□
Wema UK · Help

How to Test a Sender — Fuel, Water or Holding Tank

If your gauge has stopped reading correctly, the cause is usually the wiring — not the sender or the gauge. Here's how to work through it in order, and how to test a sender with a multimeter.

⏱ 4 min read 🔧 Fault-finding ⚓ Fuel · Water · Holding tank

When a gauge stops reading correctly, it's tempting to blame the sender or the gauge — but in practice the most common fault is the wiring between them: a bad earth, or a break or damage in the signal wire. Check that first, before you remove anything. The same approach works for any Wema level sender, whether it's reading fuel, fresh water or a holding tank.

Start hereCheck the earth and signal wire first

Before suspecting the sender or the gauge, confirm the connection between them. A poor earth or a broken, chafed or corroded signal wire is the single most common cause of a gauge reading wrongly, sitting at empty, or pinned at full.

  • Check the earth connection at both the sender and the gauge — clean, tight and to good bare metal.
  • Inspect the signal wire end to end for breaks, chafing, corrosion or a loose terminal.
  • Where you can, test continuity along the signal wire and the earth rather than assuming they're sound.

DiagnosisIs it the sender or the gauge?

If the wiring and earth are good, the next step depends on what you can reach. If you can get to the back of the gauge, start there — it's the quicker check. Disconnect the sender signal wire where it joins the gauge. With that wire disconnected the needle will rest at one end of the scale; now briefly touch the wire to battery negative. A healthy gauge will move the needle across to the other end.

If you can't reach the back of the gauge but you can reach the sender, do the same test from that end instead: disconnect the signal wire at the sender and touch it to battery negative. The gauge should still swing across — and because you're driving it through the whole run of wire, a good result also proves the signal wire itself is sound.

If the needle responds either way, the gauge and its wiring are sound and the fault is in the sender. If it doesn't move, the gauge or its supply is the problem — see how to test a gauge.

Step by stepTesting the sender with a multimeter

Test the sender on its own:

  1. Disconnect the two wires from the top of the sender.
  2. Remove the sender from the tank.
  3. Set a multimeter to resistance (Ω ohms).
  4. Connect the meter across the sender's two terminals.
  5. Move the float to the top of the sender and take a reading — this is the full value.
  6. Move the float to the bottom and take a second reading — this is the empty value.
  7. Move the float slowly through its travel and watch the reading step between values.

ReferenceExpected resistance readings

The resistance you should see at full and empty depends on whether your system uses European or American resistance:

European spec

190 → 0 Ω

190 Ω ±10 Ω at full, falling to 0 Ω ±10 Ω at empty.

American spec

30 → 240 Ω

30 Ω ±10 Ω at full, rising to 240 Ω ±10 Ω at empty.

The resistance changes in steps, not as a smooth sweep — the reading jumps at each reed switch as the float passes it, roughly every 20 mm on a standard sender and about every 10 mm on a high-resolution sender. A good sender steps cleanly through the range between the two end values.

Not sure which standard your system uses? See European vs American resistance.

ResultsWhat the readings mean

  • The reading steps cleanly between the values above as the float moves — the sender is working. If the gauge still reads wrong, the fault is the wiring, the earth or the gauge.
  • The reading is stuck on one value, jumps erratically, or shows open circuit — the sender has failed and should be replaced.
  • The gauge reads backwards (full when empty, empty when full) — the sender and gauge resistance are mismatched: a European sender on an American-spec gauge, or vice versa.

If you need a replacement, you'll want one that matches your tank depth and your gauge's resistance range — how to identify a replacement sender walks through both.

The short version Check the earth and signal wire first — that's the usual culprit. Then test the gauge by disconnecting the signal wire and touching it to battery negative — at the gauge if you can reach it, otherwise at the sender end (which also proves the wire). If the needle swings, the gauge is fine and the sender is at fault. Put a multimeter across the sender and move the float — it should step across 190→0 Ω (European) or 30→240 Ω (American), each ±10 Ω. Stuck or open means a new sender; a backwards reading means a mismatched sender and gauge.
Need a replacement?

Genuine Wema senders, made to match your gauge

Resistance-based level senders for fuel, water and holding tanks — in European or American resistance, sized to your tank. S3, S5 and the full range.

Shop senders Identify your sender

Common questions

What's the most common cause of a gauge reading wrong?
A bad earth or a break or damage in the signal wire between the sender and the gauge — more often than the sender or gauge itself. Check the earth and the wire end to end before removing anything.
How do I test the gauge if I can't reach the back of it?
Do the test from the sender end. Disconnect the signal wire at the sender and touch it to battery negative — the gauge should swing across the scale. Because you're driving it through the full length of wire, a good result also confirms the signal wire is sound.
What resistance should a Wema sender read?
On European resistance, 190 Ω ±10 Ω at full and 0 Ω ±10 Ω at empty. On American resistance, 30 Ω ±10 Ω at full and 240 Ω ±10 Ω at empty. The reading steps between these as the float moves — it doesn't sweep smoothly.
My gauge reads backwards — what's wrong?
The sender and gauge resistance are mismatched — for example a European sender paired with an American-spec gauge, or the other way round. Match the sender's resistance range to the gauge.

Related: how to test a gauge · European vs American resistance · how to identify a replacement sender.