LED indicator conversion on ute and trailer: fast flash plus a phantom glow
A customer replaced all six 21 W incandescent indicator bulbs on a 2012 ute and its box trailer with LED units measuring 0.18 A each at 12.6 V. The flasher is a standard electronic type rated 2 × 21 W plus 5 W. Now the indicators flash at about twice the normal rate with no bulb failed, the dash trailer telltale stays on constantly, and with the headlights on the left rear indicator glows dimly at about 3 V even with the indicator switched off. The trailer uses a 7-pin flat connector wired by the owner. The ute has combined stop/tail bulbs and separate amber indicators. Measured: 12.5 V at the battery, 11.9 V at the left rear lamp holder with lights on, 0.4 Ω between the trailer socket earth pin and the trailer chassis, and 12.1 V on the left indicator feed wire with the indicator off.
1. Explain the fast flash and the constant trailer telltale, and give two acceptable fixes with the trade-off for each.
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Cause
The flasher is rated 2 × 21 W plus 5 W, so it expects roughly 1.75 A per side, 3.5 A total. Two LEDs draw 0.18 × 2 = 0.36 A. The flasher's bulb-failure sensing reads that as 'a bulb is out' and doubles the flash rate. The trailer telltale is driven from the same current-sensing circuit, so it also thinks a trailer indicator is connected and lit, hence the constant lamp.
Note that a bimetallic flasher would do the opposite - slow or no flash with too little current. The fast flash tells you an electronic flasher is fitted, which is useful.
Fixes
- Fit an LED-compatible electronic flasher. Cleanest solution, keeps the 0.36 A load, no heat. Trade-off: it must match the pinout and terminal numbering exactly, and some units still need a minimum load on the trailer circuit to make the telltale behave.
- Fit 6–8 Ω 25–50 W load resistors, one per side, across the indicator feed and earth. Trade-off: each resistor dissipates about 21 W whenever the indicator is on, they get hot enough to melt trim, and they waste the current the LEDs were bought to save. Mount on metal, away from plastic and wiring.
- A parallel 21 W bulb hidden in the loom is a roadside bodge, not a repair - it defeats the purpose and will fail again.
Whichever you choose, the trailer telltale must still work correctly, because that is a legal requirement and an annual inspection item.
2. Diagnose the 3 V glow on the left rear indicator with the lights on. Give a test sequence using a meter and a known-good trailer board.
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What the symptom means
A lamp that glows with its own circuit switched off is being fed from another circuit. The dim 3 V and the 12.1 V reading on the 'off' indicator feed point to a cross-feed between the tail circuit and the left indicator circuit. Because the tail lamp is on whenever the headlights are on, the indicator filament gets a share of the tail feed through the fault resistance, and the shared earth completes the path.
Sequence
- Unplug the trailer. If the glow disappears, the fault is in the trailer or its plug. If it stays, it is on the vehicle.
- Plug in a known-good trailer board. If the fault follows the board, you have proved the trailer.
- At the 7-pin socket, plug removed, measure resistance between the tail pin and the left indicator pin. It should be open - megohms. Anything low is a bridge, typically a strand of wire between terminals in the owner's plug or a melted plug body.
- Inspect the lamp holder: corrosion bridging the two contacts, or a filament that has collapsed across the contacts, will do the same.
- Do not trust a digital meter alone for phantom voltages - repeat with a 21 W test lamp. A high-impedance meter will read 12 V through a wet connector that cannot light a bulb.
- Repair the plug properly with a new 7-pin plug and correct crimps, then re-test with lights on and indicator off.
3. The trailer socket earth reads 0.4 Ω. Calculate the drop at 1.75 A for one indicator and at 8.4 A for both stop lamps, and state whether it must be repaired.
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Drop calculations
- One indicator at 1.75 A: 0.4 × 1.75 = 0.7 V
- Both stop lamps at 8.4 A: 0.4 × 8.4 = 3.4 V
A 0.7 V loss on an indicator is already noticeable and will get worse as the joint corrodes. A 3.4 V loss on the stop lamps means the trailer brakes lights will be dim and may fail an inspection - and on a trailer with electric brakes sharing that earth, it is a safety issue.
Judgement
Yes, it must be repaired. Trailers are usually expected below 0.1 Ω on the earth pin, which would give 0.17 V at 1.75 A and 0.84 V at 8.4 A. Before you condemn it, null the meter leads and re-measure, because 0.4 Ω is close to the lead resistance of a cheap meter. Then clean the pin, replace the socket if pitted, and add a separate earth wire from the trailer chassis to the plug rather than relying on the coupling. Re-test with a load, not just a meter.