Electrical System Issues:
* Bad 4WD Shift Motor: This is a common culprit. The motor that engages the transfer case might be weak or failing, especially when cold and the lubricant is thick. The cold thickens the grease making it harder for the motor to turn.
* Wiring Harness Issues: Corrosion, broken wires, or poor connections in the wiring harness leading to the transfer case actuator motor are very common in older vehicles. Cold temperatures can exacerbate these problems. Inspect for cracks, corrosion, and loose connections, particularly near the transfer case and battery.
* Transfer Case Shift Switch (or related switch): The switch in the cab that you use to select 4WD might be faulty, causing a lack of signal to the actuator motor.
* Fuse or Relay: Check the fuses and relays associated with the 4WD system in the fuse box under the hood and inside the cab. One may be blown or failing due to cold weather affecting its contacts.
* Control Module: A faulty Body Control Module (BCM) or similar control module could be preventing the signal from reaching the actuator motor. This is less common but possible.
* Ground Issues: A poor ground connection somewhere in the 4WD system can prevent proper operation. Check all ground points near the battery and the transfer case.
* Low Battery Voltage: A low battery charge, particularly in cold weather, can affect the operation of electrical components.
Mechanical Issues (Less Likely but possible, particularly if electrical checks yield nothing):
* Transfer Case Issues: Internal problems within the transfer case itself, such as a broken linkage, worn components, or low fluid level, could prevent engagement. The cold might just make an already weak component fail entirely. Low fluid is less likely to suddenly appear but would be aggravated by cold conditions.
* Hub Issues (Manual Locking Hubs): If your Ranger has manual locking hubs, they might be frozen or sticking. Try to manually engage them (if so equipped) and check for smoothness of operation. This is less likely if the dash light isn't working, however.
Troubleshooting Steps:
1. Start with the simplest: Check the fuses and relays.
2. Inspect the wiring: Look for visual damage in the wiring harness leading to the transfer case actuator motor.
3. Test the 4WD Shift Motor: This might require some electrical testing with a multimeter to check for voltage and current at the motor connector when the switch is activated. A competent mechanic can assist with this.
4. Check the battery voltage: Make sure it's fully charged.
5. Test the switches: If you can get a service manual, you may be able to test the 4WD selector switch to determine if it's properly sending a signal to the system.
6. Check the transfer case fluid level: Make sure the fluid is at the correct level.
Recommendation: Since this is an older vehicle, and the problem seems to be electrical in nature (dash light not working), you might be better off having a mechanic familiar with 4x4 systems diagnose the problem. Trying to troubleshoot the electrical system on your own could be challenging if you lack the necessary tools and experience. The cost of a diagnosis may be worth avoiding further damage or incorrect repairs.
