If your black- or grey-water gauge has stopped behaving - lights that no longer come on, or a reading that never changes - the sender in the tank is usually the part that has reached the end of its life. These are some of our oldest products, but we still make replacements that fit, and there's a straightforward upgrade path if you'd like a better readout while you're at it.

There are two ways forward. You can stay with the same style of system and swap the sender for a modern equivalent that works with your existing three-light gauge, or you can move to a full-range analogue gauge that shows the whole tank rather than just the top of it. Both are simple jobs. This guide explains what you've got, what each option involves, and how to decide.

01 / WHAT YOU'VE GOTThe old 3-light system

The original setup is a silicone-bellows or WHT sender fitted into the top of the tank, wired to a small LED gauge with three lights - on, high and full - our three-light ISS gauge.

The important thing to understand is what this sender can and can't tell you. It only reaches into the top 200 mm of the tank and contains just two switches: a high-level switch and a full switch. The green light shows the system has power. As the tank fills, the level eventually reaches the high switch and the high light comes on; a little higher and the full light joins it. Below those switches - which is most of the tank - nothing changes on the gauge. It's a level warning, not a fuel-gauge-style reading of how much is in the tank.

02 / OPTION AStay with the 3-light gauge

The like-for-like fix is our WHTS-200 High Level Sender. It replaces the old silicone-bellows and WHT senders and works in the same way - two switches near the top of the tank driving the high and full lights on your existing gauge. It's 200 mm long, made from 316 stainless steel, and uses a 1 1/4 inch BSP thread.

How it fits depends on what you're removing. An old WHT sender uses the same 1 1/4 inch BSP thread as the WHTS-200, so it's a direct swap. An old silicone-bellows sender is bolted down over the hole, so you'll need an FL-3 flange, which converts that bolt pattern to the 1 1/4 inch BSP thread the new sender screws into - no need to cut a fresh hole. If all three LEDs on your gauge still light, the sender (plus the flange, if you need one) is all it takes. If one or more of the lights has failed, add a replacement three-light ISS gauge at the same time. You can also wire in an alarm buzzer for an audible warning alongside the lights.

03 / OPTION BUpgrade to a full-range gauge

If you'd like to actually see how much is in the tank - not just that it's getting full - move to the S3H Holding Tank Sender paired with our analogue 52 mm Holding Tank Level Gauge. The S3H reaches the full depth of the tank and the needle sweeps smoothly from empty to full, so a glance tells you where you stand.

The S3H uses the same proven electronics as our S3 and S5 senders - a free-floating magnet moving past a row of reed switches, with nothing to wear - but adds a removable 316 stainless cage the full length of the sender to shield the float in the harsh contents of a black-water tank. It reads on the American resistance standard (240-30 ohms), which matches the Holding Tank Level Gauge; if you're ever unsure which range you need, see European vs American resistance.

Fitting works the same way as the WHTS-200: the S3H uses a 1 1/4 inch BSP thread, so it's a direct swap for an old WHT sender, or takes an FL-3 flange when you're converting from a bolted-down silicone-bellows sender.

04 / FITTINGA few notes before you order

Whichever route you take, a couple of practical points make for a reliable installation. For any holding tank sensor we recommend leaving at least 2 inches (50 mm) of clearance at the bottom of the tank, so the worst of the solids don't get caught up in the sender. For the S3H, measure the internal depth of your tank before ordering, as we size the sender to suit.

If you're moving to the analogue gauge, remember that sender and gauge must share the same resistance range - the S3H and the Holding Tank Level Gauge are both 240-30 ohms, so they're matched out of the box. And as with any resistance sender, never connect it directly to 12 V: it's passive, drawing only a few milliamps from the gauge, and mains voltage will damage it.

05 / SHOPThe parts you'll need

06 / FAQCommon questions

Still not sure which option suits your tank? Use our replacement-sender guide, or get in touch - tell us what you've got and the internal depth of your tank, and we'll point you to the right sender, gauge and fitting.