Simple Checks:
* Fan Speed: Is the fan blowing at all? If not, the problem might be the blower motor, its resistor, the blower motor fuse, or a wiring issue to the fan.
* Temperature Controls: Are the controls (dial, buttons, etc.) working properly? Are they set to "heat"? A faulty control panel is a possibility.
* Coolant Level: Is the coolant level in the radiator and overflow tank low? Low coolant means the engine isn't getting properly heated, thus no heat for the cabin. Check for leaks.
* Thermostat: Is the thermostat stuck closed? A stuck closed thermostat prevents coolant from circulating, preventing the engine from heating up. This is a common issue.
* Hot Air at Vents (with fan on): If the fan blows but the air is cool even after the engine has warmed up, the problem lies in the heating system itself.
More Complex Issues:
* Blower Motor Resistor: This controls the fan speeds. A failed resistor often leads to only the highest fan speed working (or none at all).
* Blower Motor: The motor itself might have failed.
* Heater Core: This is a small radiator inside the vehicle that warms the air. A clogged or leaking heater core will prevent heat from reaching the cabin. A leak will show up as a wet carpet on the passenger side.
* Water Pump: If the water pump isn't circulating coolant, the engine won't heat up, and no heat will reach the cabin.
* Blend Door Actuator: This motor controls the amount of hot/cold air mixing. A malfunctioning actuator can prevent hot air from reaching the vents.
* Wiring and Connectors: A broken wire or corroded connector anywhere in the heating system's circuit could prevent proper operation.
* Thermostat Housing: A cracked or leaking thermostat housing can prevent proper coolant flow.
Troubleshooting Steps:
1. Start with the simple checks listed above. This often pinpoints the problem quickly.
2. Check fuses and relays. Consult your owner's manual to locate the appropriate fuses and relays for the heater system.
3. Listen for the blower motor. If you can hear a faint whirring, it may indicate a weak motor or a blockage.
4. Feel the heater hoses. With the engine running, feel the hoses leading to and from the heater core. They should both be hot if the system is working correctly. If one is cold, there's a restriction in the flow.
5. If you're not comfortable working on the vehicle yourself, take it to a mechanic. Diagnosing heating system issues can be challenging.
Remember safety first! Always work on a cool engine and disconnect the battery's negative terminal before working on electrical components. A repair manual specific to your 1999 Jeep Grand Cherokee Limited will be very helpful.
