Understanding the 953C Mag Coil

[MikePhua]

The mag-coil—short for magnetic solenoid coil—is a small but critical electrical component that controls hydraulic pressure relief on certain Caterpillar 953C machines. This coil acts like an electric switch—instead of wiring up a bulb, it lets hydraulic fluid find either a low or high-pressure path, depending on voltage applied. When 24 volts power it, the coil shifts the relief valve into a higher threshold—useful, for example, when the bucket is crowding (moving material forward). When voltage is removed, pressure reverts to the lower default setting—often used for more gentle or safer operations.
Background Story of the 953C Track Loader
The Caterpillar 953C is a durable, mid-size track loader introduced in the late 1980s and early 1990s. It combined robust hydraulics with a powerful Cat 3126B six-cylinder diesel engine, producing roughly 128 net horsepower at 2000 rpm, enabling effective digging, loading, and rough-terrain work .
These machines found homes in construction, gravel-pit operations, and industrial maintenance sites. With thousands likely sold globally, the 953C’s versatile undercarriage and numerous optional attachments made it an adaptable workhorse .
Technical Role of the Mag Coil
In early-model 953C units, the mag coil was part of a dual-pressure relief system—essentially a “two-stage” hydraulic relief valve. One setting offered baseline pressure; when the coil received 24 V, it shifted the valve to a higher pressure, enabling stronger cohesion of the bucket or loader arms under load. This provided extra pushing power or stability when needed, without permanently overstressing the hydraulics.
The coil can fail or the valve cartridge block might jam due to debris, leading to performance issues. A practical fix involves disassembling and cleaning the valve cartridge—but sometimes the root issue is a worn or failing pump, which may need replacement.
In multiple cases, mechanics replaced the oil pump outright after repeated relief-system issues. These pumps are Vickers-style hydraulic units—not always available aftermarket. Some units have been sourced directly through CAT’s parts system, though availability varies by region .
Insights from Field Experience
A longtime operator from Ontario recommended measuring hydraulic pressure at both pump outlets while applying load—lifting and tilting at low idle and full throttle—to compare performance. Noisy pumps under load often point to internal failures rather than just coil problems.
Another technician noted that the solenoid-coil setup was specific to early models—the feature was discontinued in later 953C variants. So if you own a later version, you may not even have this coil in your circuit .
Terminology Clarified

• Mag Coil: A magnetic solenoid coil that toggles hydraulic pressure relief settings electrically.
  
• Relief Valve Cartridge: The part inside the valve that shifts pressure thresholds when actuated by the coil.
  
• Pump (Vickers Style): The hydraulic pump model used in the relief system; critical for pressure generation.
  
• Crowd (Bucket Crowd): The act of pushing the bucket forward under load.
  

Recommendations and Solutions

• Diagnostic Testing: Use a pressure gauge on the hydraulic pump outlet. Activate the coil (supply 24 V) to confirm whether pressure jumps to the higher setting. Do this test under both low idle and full throttle.
  
• Cleaning Maintenance: If pressure doesn't shift, remove and clean the relief valve cartridge to clear blockages caused by grit or contamination.
  
• Pump Assessment: If cleaning doesn’t restore function, suspect a failing pump—especially if noise or pressure drops persist. Consider sourcing a good pump, ideally genuine CAT, though regional availability may vary.
  
• Upgrade Considerations: If your model lacks the mag-coil feature, you may retrofit it—but verify electrical wiring, mounting, and hydraulic compatibility before undertaking such a project.
  

Short Anecdote
On a decommissioned railway yard in midwestern USA, a 953C loader sat idle because its bucket resisted movement. The operator wired 24 V directly to the suspected mag coil—electrical "kissing" as it were—and the hydraulic pressure spiked. The dusty cartridge popped open, revealing sediment blockage. A quick clean restored the loader to service. The machine hummed back to life, saving a costly pump change and proving that sometimes the simplest fix is the best.
Summary of Key Points

• The mag-coil enables dual hydraulic pressure settings on early 953C track loaders.
  
• At rest, low pressure ensures safety and smooth operation; energizing the coil triggers high pressure for heavy work.
  
• Common issues include cartridge blockage or pump wear.
  
• Troubleshooting involves pressure testing, cleaning, or replacing parts as needed.
  
• Later machines may not include this system—a retrofit requires careful planning.
  

I hope this gives you a rich, polished, and original look into the 953C mag coil system—blending engineering insight, practical tips, and a little storytelling to bring the technical detail to life. Let me know if you want to dive into wiring schematics or regional parts sources next!