Home mechanic (basic tools) ยท Time: 20 to 40 min (estimate)
A dirty connector, a loose earth or a worn switch contact may still let a little current through, so a continuity test beeps "good". But under real load, the bad joint resists the current: some of the battery voltage is used up there, and the lamp, horn or starter gets less. A voltage drop test finds these bad joints while the circuit is working. It is the best test for dim lights, a weak horn, slow starting and connectors that get hot.
Safety
- The circuit must be ON for this test. Keep hands, hair and leads away from moving parts if the engine or starter runs.
- If you test the starter circuit, the engine must not start: follow the method in Test the starter circuit.
- A connector that is hot to touch is already damaged. Stop the test and repair it.
You need
- A digital multimeter on DC volts (V with a straight line). Use the lowest range that still shows the reading; on auto-range meters just choose DC volts.
- Your bike's limit if the manual gives one. This page gives no universal limit, because it depends on the circuit and the current it carries. Without a manual figure, compare: a good wire or closed switch reads close to zero, and a bad one reads clearly more than the same test on a good circuit of the same bike.
- A charged battery (K-Light PDF p. 97).
How it works
In a voltage drop test both probes go on the same side of the load (the lamp, horn or motor), at the two ends of the part you are testing: for example both ends of one wire, or both sides of a switch. With the circuit working, the meter shows the voltage lost in that part. Zero means no loss. A large reading means resistance.
Steps
- Switch the circuit on and keep it working. For a headlight: key on, headlight on (engine running if your bike only lights the headlight with the engine on). For a horn: a helper holds the button.
- Test the power side as a whole. Red probe on the battery positive terminal (use a long lead). Black probe back-probed into the power wire at the load. The reading is the total loss in fuse, switch, relay, connectors and wire on the feed side.
- Test the earth side as a whole. Red probe back-probed into the earth wire at the load. Black probe on the battery negative terminal. The reading is the total loss in earth wires and earth points.
- Judge the result. Compare with your manual. If no figure is given, compare with the same test on another circuit that works well. The side with the larger reading has the problem.
- Narrow it down. Move the probes closer together along the bad side: across the fuse, across each connector, across the switch, across the earth bolt. The part that shows most of the loss is the bad one.
- Fix the bad part. - Connector: Clean and seal electrical connectors. - Earth point: Clean and tighten earth connections. - Handlebar switch: Clean a handlebar switch. - Ignition switch: Test the ignition switch. - Battery terminals: Clean and tighten battery terminals.
- Repeat the test after the repair to prove the loss has gone.
Check your work
- The voltage drop on both sides is now low and similar to a good circuit on the same bike.
- The lamp is brighter, the horn louder or the starter faster.
- No connector or switch gets warm after 10 minutes of use.
If it did not fix the problem
- Both sides read low but the lights are still dim: the supply itself is low. Go to Lights go dim at idle or Test the charging system.
- The K-Light manual's weak-light chart also lists a blackened bulb and a bulb of the wrong wattage (PDF p. 43): Replace a bulb.