If you’re running a legacy Mercury Marine, Mercury Racing or Mariner outboard, accurate temperature monitoring is essential for protecting your engine from costly overheating damage.
 
The Temperature Sending Unit from Buckshot Racing #77 is a direct replacement for Mercury/Quicksilver OEM Part Number 12415 and Sierra 18-5896. Built to precise OEM specifications, it’s designed to work seamlessly on Mercury and Mariner outboards from 35 HP up to 300 HP, manufactured between 1976 and 1999.
 
This sender delivers reliable readings and installs easily, making it a go-to solution for boaters who want performance and peace of mind. The Temperature Sender Unit is features a 1/8-27 NPT threaded body, the most common thread size in the marine industry for temperature senders. That means it can be threaded directly into the existing port on your engine’s water jacket, cylinder head, exhaust plate, or block, depending on your specific outboard.
 
This unit is also calibrated for use with 240°F full-scale temperature gauges and is manufactured to follow the “Code D” resistance curve. This industry standard helps ensure compatibility with popular marine gauges from Mercury, Quicksilver, Sierra, Faria, Teleflex, and SeaStar. The electrical connection is made using a 10-32 stud terminal, which provides a secure and vibration-resistant connection point for your gauge wire.
 
How the Temperature Sender Works
Inside the sender is a negative temperature coefficient (NTC) thermistor. As engine temperature rises, the resistance of this thermistor drops. Your analog gauge measures this resistance change and moves the needle accordingly, giving you a real-time view of your engine’s temperature.
 
The brass housing of the sender doubles as the ground connection, which is why clean threads and good contact are so important. If thread sealant or corrosion prevents proper grounding, the gauge may read inaccurately or fail to respond. A light application of non-hardening sealant is enough to prevent leaks while still maintaining the ground path.
 
Gauge and System Compatibility
This temperature sender is designed specifically for use with analog marine temperature gauges rated to 240°F that are marked for Code D senders. Using a gauge with a different resistance curve may cause false readings, so confirming the match is key to accuracy.
 
For boaters running aftermarket digital monitoring systems, the sender can be integrated as long as the system accepts an NTC thermistor input. In those setups, the sender’s resistance curve may need to be programmed into the digital display.
 
Installation Guide
Installing the Temp Sender is straightforward. First, make sure the outboard is completely cooled down and the battery is disconnected. Locate the threaded 1/8-27 NPT port on the head, exhaust plate, or block where the sender will mount. If you’re replacing an old sender, remove it and clean the threads down to a bright metal finish.
 
Next, apply a thin coat of non-hardening thread sealant to the sender’s threads. Avoid heavy wraps of PTFE tape, which can block the sender’s ability to ground. Thread the new sender in by hand to avoid cross-threading, and then snug it gently with a wrench. Because it’s a small NPT fitting, a snug seal is all that’s required — overtightening can cause damage.
 
Once installed, connect the signal wire to the sender’s 10-32 stud terminal using a marine-grade ring terminal. Reconnect the battery and run the engine either on a flush kit or in the water. As the engine warms up, you should see the temperature gauge needle climb smoothly, confirming the sender is working properly.
 
Wiring Guidance
For best performance, connect the sender to the gauge using 18 AWG tinned copper marine-grade wire. This is the industry standard for marine gauge wiring because it resists corrosion and vibration damage. If the wiring run from the engine to the dash is unusually long — more than about 25 feet — stepping up to 16 AWG wire can provide extra reliability.
 
Always use marine-grade heat-shrink terminals to make the connection at the sender’s stud terminal. This prevents corrosion and ensures a secure, waterproof electrical path in the harsh marine environment.
 
Our Temperature Sender is a proven replacement for Mercury/Quicksilver part 12415 and Sierra 18-5896 for boat owners looking to protect their engine, prevent overheating, and maintain dependable instrumentation.
 
For technical support, contact Mike Hill at +1-714-697-1716 or mike@buckshotracing77.com.