* Low Battery Voltage: This is the most straightforward explanation. A weak battery can't supply the surge of current needed to energize the compressor clutch's electromagnet. The jump-start temporarily provides the necessary voltage. Test your battery's voltage both at rest (should be around 12.6V) and under load (with the engine running and the headlights on, should be above 13V). A weak or failing battery needs replacement.
* Bad Battery Cables/Connections: Corroded, loose, or damaged battery cables (positive and negative) can restrict current flow. Clean the terminals thoroughly, check for broken or frayed wires, and ensure tight connections.
* Faulty Alternator: If the alternator isn't charging the battery adequately, the voltage will drop, especially under load. Have the alternator tested at an auto parts store. A low output alternator won't provide enough power to keep the battery charged and the compressor running.
* Failing Compressor Clutch Relay: The relay acts as a switch, completing the circuit to the compressor clutch. A weak relay may not close reliably with low voltage, working only after the jump start provides a temporary voltage boost. Replace the relay – they're inexpensive.
* Low-Voltage Cutout Switch (if equipped): Some systems have a low-voltage cutout switch that prevents the compressor from engaging if the voltage drops too low to protect the electrical system. This could be malfunctioning or set too high.
* Wiring Issues: Look for corroded, frayed, or loose wires in the circuit from the battery, through the relay, to the compressor clutch. A poor connection anywhere in the circuit can cause voltage drop.
* Compressor Clutch Coil: While less likely than the other issues, the coil in the compressor clutch itself could be failing. It may have high resistance, requiring a higher voltage to energize. This is less probable since it typically causes complete failure rather than intermittent operation.
Troubleshooting Steps:
1. Check Battery and Cables: Start with the simplest and most common causes first. Clean and tighten the battery terminals, inspect the cables, and test the battery voltage.
2. Test the Alternator: Have the alternator tested to ensure it's charging properly.
3. Replace the Compressor Clutch Relay: These are cheap and easily replaced. It's a good preventative measure even if you've already checked other areas.
4. Inspect Wiring: Carefully check all wiring in the AC system circuit for damage or corrosion.
5. Voltage Drop Test: Use a voltmeter to check the voltage at various points in the circuit (battery, relay, compressor clutch) while trying to engage the compressor. A significant voltage drop indicates a problem in that section of the wiring.
If you've checked all these and still have the issue, you might need a professional mechanic to diagnose the problem more thoroughly. The AC system is complex, and a multimeter is helpful for accurate testing.
