Why Your RV Generac Diesel Generator Shuts Off After 2 Minutes Under Load: Troubleshooting Guide

The Quick Answer

Ed, your Generac 7.5kw diesel generator on your 2007 Alfa is most likely shutting down due to overheating when the AC load is applied. Since it runs fine with no load but consistently shuts off after 2 minutes with your basement AC running, and restarts immediately but shuts down right away on the second attempt, this points to a thermal protection shutdown. The fact that you just serviced it with new filters and oil suggests the issue isn't fuel starvation or lubrication problems. Without error codes flashing, we're looking at either a cooling system blockage, faulty temperature sensor, or the cooling fan not operating properly under load.

What's Causing This

Your generator's diesel engine is designed to handle the 7.5kw load, but something is preventing proper heat dissipation when that AC kicks in. The most common culprits are a clogged radiator or cooling fins, which restrict airflow and cause rapid overheating under load. Even though you performed recent maintenance, debris can accumulate quickly in RV environments.

Another possibility is the cooling fan isn't engaging properly or the fan belt is slipping when the engine works harder. Some Generac units have a thermostatic fan clutch that may be failing. Additionally, the coolant level might be low, or there could be air pockets in the cooling system that weren't properly bled after your service.

The temperature sensor sending false readings to the control board is also a suspect. If it's reading higher temps than actual, it will trigger premature shutdown. On your 2007 model, these sensors can degrade over time and become overly sensitive.

Less likely but possible causes include a failing voltage regulator causing the engine to work harder than necessary, or carbon buildup in the combustion chamber creating hot spots that trigger thermal protection even with adequate cooling.

Tools You'll Need

• Digital multimeter for electrical testing
• Infrared temperature gun ($25-40) to check actual engine temps
• Compressed air source or leaf blower for cleaning
• Basic hand tools (wrenches, screwdrivers)
• Garden hose for radiator flushing
• Coolant system pressure tester (rental from auto parts store, $20-30)
• Wire brush for cleaning cooling fins
• Shop vacuum for debris removal

Step-by-Step Fix

WARNING: Always allow the generator to cool completely before working on the cooling system. Engine components can remain dangerously hot for extended periods and cause severe burns. Never remove radiator caps or touch cooling system components while hot.

Start by letting the generator cool completely, then remove the access panels to expose the cooling system. First, check the radiator and cooling fins for debris buildup - RV generators are notorious for collecting grass, leaves, and dirt that blocks airflow. Use compressed air to blow out the radiator from the inside out, working systematically across all fins.

1. With the engine cool, check coolant level in the radiator and overflow tank. Top off if low and look for signs of leaks around hoses and connections.
2. Start the generator with no load and use your infrared temperature gun to establish baseline temperatures at the thermostat housing and radiator inlet/outlet.
3. Watch the cooling fan operation - it should engage according to manufacturer specifications. If it doesn't engage or runs constantly, the fan clutch or temperature switch needs replacement.
4. With the generator running unloaded, check belt tension on the cooling fan. Check deflection according to your Generac model specifications (consult manual for exact values).

Now test under load by turning on your AC. Monitor temperatures closely and note exactly when shutdown occurs. If temps spike rapidly past 200-210°F, you've confirmed overheating. If the temperature gun shows normal temps but shutdown still occurs, suspect a faulty temperature sensor.

To test the temperature sensor, locate it near the thermostat housing (usually a single wire connector). WARNING: Shut off the generator and allow to cool before electrical testing to avoid shock hazards. With the engine at normal operating temperature, use your multimeter to check resistance - consult your Generac service manual for the specific resistance values at various temperatures for your model. If readings are erratic or way off spec, replace the sensor with the correct part for your model (verify with dealer).

When to Call a Pro

Call a certified Generac technician if you find no obvious cooling system problems but the unit continues shutting down under load. Internal engine issues like carbon buildup, valve problems, or fuel injection timing require specialized diagnostic equipment and expertise. Also, if you discover coolant leaks from internal engine components or the water pump, these repairs typically require significant disassembly.

You should also seek professional help if electrical testing reveals voltage regulation problems or control board issues. These components can be expensive to replace incorrectly, and warranty coverage may apply depending on your maintenance history. A qualified tech can also perform a complete load bank test to verify the generator's actual output capacity and identify any internal electrical faults.

Given that your 2007 Alfa represents a significant investment, don't hesitate to get professional diagnosis if initial cleaning and basic checks don't resolve the issue. Many RV service centers offer diagnostic services for around $100-150, which can save money compared to parts replacement guesswork.