Chevy C70 2 speed rear stuck in high

[JRP3]

We have a 1980's vintage C70 with 2 speed rear at work, the rear end shift wire corroded and shifted into high, and then I guess shorted out because it's stuck in high. We only use it in the yard and I need it to be in low all the time. I ran new wires to the switch but it's not shifting. The old wires were so corroded the insulation fell off and I wasn't sure of the color coding, looked like one red wire and two black wires. Maybe a blown fuse? Haven't found it yet. Is there a way to manually get it to shift into low on the rear end itself? Seems to be a solenoid that applies air pressure to the axle.

 

[bvfdfire]

depends whether you have a rockwell rear versus an eaton, the rockwell uses a vacuum motor to pull the two speed shift mechanism between high and low range. It will have a large hose going the shift motor. It seems that most of these went away in the mid 70's? The eaton uses 3 wires that extend all the way to the 3rd member. Best of my recollection, one red, one black, and one green. the easiest place to check the power going to the motor is on the splitter panel on the firewall, red wire should be hot all time. If you are getting power back to the motor, on the eaton shifter, the most common issue that they had were, that they have a set of contactors inside the housing that would arc and corrode which would leave them in whichever range it happened to be in at that time. Eaton used to have a replacement contactor set repair set readily available from mfr. In fact, we used to be able to get them from Standard ignition. There are only two wires inside the housing which run the motor, so you should be able to jump the center wire, which should be hot all time to either the upper wire or the lower wire, if the points are not stuck, you should hear the motor shift when you apply power. If you want to shift it to low range and leave it there, you could pull the motor out of the housing and screw the shift mechanism with a screwdriver, however, you still have to pull the front cover off the shifter assembly to remove the motor wires and allow you enough slack to completely remove motor from housing. If you wish to bypass all this and just block it into low for good, pull the two shifter motor off and observe which way the fork that you will see sticking out of the rear end assembly is pointing. it should be pointing toward the left front of the vehicle to for low range, toward the right front for high range. Take a pry bar and pry the fork to the opposite direction until you feel it make a dull thunk. be sure that there is no chance that it can roll, because the fork will not be able to move if there is pressure against it. Once it moves about an inch, you can put the shift motor back on it to hold it in that range and you should be good to go.

 

[JRP3]

Here's what it looks like. Single wire going to the solenoid.

 

[JRP3]

Here's the switch with the wiring I put on, so the colors don't mean anything. I have 12V going to the middle terminal, the other two go to the two upper right hand terminal blocks on the firewall, blue wires if you can tell from the firewall pic.

 

[bvfdfire]

Here's a link to the factory service manual........ http://www2.dana.com/pdf/AXSM-0029.pdf hopefully it can of use to you, I would be suspect of the shift valve. That is if you have checked the fire at the switch. The speedometer sensor changes the ratio inside itself to compensate for the variance in speed of the different ratios between low and high range.

 

[JRP3]

Thanks, that's sort of helpful, but my setup seems to be different, (of course). In mine the solenoid valve is plumbed to the tank through a small fitting of some sort, maybe a pressure relief valve, and the solenoid goes directly to the axle shift unit with nothing else in between, no 2 speed air shifter valve and no pressure switch with quick release valve or speedo adapter.

I have power to the solenoid with the switch "in", which should be low, so I'm guessing the solenoid is stuck, or the axle shift unit itself. I assume the solenoid puts air pressure to the shifter in one position and releases it in the other?

 

[JRP3]

Reading further, this is definitely a different system, since the manual talks about an air valve on the shifter, but mine is an electrical switch.

In operation, the air shifter valve (in gear shift knob) supplies
air through the quick release valve to the axle shift unit. Air
pressure in the shift unit shifts axle to High range. To shift
axle to Low range, air pressure is exhausted at shifter valve,
which exhausts air pressure at quick release valve, allowing
shift unit to return to Low range.
Low Range Operation — Shifter knob is in “LO”, shifter valve
is closed and no air pressure is present to shift unit. Axle is
held in Low range by shift unit return spring.

I wonder if I physically disconnect the air line to the axle shifter if that would release the pressure and move it into Lo. What would happen if it shifted while sitting still, would it actually shift when I started driving?

 

[bvfdfire]

I noticed the difference in the shift valve as well. You could pull the nuts on the motor in your picture and shift it manually then put the motor back on to hold it there.

 

[bvfdfire]

or better yet; pinch the air line off between the solenoid and motor and that should prove your query pretty quick.

 

[JRP3]

Wouldn't I want to open the line up and release the pressure, to shift it into low? Or do you mean I should pinch it off so it doesn't build pressure when I start the truck up?

 

[bvfdfire]

If you pinch it closed, that will not allow the pressure to get to the shift motor. So either way in your post will work, however you will have an air leak that will have to deal with.

Easiest way to picture it; the shift motor is in low gear and held there by spring pressure until you apply pressure to the motor, when you apply pressure to the motor, it overcomes the spring pressure and allows the shifter into high range.

 

[JRP3]

Fixed! The lines were hard plastic so I couldn't really pinch them off, so I just took off the fitting on the tank and put in a pipe plug. That did the trick. Thanks for all the help :notworthy