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two speed

All the two speed axles I've worked with require the actuator servo to keep them in high gear. If power or vacuum is lost to the servo while in high gear they will pop out and attempt to shift into low. That's why it's important to park with the axle in low gear, otherwise the truck is likely to roll away. That said, I would assume the problem is NOT internal to the axle. With vacuum units, it's likely there is a leak in the servo diaphragm. Electric, I'd look for a bad electrical connection.
 
If I remember right GMC was still using vacuum shift at that time and the shift is by spring on way. I like the other poster suspect in high you have very little vacuum and it is letting the spring pull it back to low. Need to check your lines and the diaphragm there in the shift unit.
 
I have worked on dozens of these things. Still have one on my International. The older ones had a shift motor- a screw - with a block the loaded a spring that actually did the shifting and a set of points( actually a little knife switch that stopped the motor from running, a shift fork and a collar in the differential. They are supposed to hold a couple of ounces of motor oil for lubrication. The new design has two limit switches and a circuit board that constantly fries itself. They are not very complicated. Take a test light and check the shift switch and the wiring first. Then pull the cover off the unit and look inside. You'll figure it out.
 
Sounds like maybe it just needs freeing/cleaning up,those electric 2 speeds would stay in whatever range they were put in.I had one in a Louisville Ford that would get to
sticking every now and then,it liked to stay in high range and not shift to low range.
 
Had a truck that did the same thing, problem is it would only do it randomly and sometimes it would go for months without a problem.

Had checked all the connections and obvious things.

Finally one day while out on the road, just as I shifted gears the rear axle popped into neutral and at that point it would not go back into high or low.

Being dead in the water on the side of the road I crawled under and removed the cover from the box of the shift motor, turned the worm screw until the block that rides on it was at the end of it's travel then stuffed a rag in tight so the block could not move and put the cover back on.

It got me home and I decided that for what I use the truck for I really did not need to have low range, that was 3 or 4 years ago and it has never acted up since.

Not by any means a fix if you need to have the 2 speed functioning but it will get you home in a pinch.
 
(quoted from post at 07:36:16 09/21/19) I have worked on dozens of these things. Still have one on my International. The older ones had a shift motor- a screw - with a block the loaded a spring that actually did the shifting and a set of points( actually a little knife switch that stopped the motor from running, a shift fork and a collar in the differential. They are supposed to hold a couple of ounces of motor oil for lubrication. The new design has two limit switches and a circuit board that constantly fries itself. They are not very complicated. Take a test light and check the shift switch and the wiring first. Then pull the cover off the unit and look inside. You'll figure it out.
Click to expand...

Ditto on the shift circuit board, if it's the newer style that uses one.

They get "flaky" and do some strange stuff.

REALLY simple device, two SPDT microswitches, a diode, and a resistor for braking/stopping the motor quickly at the end of the shift, but a cracked solder connection or a failing switch will cause them to act up.

Lower drawing shows what the newer-style unit that uses a circuit board looks like.



w400_5b56a15b4d1a33e562b51971cd0aeb79954b8dcc.png



mGRtMxw.jpg
 
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