A Murphy switch, also known as a magnetic switch or engine protection relay, is a safety device used to protect diesel engines from damage due to low oil pressure or overheating. As a professional diesel engine parts supplier, I will share a detailed look at what a Murphy switch is, why it’s important, and exactly how it works to shut off a diesel engine when there’s a mechanical issue.
What Is a Murphy Switch?
A Murphy switch is an electromechanical device that serves as an automatic safety shutdown control for diesel engines. It was originally designed and manufactured by the Murphy company (now known as FW Murphy), but “Murphy switch” has become a generic term for any type of diesel engine safety shutdown device.
The primary purpose of a Murphy switch is to protect the engine from catastrophic damage in the event of:
- Low oil pressure
- High coolant temperature
- Alternator belt failure
Without an automatic shutdown system, these mechanical issues could destroy an engine within minutes due to lack of lubrication or overheating. But with a Murphy switch installed, the engine will safely shut off before any permanent damage can occur.
Why Are Murphy Switches Important for Diesels?
Diesel engines are highly durable, but they also operate under extreme internal pressures and temperatures. This means they have less tolerance than gasoline engines if something goes wrong mechanically.
Some of the key reasons why an automatic shutdown control like a Murphy switch is essential equipment on any diesel engine include:
Prevents Bearing Damage
If oil pressure gets too low, engine bearings won’t receive adequate lubrication and will overheat, seize up, and fail catastrophically. The Murphy switch prevents this by cutting off fuel to the engine when it senses low oil pressure.
Avoids Piston Scoring
Without oil, pistons can seize against cylinder walls, causing deep grooves and scratches called “piston scoring”. This usually necessitates a full engine rebuild. The Murphy switch shuts the engine down before scoring occurs.
Protects Against Overheating
On liquid-cooled diesels, coolant circulation is critical for carrying heat away from the cylinder and head. If coolant flow stops due to a belt failure or leak, temperatures inside the engine can exceed 2500°F within minutes, warping metal components beyond repair. The Murphy switch prevents this kind of overheat damage by shutting the engine down if it senses high coolant temp.
Saves the Turbocharger
Turbocharged diesel engines are especially susceptible to damage from oil starvation or overheating. By shutting the engine off promptly, the Murphy switch prevents catastrophic turbo failure.
So in summary, Murphy switches are a simple but incredibly useful protective device for diesel engines operating in harsh conditions. The automatic emergency shutdown they provide can literally save an engine from destruction and prevent very costly repairs.
How Does a Murphy Switch Work?
The key part of understanding how a Murphy switch works is to grasp these basics:
- It’s an electromechanical device, meaning it uses a combination of electrical and mechanical components to sense issues and trigger a shutdown
- There is typically a central control box with shutdown buttons wired to various engine sensors
- When a sensor detects low oil pressure, high coolant temp, etc., it completes an electrical circuit back to the Murphy box which then kills the engine
Here are more details on the major functions and components that make up a diesel engine Murphy switch system:
Sensors
The sensors that feed data back to the Murphy switch are simple electromechanical devices screwed into various parts of the engine. Common sensors include:
Oil Pressure Sensor
Oil pressure sensors contain an internal diaphragm switch that closes when oil pressure drops below 5-10 PSI. This completes a ground circuit back to the Murphy box, initiating shutdown.
Coolant Temperature Sensor
These sensors operate based on the expansion of wax inside a sealed capsule. As coolant reaches a certain high temp (usually 205°F), the wax pushes a switch closed, completing the shutdown circuit.
Alternator Wire Sensor
This is a current-sensing device that clamps around the alternator’s charging wire. If the alternator belt breaks or fails, it will stop generating electricity. The sensor detects this drop in current and completes the shutdown circuit.
Central Murphy Box
The central Murphy control box contains the main decision-making logic for the system. Inputs from all the sensors around the engine feed into this box. Inside is circuitry that decides whether the incoming signals warrant an engine shutdown.
There is also typically some kind of manual reset button or switch on the Murphy box that the operator must activate before restarting the engine once a shutdown has occurred.
