1. Coolant Flow:
* Thermostat: The most common cause. A stuck-closed thermostat prevents coolant from circulating to the heater core. This is a relatively easy and inexpensive part to replace. Feel the upper and lower radiator hoses when the engine is warm. If the upper is hot and the lower is cool, the thermostat is likely the culprit.
* Heater Core: The heater core is a small radiator inside the car that warms the air. It can become clogged, leak, or fail entirely. A leak would be evident by a sweet smell in the cabin or damp carpets. A clogged core requires replacement.
* Water Pump: A malfunctioning water pump won't circulate coolant effectively, impacting both heating and engine temperature. Listen for unusual noises from the pump.
* Airlocks in the Cooling System: Air trapped in the cooling system can prevent coolant from reaching the heater core. Bleeding the system (often a process of carefully opening bleed valves or raising the front of the car) might resolve this. Consult your owner's manual or a Jaguar XJS specific repair manual for the correct bleeding procedure.
2. Heater Control Valve:
* Actuator: The heater control valve, often electrically operated, directs coolant flow to the heater core. A faulty valve or its actuator (the motor that operates it) will prevent hot coolant from reaching the heater core. This usually requires testing the valve's electrical operation and the actuator's movement with a multimeter or visual inspection.
3. Blend Door:
* This mechanism controls the proportion of hot and cold air. A malfunctioning blend door could be stuck in the cold air position even if the heater core is receiving hot coolant. This often requires accessing the HVAC system to diagnose and repair.
4. Low Coolant Level:
* Check the coolant level in the expansion tank (usually a translucent reservoir). Low coolant would affect heating capabilities.
Troubleshooting Steps:
1. Check Coolant Level: Ensure the coolant is at the correct level. Top it off if necessary, but be cautious as the engine may be hot.
2. Feel Hoses: Once the engine is warm (but not overheating), feel the upper and lower radiator hoses and the heater hoses (if accessible). They should both be hot if the system is working correctly. If the heater hoses are cold, the problem is likely the thermostat, heater core, water pump, or an airlock.
3. Check Thermostat: This usually requires removing the thermostat housing, which is straightforward on most vehicles but could vary with the XJS.
4. Listen for Unusual Noises: Listen for any unusual noises from the water pump. A failing pump often makes a whining or rumbling sound.
5. Test the Heater Control Valve: This usually involves checking its electrical continuity and whether it moves when commanded by the HVAC system. A detailed wiring diagram is essential for this.
6. Consult a Repair Manual: A Jaguar XJS-specific repair manual is invaluable. It contains detailed wiring diagrams, component locations, and repair procedures. Haynes and Bentley manuals are popular choices.
Important Note: Working on the cooling system involves hot coolant and potentially pressurized components. Allow the engine to cool completely before attempting any repairs. If you're not comfortable working on your car's cooling system, it's best to consult a qualified mechanic specializing in classic Jaguars. They have the experience and tools to diagnose and repair the issue safely and effectively.
