Shift on the Fly 4WD Problem??? - Ford Powerstroke Diesel Forum
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post #1 of 5 Old 02-11-2007, 02:17 PM Thread Starter
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Shift on the Fly 4WD Problem???

I've got an 01 F250 crew cab 7.3 power stroke. It's got the shift on the fly 4WD with the knob on the dash. When I place the switch in 4X4 High or Low I here the motor on the transfer case shift, but the front hubs dosen't lock in automatic. I can place them in the lock position and it'll work. The vacuum switch on the inside of the passenger side fender is not opening. I've got vacuum going to the vacuum switch, but nothing out. I pulled the three wire electrical plug on the vacuum switch to check voltage, I had 14.6 volts on the center plug with the dash switch on or off. I couldn't find the wire diagram in my Hayes manual for this circuit. In the truck's owners manual it show this on the same fuse as the backup lights and its not blown. I guess it could be in the dash switch, but I'm not sure if there's anything between it and the vacuum switch that may keep this from working. Has anyone had this same issue?
Thanks for the help,
Jim
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post #2 of 5 Old 02-11-2007, 02:20 PM
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Im just guessing here but it could be your seals are leaking on your hubs.

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post #3 of 5 Old 02-12-2007, 02:05 AM
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Welcome to Powerstroke, glad your here. See if you can blow through the lines and feel it on the other end, it may somehow have gotten plugged up. When you switch it over, does your defrosters come on? If so it's a vac leak somewhere.

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post #4 of 5 Old 02-12-2007, 05:24 PM Thread Starter
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Thanks for the help guy's. I've not check for leaks in the hub yet, but I did check vacuum on the line before it tees off to the two hubs. I didn't have vacuum on that line when its in 4WD, but it did have it on the line going to the valve/switch. The line coming from the valve could be plugged, I've not check it yet. You would think it would have to energize the vacuum switch/valve before you get vacuum to the hubs. No matter which position I have the 4WD dash switch in, the center terminal on the plug has 14.6 +VDC, but I didn't think to check if it was connecting one of the other two wires to ground. I'll check that next.


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post #5 of 5 Old 03-17-2007, 07:58 AM
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I had the same problem on my 2000 F-350 .. Mine was the seals on the hubs were cracking loosing vacuum.. Replaced both sides and has been working ok
About 2 weeks after replacing my seals my vacuum pump went out .. Replaced it also.. I'm thinking the seals were so bad that the pump could have pulled moisture in the lines,,
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