Sierra TS25101 Coolant Fan Temperature Switch 210°F

If your inboard or repower runs an electric cooling fan, something has to decide when that fan spins up. That's the job of the Sierra TS25101. It's a simple temperature switch that closes its contacts once the coolant hits 210°F and opens them again at 200°F. No control module, no guesswork. The engine tells the fan what to do, and the fan only runs when it actually needs to.

We like parts like this because there's nothing to overthink. The switch threads into a 3/8-inch by 18 male port, usually on a manifold, a thermostat housing, or an adapter tapped for it. One 1/4-inch blade terminal carries the signal. When the coolant is below 200°F the contacts sit open and the fan stays off. Cross 210°F and the contacts close, feeding your fan circuit. That 10-degree gap between on and off is deliberate. It keeps the fan from chattering on and off every few seconds when you're sitting right at the threshold.

Who's this for? Anyone running an aftermarket or replacement electric fan on a marine engine, a repowered inboard, or a closed-cooling conversion where the original setup is gone. If you've got a relay-driven fan and you want it to cycle on engine temperature instead of a manual dash switch, this is the piece that automates it. It's also a straight replacement when an old switch starts sticking, cycling late, or not closing at all.

Wiring is about as basic as it gets. Most installs run the blade terminal to the trigger side of a fan relay, with the relay handling the heavy fan current. That keeps the switch contacts happy and lets you run a bigger fan without cooking a thin signal wire. Because the sensing element grounds through the threads, get a clean metal-to-metal seat in the port and use an appropriate thread sealant rated for coolant and heat. Snug it down. Don't gorilla it. These brass-bodied switches strip or crack if you lean on the wrench. If you're not sure the old switch is the culprit, you can bench-test it. Heat it in a pot of water with a thermometer and check for continuity between the terminal and the body as it crosses the trip point.

A couple of honest notes so you order right. This switch doesn't come with a wiring harness or a relay, so if you're building a fan circuit from scratch, plan on sourcing those separately. And it's a fixed-temperature switch, not an adjustable one. The 210°F on and 200°F off points are set at the factory and can't be dialed. If your engine builder or fan kit calls for a different trip point, match the part to that spec rather than trying to make this one do it.

On the cross-reference side, the TS25101 interchanges with a few common industrial part numbers, so if your existing setup was built around one of those, it drops right in. Check the thread size and terminal against what's in the port before you pull the old one, since ports and adapters vary from one engine to the next.

Sierra has been supplying marine replacement parts to boat builders and engine manufacturers for decades, and their gear is engineered to meet or beat the original equipment it replaces. For a small part that lives in a hot, damp, vibrating spot on the engine, that track record matters. A cooling fan that won't switch on is how you end up with an overheat alarm halfway across the bay, and a sticking switch is a lousy reason to cut a trip short. Keep a spare in the parts box if your fan is critical to holding temps in range on hot days or at low idle speeds.