Shared Earth Return on a Light Truck
A light truck owner complains that the headlamps are dim at idle and brighten when the engine is revved. He also notices that the blower slows noticeably whenever the headlamps are switched on. With the engine off, the battery measures 12.6 V. With the engine running at 1,500 rpm, the alternator B+ terminal reads 14.1 V, the battery positive post 13.8 V, the main fuse-box feed 13.7 V and the headlamp supply at the lamp 13.6 V. The headlamp earth terminal, measured from the lamp earth pin to the battery negative post with the lamps on, reads 0.7 V. The blower earth reads 0.6 V under the same condition. A cranking voltage-drop test from the engine block to the battery negative post shows 0.04 V. The headlamp pair draws 8 A. The earth for the lamps and blower is a single bolt on the inner wing.
1. Calculate the resistance of the headlamp earth return from the measured values and state, with reasons, whether the earth path is acceptable. Use typical limits.
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Calculation
R = V / I = 0.7 V / 8 A = 0.0875 Ω, or 87.5 mΩ.
Power wasted in the earth path = 0.7 V × 8 A = 5.6 W.
Judgement
A healthy earth return on a lamp circuit should drop far less than this - typically no more than about 0.2 V at full load, with each connection well under 0.1 V. A reading of 0.7 V is more than three times that guide. The earth path is therefore not acceptable: it behaves as a real resistance in series with the lamps rather than as a near-perfect conductor. The 0.04 V block-to-battery reading shows the main engine earth is good, so the fault is localised to the inner-wing earth bolt, its strap or its contact surfaces. The lamp current is raising the earth pin above battery negative, which reduces the voltage actually available across the lamp.
2. Explain, using circuit theory, why the blower slows when the headlamps are switched on even though the blower motor is not faulty.
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The lamps and the blower are in parallel, but they share a common return resistance at the inner-wing bolt. When the lamps are off, only a small blower current flows through that return, so the earth offset is small. When the lamps are switched on, 8 A flows through the corroded joint and produces about 0.7 V across it. The blower supply remains roughly 13.6 V, but its earth terminal is lifted by about 0.6 V, so the voltage across the motor falls. Motor speed depends on applied voltage, so the blower slows. This is a shared-impedance effect, not a blower fault. Repairing the common earth should restore both the lamp brightness and the blower speed.
3. Describe the voltage-drop test sequence and the repair you would authorise, including one check that prevents a repeat failure.
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Test sequence
- Set the meter to DC volts, using the lowest suitable range.
- With the lamps on, connect the negative lead to battery negative and the positive lead to the lamp earth pin. Record the reading.
- Move the positive lead along the return path: lamp pin to body, body to inner wing, inner wing to engine block, engine block to battery negative. The largest single jump identifies the faulty connection.
- A good connection should read close to 0 V; the total earth drop should be under about 0.2 V.
Repair
Remove the earth bolt, clean the lug and body to bare metal, fit a star washer, tighten securely and seal with petroleum jelly. Replace the strap if it is corroded internally. Recheck under load.
Repeat-failure check
Inspect the battery-to-body earth strap as well, because a poor body earth can force current through unintended paths and overheat the new joint.