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I found a 2001 Ford Ranger 4x4 on craigslist that the owner said was stuck in 4-wheel drive low range.
4WD Light Operation: When you turn the ignition on, the dash will briefly display the 4WD and 4WD LOW lights, and then go out. If the vehicle is in 4Hi or 4Lo, then that light will stay on. If there is a problem in the 4WD, the 4WD lights will flash intermittently
Shifting The 4WD: The 4WD can be shifted from 2Hi to 4Hi as long as the vehicle is below 45 MPH. You CANNOT shift from 2Hi straight to 4Lo. The vehicle must be shifted in to 4Hi before it can be shifted in to 4Lo. In order to go from 4Hi to 4Lo, the truck needs to be placed in neutral, and you have to hold the brake pedal down. As long as the truck isn't moving, the transfer case will shift in to 4Lo. There is generally a brief moment before the transfer case shifts.
When you shift the transfer case, you should be able to hear a clicking sound coming from the transfer case control module.
When I started the Ranger, the 4WD lights came on like they should, went out, and then displayed the 4Lo light indicating that the truck was in 4Lo. When putting the truck in neutral and holding down the brake pedal, the transfer case refused to come out of 4Lo. I did not hear the transfer case control module clicking either.
Manually Shifting The Case In To 2HI:
I keep 'The Shiftster' in my toolbox in case I should ever have a problem with a transfer case shift motor. Using one is very easy. There's (4) 10mm bolts that mount the transfer case motor to the transfer case. The Shiftster didn't come with any bolts, but the transfer case is held on with (3) long bolts, and (1) short one. I was able to use the (1) short bolt, and (2) of the long ones to hold the shiftster in place.
When you remove the transfer case shift motor, you will see the the case has 2H, 4H and 4L molded right in to the case. The shaft for the shift motor is actually shaped like an arrow pointing to the position the transfer case is in. I actually took the photo below after I removed The Shiftster to put the new shift motor on. That's why it's in 2H.
Once The Shiftster was mounted, I was able to turn the knob and lock the transfer case in 2H. Then I was able to drive the vehicle home.
Check The Fuses:
This should be the first thing you do. Unfortunately, I didn't have the fuse information on hand when I first looked at the vehicle. And honestly, fuses don't just go bad. If there would have been a blown fuse, there would have been a bigger problem.
Once the vehicle was home, I started diagnosing the problem.
There are (2) fuses that control the 4WD. One in the passenger compartment, and (1) under the hood.
Check the # 5 (4x4 control module) 15 amp fuse in the passenger compartment.
Also check the #13 (4x4 Motor) 20 amp fuse under the hood.
Replace these fuses if they're bad, and retest the 4WD if necessary.
Test The 4WD Switch:
Next, test the 4WD switch to make sure it's functioning properly.
You'll need a multimeter. You can see where I used the ground wire on the cigarette lighter / 12 volt jack for the multimeter ground. Next, I placed the meters probe in to the second pin slot as shown in the photo below. I believe it's a white wire with a blue stripe.
With the meter hooked up, I cycled the switch between 2Hi, 4Hi and 4Low. I checked the voltage (1.78, 2.94, and 3.85 volts) as shown in the photos below. This showed that the switch as functioning properly.
The two common failures in the 4WD system is the shift motor, and the control module. I went ahead and replaced the shift motor. This may seem like an expensive part, but you can actually get a new transfer case shift motor from Rock Auto for $64.00. For that price, I went ahead and just replaced it.
(FYI, once the 4WD was fixed, and reconnected the old shifter motor, and it didn't work.)
The 4WD system will not allow the transfer case to shift in to 4Lo unless it sees 0 MPH. I noticed that the speedometer never went above 38 MPH when I was driving the truck home. It could have been because the truck still thought it was in 4Lo. Irregardless, these sensors range from $20 - $40, which is much less than what you would pay a ford service tech to diagnose your 4WD.
The speed sensor is mounted on top of the transmission (not the transfer case) near the rear. It gets its signal from the transmissions output shaft.
When I unplugged the sensor, I noticed that the wires were bare near the plug. This creates a risk for a short, so before it was plugged back in, the wires were coated with a liquid electrical tape that you apply with a brush. The sensor is held on with a 10 mm bolt. The sensor was removed and replaced with a new one.
Transfer Case Control Module:
At this point, the transfer case would shift in and out of 4Lo, but it was very intermittent. Actually, the new case only shifted a couple of times and quit.
The next step was to replace the transfer case control module.
The control module is behind the passenger side kick panel under the dash. You can find these used, but you can get a new one from Amazon for $150.00.
The original control module was P/N 1L54-7H417-AC. The new module is P/N 1L54-7H417-AE.
Alternate/OEM Part Number(s): 1L54-7H417-AB, 1L547-H417-AC, 1L54-7H417-AD, 1L54-7H417-AE, 1L5Z-7E453-AB, 1L5Z-7E453-AC
Once the control module was replaced, the 4WD worked perfectly.
I purchased A new shift motor, control module, and a speed sensor. The total cost was approximately $250.00, but it basically replaced every vulnerable part in the system. Also, it included new parts. Why risk the reliability using used parts.