Hot No-Crank With a Dashcam Drain
A 2020 1.5 L petrol hatchback with stop-start (54,000 km) intermittently fails to crank when hot. The dash lights dim slightly but there is no click; after 10-15 minutes it restarts normally. The battery is six months old: 12.55 V at rest, 9.8 V during cranking, and starter draw is 180 A. Charging is 13.8 V at idle. Voltage drop during cranking is 0.22 V on the positive cable and 0.15 V on the engine-to-battery negative cable. A dashcam is hardwired with a fuse tap at the BCM. Parasitic draw is 38 mA after 45 minutes (specification 25 mA). No DTCs are stored, and live data shows a starter request when the key is turned. The shop proposes a new starter and battery.
1. Is replacing the starter and battery justified by these readings? Explain what else must be tested and why.
Read the worked answer
The readings do not justify throwing a starter and battery at the car. A rest voltage of 12.55 V is about 80% state of charge, not a failed battery. A cranking voltage of 9.8 V is low but not conclusive; the specification depends on temperature and measurement point. Starter draw of 180 A is not abnormal for a small petrol engine, and the voltage drops of 0.22 V and 0.15 V are within a usual 0.5 V limit per cable.
The symptom is the key: no click and dimming lights when hot, with a starter request shown in live data, points to a control-circuit or solenoid problem rather than a seized starter. Heat increases resistance at a poor connection. A high-impedance multimeter can show 11.6 V at the solenoid control terminal while almost no current can flow through a corroded fuse, relay contact or splice.
Test the control circuit under load. Back-probe the starter control terminal with a test lamp or load resistor while cranking, and measure voltage drop across the ignition switch, starter relay, neutral/range switch, fuse and aftermarket wiring. Wiggle-test the harness while hot. Check engine and body grounds under load. Perform a battery conductance test and confirm the battery is registered to the BCM if the vehicle uses smart charging; without registration, stop-start and charging strategy may be incorrect. Replace only a part that fails a test.
2. The fault is traced to a corroded 20 A fuse in the starter control circuit. Explain why a voltmeter at the starter showed normal voltage when cold but the engine would not crank when hot, and how you would verify the repair.
Read the worked answer
A voltmeter has very high resistance, so it draws almost no current. Across a corroded fuse or terminal it can read nearly battery voltage because the meter completes only a tiny current path. When the starter solenoid tries to draw several amps, the corrosion resistance drops most of the voltage and the solenoid receives too little current to pull in. Heat raises resistance further, which is why the fault appears only when hot.
The correct test is a voltage-drop test with the circuit loaded, not an open-circuit voltage reading. Connect the meter across the fuse or connection and crank; a good fuse should show near 0 V, while more than about 0.1-0.2 V across a fuse indicates resistance. A test lamp connected at the solenoid control terminal should glow brightly during cranking; a dim lamp confirms a poor supply.
Repair by replacing the fuse and the corroded fuse holder or terminal, not by cleaning the fuse link alone. Check for water ingress into the fuse box and for an aftermarket fuse tap that has spread the terminal. Verify with a hot-soak test: run the engine to temperature, stop it, and crank at least 10 times with a 5-minute heat soak between attempts. Measure control-circuit voltage drop under load and confirm the starter engages every time. Clear any adaptation and check stop-start operation.
3. The dashcam is causing a 38 mA parasitic draw against a 25 mA specification. The customer wants it hardwired. What is the correct installation and what would you tell the customer?
Read the worked answer
A 38 mA draw will not flatten a healthy battery overnight, but it is above specification and will matter if the vehicle stands for several days. Over 72 hours it consumes roughly 2.7 Ah, which can tip a marginal battery into a no-start, especially in cold weather.
Install the dashcam on a dedicated fused circuit, not by spreading a BCM fuse terminal. Use an inline fuse rated for the camera cable and a proper fuse tap of the correct size. Prefer an ignition-switched supply unless the customer needs parking mode. If parking mode is required, use a voltage-monitoring cutoff set around 11.8-12.0 V and a timer, and confirm the camera actually enters sleep mode.
After installation, measure current draw with a clamp meter or a series ammeter after the vehicle has gone to sleep, which may take 30-60 minutes. Compare with the 25 mA specification. Check that the camera is not keeping the BCM or infotainment awake. Record the before-and-after figures and explain to the customer that a hardwired camera can still discharge the battery if the car is parked for long periods. Offer an ignition-switched installation and a battery guard as the safer options, and document the customer's choice.