Here's a systematic approach to troubleshooting and fixing this:
1. Check the Blower Motor Resistor:
* Location: The blower motor resistor is a common culprit. Its location varies slightly depending on your specific trim level, but it's usually located somewhere under the dash, often near the blower motor itself. Consult a repair manual or online forums specific to your 1998 Grand Cherokee (ZJ) to find its precise location.
* Function: This resistor regulates the fan speed. A faulty resistor can cause intermittent operation, or, in your case, only allow the motor to run when the voltage spikes (during acceleration). Look for burned or discolored components on the resistor. Replace it if you find any damage.
2. Inspect Wiring and Connections:
* Blower Motor Wiring: Carefully trace the wiring harness from the blower motor to the resistor and then to the control panel. Look for any loose connections, corrosion, broken wires, or areas where the insulation is damaged. Pay particular attention to the ground wire. A poor ground connection is a common cause of intermittent electrical problems. Clean any corrosion with a wire brush and reconnect securely.
* Fuse and Relay: Check the fuse and relay associated with the blower motor. The fuse might be blown, or the relay might be failing intermittently. Replace both if necessary. Again, consult a repair manual to identify the correct fuse and relay.
3. Check the Blower Motor Switch:
* Testing: While less likely given the symptom, a failing blower motor switch could also cause this issue. Testing it requires a multimeter to check for continuity at different fan speed settings. This is more advanced and requires electrical knowledge.
4. Voltage Testing:
* At the Blower Motor: Use a multimeter to measure the voltage at the blower motor terminals while the ignition is on and the fan is set to a speed. If the voltage is significantly lower than the battery voltage (around 12V), you have a voltage drop problem somewhere in the wiring. This confirms a bad connection or high resistance somewhere in the circuit.
* At the Blower Motor Resistor: Measure voltage at the input and output of the resistor to see if the resistor is dropping too much voltage even when it appears okay visually.
5. Ground Check:
* Blower Motor Ground: Ensure the blower motor has a good ground connection. A poor ground can cause erratic operation. Clean the ground connection and make sure it's tight. Consider running an additional ground wire directly from the motor to a known good chassis ground point for a definitive test.
Important Considerations:
* Repair Manual: A factory service manual for your specific Grand Cherokee model is invaluable for this type of repair. It shows wiring diagrams, component locations, and detailed troubleshooting procedures.
* Safety: Disconnect the negative battery terminal before working on the electrical system.
* Professional Help: If you're not comfortable working with car electrical systems, it's best to take your Grand Cherokee to a qualified mechanic.
By following these steps methodically, you should be able to isolate the cause of the problem and fix your blower motor. Remember to test after each step to see if the problem is resolved.
