1. Missing Fuse or Relay (The Most Likely Cause)
* Check the Charging System Diagram: You mentioned a diagram showing a fuse or contacts in the power distribution box. Double-check that diagram to ensure you're looking in the right place. Some motorhomes might have slightly different wiring configurations.
* Locate the Fuse Box: The power distribution box in an E350 might be in several locations:
* Under the Hood: On the driver's side.
* Inside the Cabin: Near the steering wheel, under the dash, or by the driver's seat.
* Underneath the Motorhome: In the storage compartment.
* Inspect All Fuses: Even if the diagram indicates a specific fuse, other fuses in the box might be related to the auxiliary battery charging system. Look for blown or corroded fuses.
* Check for a Relay: Some systems have a relay that controls the auxiliary charging circuit. It might be located in the fuse box or in a separate relay box.
2. Faulty Alternator
* Test the Alternator Output: Use a voltmeter to test the voltage output of the alternator while the engine is running. It should be between 13.5-14.5 volts. If the voltage is lower, the alternator might be failing.
3. Issues in the Charging Circuit
* Inspect Wiring: Check the wiring between the alternator, the fuse box, and the auxiliary battery. Look for any signs of damage, corrosion, or loose connections.
* Test the Battery: A faulty auxiliary battery can prevent it from charging. Use a voltmeter to check the battery voltage when the engine is running. It should be increasing toward the alternator's output voltage.
* Check the Isolator: Some motorhomes use a battery isolator to separate the house battery (auxiliary) from the starting battery. Ensure the isolator is working correctly and connecting the auxiliary battery to the charging circuit.
3. Other Possible Causes
* Low Voltage Sensor: Some systems have a sensor that prevents the auxiliary battery from charging if the voltage drops below a certain threshold. This could be a faulty sensor or incorrect settings.
* Ground Fault: A poor ground connection somewhere in the charging circuit can disrupt the flow of electricity.
Steps to Take
1. Review the Charging System Diagram: Start by carefully studying the diagram for your specific 1994 Ford E350 motorhome. This will provide valuable information about the circuit components and their locations.
2. Inspect Fuses and Relays: Thoroughly check all fuses and relays in the power distribution box, even if they are not specifically mentioned in the diagram.
3. Test the Alternator: Use a voltmeter to test the alternator output while the engine is running.
4. Inspect Wiring and Connections: Examine all the wiring related to the auxiliary battery charging system for any signs of damage or corrosion.
5. Check the Battery and Isolator: Test the auxiliary battery voltage and ensure the battery isolator is working properly.
Important Safety Tips
* Disconnect the Battery: Always disconnect the negative terminal of the battery before working on the electrical system.
* Avoid Shorts: Be extremely careful when working with electrical components, as a short circuit can cause damage or injury.
* Seek Professional Help: If you're not comfortable troubleshooting electrical systems or are unsure about any of the steps, consult a qualified automotive technician.
Let me know if you have any other questions!
