Sierra TS26621 Marine Coolant Temperature Sender 240°

The Sierra Marine TS26621 is a coolant temperature sender, the small brass fitting that threads into your engine and tells the dash gauge how hot things are running. It's a wear part. When it fails, your temperature gauge either goes dead, pegs to the top, or bounces around for no reason. Swapping in a fresh sender is one of the cheapest, most satisfying fixes on a marine gas engine.

This one uses a 3/8-18 pipe thread, so it screws into the standard sender port on most gas sterndrive and inboard engine blocks or thermostat housings. The single terminal is a 10-32 stud, the same small threaded post you'll find on a factory sender, so your existing gauge wire and ring terminal land right back on it. And it's matched to a 240 degree gauge, which is the common sweep for marine temperature gauges. That last part matters more than people think. A sender has to speak the same resistance language as your gauge or the needle reads wrong all day.

Sierra builds these to meet or exceed the original equipment part they replace. If you've got a factory sender that's aged out, this is a like-for-like swap in most cases, no rewiring, no adapter, no gauge recalibration. That's the whole point of a quality aftermarket sender. It should disappear into the job.

Install is straightforward if you've done any engine work. Let the engine cool, back out the old sender, and clean the port threads. We like a light wrap of thread sealant or a dab of conductive sealant on the threads. Go easy on standard Teflon tape if the sender grounds through its body, because too much tape can insulate the ground path and give you a flaky reading. Thread the new one in by hand first so you don't cross-thread the port, then snug it with a wrench. Don't gorilla it. Brass is soft and the port is usually aluminum or cast iron, and an over-torqued sender can crack a boss or strip a thread. Land your gauge wire on the 10-32 stud, tighten the small nut, and you're done.

How do you know the old one's the problem and not the gauge? Classic symptoms are a temperature needle that reads cold when the engine is clearly warm, a needle pegged hot when the engine is fine, or a reading that jumps around with rpm or vibration. A quick check is to disconnect the sender wire with the key on. If the gauge swings full one way, the wiring and gauge are usually fine and the sender is the suspect. From there it's a five minute part swap.

This is a sender only. It doesn't come with a wiring harness, a gauge, or an adapter bushing, so if your port is a different thread you'll want the right reducer, and if you're building a fresh gauge setup you'll source the gauge and harness separately. For a straight replacement of a tired factory sender though, this is usually all you need.

Sierra has been supplying marine engine and drive parts for decades, and they build to survive the environment they live in. Heat cycling, moisture, salt air, and constant vibration are hard on any sensor, and a temperature sender sits right in the thick of it, threaded into a hot engine and splashed by whatever's in the bilge. That's why we stick with proven marine-grade parts here instead of a generic automotive sender that wasn't built for the water.

Who's this for? Anybody running a gas sterndrive or inboard who's chasing a temperature gauge that's gone squirrelly, doing a repower, or refreshing an older rig's engine instrumentation. It's a small, honest part that does one job well. And at this price, it's worth keeping a spare in the boat's parts box. A dead temp gauge on the water is the kind of thing that turns a good day into a nervous one, and this is cheap insurance against it.