Shutdown Module
The final output component that actually cuts off fuel flow and stops the engine is called the shutdown module (some Murphy box units have this functionality integrated internally). It contains a high-current relay that interrupts power to the electric fuel solenoid when it receives the shutdown signal from the Murphy box.
On some engines, the shutdown module may interface with the ECU instead of directly de-energizing the fuel solenoid. But the effect is the same – fuel stops flowing and the engine grinds to a halt within seconds.
Delay Timers
An optional but useful feature on some Murphy systems is a delayed timer module. This adds an adjustable 5-60 second shutdown delay into the system.
The benefit is that brief drops in oil pressure or coolant flow on engine startup don’t immediately kill the engine before oil starts circulating or the thermostat opens. The delay timer overrides shutdown signals from sensors momentarily to give the engine time to stabilize.
Real-Life Example
Here’s a real-world example of how all the Murphy switch components work together to automatically shut off a diesel engine in an emergency situation:
The engine is up and running with oil pressure normal, coolant temperature 185°F, and the alternator belt intact. All sensors are reading good status back to the Murphy box, so there are no shutdown signals being sent.
Suddenly, the alternator belt breaks or jumps off its pulley while the engine is under load. The alternator quits generating electricity. The alternator wire sensor detects this and closes its internal contact, completing a circuit back to the central Murphy box.
Logic inside the Murphy box recognizes this as an emergency shutdown condition since the engine can’t run without an alternator charging the battery. It sends a signal forward to the shutdown module relay.
The shutdown module relay opens, cutting off power to the electric shut-off solenoid on the engine’s fuel pump. Fuel flow stops immediately and the engine begins rolling to a halt.
In this way, the Murphy system automatically sensed the critical alternator failure and took action to safely shut down the engine before damage could take place. It functioned exactly as designed!
Variations Between Murphy Switch Setup
While the basics of how Murphy switches work is pretty consistent across most diesel engine applications, there can be some important variations in terms of:
- Number of sensors
- Specific shutdown logic
- Integration with engine ECU
- Manual reset functionality
So if you’re working on an unfamiliar engine equipped with a Murphy system, it’s wise to carefully trace out all the wiring, input sensors, shutdown triggers, and control modules before making any assumptions about how that particular unit operates.
The basics covered in this article will apply, but there may be additional engine-specific protective shutdown measures in place as well. Slow down, follow each wire, and build a complete picture of the system before making any repairs or troubleshooting blindly. Understanding exactly how that engine’s Murphy switch is wired up is key!
Signs of a Failing Murphy Switch
If you notice any of the following symptoms, it could indicate that your diesel engine’s Murphy switch (or related sensor components) are starting to fail:
- Warning lights activating on the Murphy box itself
- Engine shutting down randomly without any obvious cause
- Having to press the manual reset button frequently to keep engine from quitting
- Hard starting, followed by immediate engine shutdown after start
Any of these issues point to a potential problem within the Murphy shutdown control system. It’s smart to thoroughly inspect and test all aspects of the system including sensors, wiring, controller box, shutdown module, etc. to identify the source of the failure.
Catching Murphy switch issues early on can allow for less expensive repairs. Ignoring the symptoms and continuing to reset or jump-start the engine will almost certainly lead to a full breakdown and very costly damage if an overheat, low lube condition, or runaway overspeed event were to occur.
Conclusion
Understanding how to work on and troubleshoot Murphy switch engine shutdown systems is an important skill for any diesel mechanic or operator. Hopefully this overview has provided useful insight into what a Murphy switch is, how it functions, why it’s so essential for engine protection, and how to start diagnosing issues if your shutdown system isn’t working properly.
A Murphy switch is a simple but extremely valuable device that serves as the last line of defense to prevent catastrophic engine failures. Taking the time to learn how it works and integrate it into your standard maintenance program is well worth the effort to avoid damaged parts, downtime, and expensive repairs down the road.
If you have any other questions about Murphy switches or engine safety shutdown systems in general, don’t hesitate to reach out to a professional diesel technician for assistance. Properly maintaining your emergency shutdown controls should be a top priority for reliable operation and maximum engine life.