Dull headlamps on a 24 V distribution truck
A 24 V distribution truck has 'dull headlamps'. With the engine off and the batteries at 25.4 V, you measure voltage drops on the left low-beam circuit with the lamp lit: 0.06 V from battery positive to fuse input, 0.05 V across the fuse, 0.09 V from fuse output to relay terminal 30, 0.82 V across the relay contacts (30 to 87), 0.18 V from relay 87 to the lamp connector, 0.35 V from the lamp earth pin to the local body earth point, and 1.10 V from that body earth point to the battery negative post. The right-hand lamp, on its own fuse and relay but the same body earth, measures 23.8 V at the connector. Both lamps are the specified 24 V 70 W units. The body-to-chassis braid is behind the cab, exposed to road spray.
1. Calculate the voltage the left lamp actually sees, identify which drops are faults against a sensible limit, and explain why measuring resistance instead would have misled you.
Read the worked answer
Left lamp voltage: Total unwanted drop = 0.06 + 0.05 + 0.09 + 0.82 + 0.18 + 0.35 + 1.10 = 2.65 V Lamp sees 25.4 − 2.65 = 22.75 V (89.6 % of supply).
Halogen output varies roughly with V^3.4: 0.896^3.4 ≈ 0.69 - about 30 % less light, which is exactly what the driver reports.
Against a working limit of ≤0.1 V per connection/contact and ≤0.5 V total per circuit:
- 0.06, 0.05, 0.09, 0.18 V - acceptable.
- 0.82 V across the relay contacts - failed relay (burnt/pitted contacts).
- 0.35 V across a single earth point - excessive; should be under 0.1 V.
- 1.10 V across the body-to-chassis braid - badly corroded, a major fault.
Why not ohms: a multimeter's test current is milliamps, so a corroded joint can read a fraction of an ohm and look perfect. Voltage drop must be measured with the circuit loaded, because resistance that matters only shows up when real current flows. Always measure drop, not resistance, on high-current circuits.
2. The right lamp is also low at 23.8 V. Explain why, and why the repair order matters.
Read the worked answer
The right lamp shares the return path. Its own fuse, relay and wiring account for only about 0.3–0.4 V; the remaining ~1.2 V is largely the same 1.10 V body-to-chassis braid drop plus its local earth. Both lamps' return current flows through that one braid, so its resistance is common to both circuits.
Why the order matters: the shared braid skews every reading you take on either side. If you repair the left local earth and the relay first and re-measure, you will see an improvement and may conclude the job is done - or chase a phantom fault on the right - while the real common cause is still there. Also, repairing the braid changes the load current slightly (lamps draw more when they get more volts), so every subsequent drop reading shifts.
Sequence:
- Isolate the battery.
- Clean/replace the body-to-chassis braid and its fixings (bare metal to bare metal, star washers, corrosion protection, sealed) - biggest single fault, affects both sides.
- Re-measure both sides.
- Replace the left relay (0.82 V across contacts).
- Clean the left local earth point and its terminal.
- Re-measure.
3. After all repairs the left lamp is better but still not matching the right. What is your next line of enquiry, and how do you verify the finished job?
Read the worked answer
Next checks, cheapest and most likely first:
- Bulb: confirm it is the correct 24 V 70 W unit, not a 12 V bulb, not a higher/lower wattage substitute, and that the filament is not blackened or the bulb seating corroded.
- Connector and terminal: look for a pushed-back or green terminal in the lamp connector, and measure drop across the connector itself under load.
- The lamp's internal earth path: many lamp housings earth through a stud in the housing; corrosion inside the housing or a poor path through rubber mounts adds resistance that does not appear at the earth pin.
- Lens/reflector: clouded or peeling reflector and a yellowed lens cost as much light as 2 V of drop - compare the two lamp assemblies visually side by side.
- Switch/column stalk contacts if the circuit still passes through the light switch rather than a relay.
- System voltage: confirm ~28 V at the batteries with the engine running; 25.4 V engine off is fine, but a low charging voltage makes every lamp dull.
Only then consider uprating cable or relays - an upgrade fitted over an existing fault just hides it.
Verification of the finished job: with lamps lit, record volt drop at each lamp connector and each earth leg (both ≤0.5 V total per circuit); check the lamp voltage matches side to side within 0.2 V; run for 5 minutes and check relay, fuse and terminal temperatures by hand; confirm the braid and earth fixings are sealed and torqued; road test and confirm the customer sees the improvement at night; record the before/after values on the job sheet.