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"stuck in 4wd" problem is electric/sensors

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Old 09-27-2012, 03:40 PM   #1
pcrussell50
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1997 expolrer xlt
 
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"stuck in 4wd" problem is electric/sensors

1997 explorer xlt, 4.6 sohc: transfer case permanently locked in 4wd

what i've done so far:

1] read a bunch of threads on here before i joined. it helped me learn a LOT about how the Awd system works on my car

2] went back and performed the proper process with the switch knob on the dash, for putting it back into "auto", where the transfer case should only lock if the system detects slipping--this did not work. with the switch in "auto" the case was locked and i got scrubbing on dry pavement

3] pulled the fuse for the 4wd system--this did work, but i could not live with it, because the power windows are on the same fuse. so i put the fuse back in, went under the car, and...

4] ...pulled the pigtail off the motor that actuates the transfer case lock--this worked. and now i can drive without scrubbing tires and stressing driveline. BUT, i want to fix this.

based on these steps, i think i can assume that the problem is not mechanical, because the the transfer case will lock and unlock and the motor that does this also works.

so i think the problem is electronic, with one of the sensors that tells the pcm to send a "lock" signal to the case-actuator-motor.

periodically, i get the the amber messages on the dash: "4wd" and "4wd lo" that flash a few times, then go away. either this is because i pulled the pigtail off the case-locker-motor, or the number of flashes is telling me which sensor has gone bad--i don't know which.

oh, by the way, the problem is not in the pcm/engine control computer, because i have swapped that out too, and the problem remains identical.

thanks, and HELP!

-peter

Last edited by pcrussell50; 09-27-2012 at 04:37 PM. Reason: clarity
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Old 09-28-2012, 06:34 PM   #2
Chikar
Ladysmith, British Columbia
1999 Explorer Sport 4X4
 
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If you've got a switch knob on the dash you have Automatic 4WD.

There are two Hall effect speed sensors on the transfer case. 1 each on the front and rear output shafts.

The flashing 4Hi/4Lo lights is the system telling you it's not getting a signal from the sensors which it wouldn't with the connector unplugged. That's because the speed sensor wires are in the connector to the shift motor.

This part gets confusing.

The shift motor doesn't actually put the case into 4WD, that's accomplished by an electromechanicall ball ramp clutch in the case controlled by the GEM (Generic Electronic Module)

In Auto mode the speed sensors detect a speed difference, send the difference signal to the GEM, and the GEM engages the clutch.

in 4Hi the switch sends a signal to the GEM you want 4Hi to stay on and the GEM sends a signal to the clutch regardless what the speed sensors on the output shaft are telling it about axle speeds.

In 4WD Low Range, the transfer case electromechanical clutch locks the front and rear driveshafts for maximum 4WD traction. The transfer case motor also rotates the shift cam to move the reduction fork to the 4WD low range position. This low range shift is accomplished through a planetary gear set which changes torque to the driveshaft from 1:1 to 2.48:1 ratio.

Clear as mud isn't it. It took me a while to wrap my head around this when I went looking for the cure to my problem.

My thought is you've likely got a bad front output speed sensor. Either completely dead or not accurate. The GEM thinks the rear wheels are turning faster than the front and engages the ball clutch. Or you've got a wiring problem.
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Old 09-28-2012, 11:47 PM   #3
pcrussell50
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awesome! and not confusing at all. great description. thanks for clarifying that. i was beginning to see the error in how i described the operation of the system and now you have confirmed it. makes much more sense than how i described it.

can i change the hall effect speed sensors without cutting and splicing?

-peter
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Old 09-29-2012, 12:27 AM   #4
Chikar
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You won't have to do any cutting and splicing but will need a pin extraction tool.

You need to extract the pins for the speed sensor you're replacing from the connector on the shift motor. Make SURE you record which wire goes in which slot, backwards would be a bad idea :-)

Quote from the Ford manual:

"Remove the front output shaft speed sensor wires from the electrical shift motor connector.
Use the proper electrical connector pin remover."

The only problem is they don't say what is the "proper" pin remover. They also say to remove the shift motor for some reason.

Ideally you want to extract the pins but if you can't get the proper extractor I suppose you could cut and splice the new wires in. Just make sure you used good quality tubular crimp connectors with a crimping tool and use quality heat shrink tubing over the splice as well. It's a nasty environment under the truck. A corroding splice will give you future problems.

I have read in some posts people have had success removing and cleaning the speed sensor rather than replacing it. Metal particles and general crud buildup over the years can block them. But they're 15 years old, your call.

I'd also recommend you change the case oil if you don't know how long it's been since it's last change, if ever. Just be sure you can get the upper fill plug out (rusted in, etc) before you open the lower drain plug.

Last edited by Chikar; 09-29-2012 at 12:28 AM. Reason: Add info
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