08-29-2025, 10:35 PM
The Bobcat 825 and Its Mechanical Simplicity
The Clark Bobcat 825, introduced in the late 1970s, was a compact skid steer loader built for rugged utility work. Powered by a Perkins 4-cylinder diesel engine, it relied on mechanical systems for fuel delivery, ignition, and shutdown—long before electronic control modules became standard. These machines were designed for reliability and field serviceability, often used in municipal yards, farms, and small construction outfits.
Perkins engines of that era, particularly the 4.108 and 4.236 variants, were known for their simplicity. They featured mechanical injection pumps and manual fuel shutoff levers, making them ideal for equipment that needed to run in remote or low-tech environments.
Symptoms of a Non-Shutting Engine
In one case, a 1978 Bobcat 825 was rebuilt after its electrical system had been completely severed. Once rewired, the engine started successfully—but refused to shut off when the ignition key was turned. This raised questions about whether the engine used an electric fuel solenoid or a manual shutoff mechanism.
The team working on the machine identified a small cylindrical device mounted between the fuel tank and the injection pump. Initially suspected to be a fuel shutoff solenoid, it was later confirmed to be a Facet electric fuel pump—used to prime and deliver fuel, not to stop it.
Understanding Perkins Fuel Shutoff Systems
Most Perkins engines from the 1970s do not use electric solenoids for fuel cutoff. Instead, they rely on one of two methods:
Why the Ignition Key Doesn’t Stop the Engine
The ignition switch in a diesel-powered Bobcat typically controls the starter circuit and auxiliary power—not the fuel system. Unless the machine has been retrofitted with an electric shutoff solenoid wired to the key, turning off the ignition will not stop the engine.
In the case of the Bobcat 825, the ignition switch was wired to power the Facet fuel pump. When the key was turned off, the pump stopped—but the injection pump continued to deliver fuel from residual pressure and gravity feed, allowing the engine to run.
Solutions for Reliable Shutdown
To ensure proper engine shutdown:
Educational Value and Field Learning
This repair was performed by students in a secondary automotive program, offering hands-on experience in:
Conclusion
A 1978 Clark Bobcat 825 with a Perkins diesel engine that won’t shut off is not a mystery—it’s a reminder of how mechanical systems function without electronic intervention. By identifying the correct fuel shutoff method and understanding the role of auxiliary components like electric pumps, operators and technicians can restore reliable control. In a world of digital diagnostics, sometimes the solution is as simple as pulling a cable.
The Clark Bobcat 825, introduced in the late 1970s, was a compact skid steer loader built for rugged utility work. Powered by a Perkins 4-cylinder diesel engine, it relied on mechanical systems for fuel delivery, ignition, and shutdown—long before electronic control modules became standard. These machines were designed for reliability and field serviceability, often used in municipal yards, farms, and small construction outfits.
Perkins engines of that era, particularly the 4.108 and 4.236 variants, were known for their simplicity. They featured mechanical injection pumps and manual fuel shutoff levers, making them ideal for equipment that needed to run in remote or low-tech environments.
Symptoms of a Non-Shutting Engine
In one case, a 1978 Bobcat 825 was rebuilt after its electrical system had been completely severed. Once rewired, the engine started successfully—but refused to shut off when the ignition key was turned. This raised questions about whether the engine used an electric fuel solenoid or a manual shutoff mechanism.
The team working on the machine identified a small cylindrical device mounted between the fuel tank and the injection pump. Initially suspected to be a fuel shutoff solenoid, it was later confirmed to be a Facet electric fuel pump—used to prime and deliver fuel, not to stop it.
Understanding Perkins Fuel Shutoff Systems
Most Perkins engines from the 1970s do not use electric solenoids for fuel cutoff. Instead, they rely on one of two methods:
- A manual cable connected to a lever on the injection pump
- A mechanical linkage that pulls the fuel rack to the “off” position
- In some military or industrial applications, an electric solenoid may be used to actuate the shutoff lever
Why the Ignition Key Doesn’t Stop the Engine
The ignition switch in a diesel-powered Bobcat typically controls the starter circuit and auxiliary power—not the fuel system. Unless the machine has been retrofitted with an electric shutoff solenoid wired to the key, turning off the ignition will not stop the engine.
In the case of the Bobcat 825, the ignition switch was wired to power the Facet fuel pump. When the key was turned off, the pump stopped—but the injection pump continued to deliver fuel from residual pressure and gravity feed, allowing the engine to run.
Solutions for Reliable Shutdown
To ensure proper engine shutdown:
- Install a manual shutoff cable connected to the injection pump lever
- Mount the cable within reach of the operator and label it clearly
- If an electric solenoid is desired, use a pull-type solenoid rated for continuous duty
- Wire the solenoid to the ignition switch or a dedicated kill switch
- Test the system by starting and stopping the engine multiple times
Educational Value and Field Learning
This repair was performed by students in a secondary automotive program, offering hands-on experience in:
- Diesel fuel systems
- Electrical continuity testing
- 12V circuit design
- Mechanical diagnostics
- Collaborative problem-solving
Conclusion
A 1978 Clark Bobcat 825 with a Perkins diesel engine that won’t shut off is not a mystery—it’s a reminder of how mechanical systems function without electronic intervention. By identifying the correct fuel shutoff method and understanding the role of auxiliary components like electric pumps, operators and technicians can restore reliable control. In a world of digital diagnostics, sometimes the solution is as simple as pulling a cable.
