usually the problem 90% of time but recheck all of the wire harness just to maker sure there is no short before replacing any parts.
Posted on Jan 02, 2010
Testimonial: "This is exactly the info i need to troubleshoot and repair efficiently complete with wire numbers and colors thanks for the help buddy"
then the seloniod actuator if the front diff may be bad, this is common,heres a diagram to show you what it is and where it is, hope this helps.
Posted on Jan 27, 2010
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