1. Ignition System Problems:
* Ignition Switch: The switch itself could be faulty, preventing power from reaching the starter solenoid. This is a common failure point in older vehicles.
* Ignition Coil: A weak or failing ignition coil won't provide the necessary spark to ignite the fuel. You might not hear *anything* if this is the problem.
* Distributor (if equipped): If your 7.5L engine has a distributor, problems within it (rotor, cap, points/electronic ignition module) can prevent the spark from reaching the spark plugs. This is less likely on a 1986 but possible.
* Wiring: Corrosion, loose connections, or broken wires in the ignition system can interrupt the electrical flow to the starter or other critical components. Check all wiring, especially near the battery and starter.
* Neutral Safety Switch (NSS): This switch prevents the engine from starting unless the transmission is in park or neutral. A faulty NSS will prevent the starter from engaging.
* Inertia Switch: Some vehicles have an inertia switch that cuts power to the fuel pump and/or ignition system in a collision. Check if yours is tripped.
2. Fuel System Problems:
* Fuel Pump: A faulty fuel pump won't deliver fuel to the engine, preventing it from starting. You might hear the fuel pump prime when you turn the key; if not, this is a strong suspect.
* Fuel Filter: A clogged fuel filter can restrict fuel flow sufficiently to prevent starting.
* Fuel Lines: Check for cracks or blockages in the fuel lines.
3. Other Possibilities:
* Starter Solenoid: While you say the starter is good, the *solenoid* (the part that engages the starter motor) could be the issue. It's a separate component and can fail even if the starter motor itself is fine. You might hear a click from the solenoid when you turn the key, but not the engine cranking.
* Flywheel Issues: While less common, problems with the flywheel (e.g., stripped teeth) can prevent the starter from turning the engine.
* Security System (if equipped): An aftermarket or factory security system could be preventing the engine from starting.
* Low Compression: If the engine has low compression (due to worn rings, valves, etc.), it might crank but not start. This would usually be accompanied by other symptoms like lack of power or excessive smoke.
Troubleshooting Steps:
1. Listen Carefully: When you turn the key, listen for any clicks, whirs, or other sounds. This can help pinpoint the problem area.
2. Check the Battery Connections: Ensure the battery terminals are clean, tight, and corrosion-free.
3. Check the Starter Solenoid: See if you can hear a click from the solenoid when you turn the key. If so, you might need to replace it.
4. Test the Ignition System: Check spark at the spark plugs (safely!).
5. Check the Fuel Pump: Listen for the pump to prime when you turn the key. If not, this is prime suspect.
6. Check for Codes (if applicable): Does your motorhome have any diagnostic capabilities? This is unlikely on a 1986, but worth checking.
This situation requires systematic troubleshooting. Start with the simplest things (battery connections, solenoid), then move to more complex areas (ignition system, fuel system). A multimeter is an invaluable tool for diagnosing electrical problems. If you're not comfortable working on your motorhome's electrical system, it's best to consult a qualified mechanic specializing in older vehicles or RVs.
