1. Power Issues:
* Fuses: Check the fuses in the interior fuse box and under the hood. Your owner's manual will show you the fuse layout and which fuses relate to the power locks. Look for blown fuses (broken filament). Replace any blown fuses *after* addressing the underlying problem (if you find one), or they'll blow again immediately.
* Battery: A weak battery can sometimes cause intermittent or complete failure of power accessories like power locks. Check the battery voltage with a multimeter. It should be around 12.6 volts when fully charged. If it's low, charge or replace the battery.
* Wiring: Check the wiring harness for any damage, corrosion, or loose connections, particularly around the door switches, the central locking module (if equipped), and the locks themselves. Look for broken, frayed, or corroded wires. This can be difficult, and you may need a wiring diagram to trace the circuits.
2. Actuator Problems:
* Door Lock Actuators: The actuators are the motors inside the doors that actually move the locking mechanism. They can fail due to wear, age, or damage. You'll likely need to remove the door panel to access and test the actuator. Sometimes you can hear a faint clicking sound if the actuator is getting power but failing to move. Replacement is usually the solution.
* Trunk/Hatch Release Actuator (if applicable): The same applies to the actuator controlling the rear hatch/cargo area lock.
3. Switch Issues:
* Door Lock Switches: The switches in the driver's and passenger's doors might be faulty. Try testing them with a multimeter to see if they're sending a signal when pressed. A simple test is to see if the other door locks operate when pressing the opposite door switch. If only one door switch is malfunctioning, that is most likely the culprit.
* Key Fob (Remote): If your Astro has remote keyless entry, check the batteries in the key fob and test its range.
4. Central Locking Module (Body Control Module - BCM):
* BCM Failure: Some Astro vans have a central locking module (often part of a larger Body Control Module). A faulty BCM can cause issues with all the power locks. This is more difficult to troubleshoot and often requires professional diagnosis with a scan tool.
5. Vacuum System (Less Likely):
* Vacuum Leaks: Older vehicles sometimes use vacuum to operate the locks. This is less common in 1999, but a vacuum leak could prevent them from working correctly.
Troubleshooting Steps:
1. Start with the simplest: Check the fuses first.
2. Test the switches: Try the switches to see if they respond.
3. Check the battery voltage.
4. Inspect the wiring: Look for visible damage.
5. Listen for clicking sounds: This might indicate a faulty actuator.
6. Test the actuators (if accessible): This usually requires removing the door panel.
7. Check the key fob (if applicable).
8. Consider a professional diagnosis: If you cannot isolate the problem, take it to a mechanic or auto electrician.
Remember safety first! Disconnect the battery negative terminal before working with any electrical components to avoid shocks. Consult a repair manual specific to your 1999 Astro van for wiring diagrams and detailed repair procedures.
