Spannungsfall unter Last
About this app
Fehlersuche am 12-V-Bordnetz: Schaltplan, Multimeter und Spannungsgebirge zeigen, warum ein korrodierter Kontakt erst unter Last auffällt.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
Why voltage drop under load matters
In a closed circuit the battery voltage is shared among all resistances: U = I · R. The load should get almost all of it; wires, contacts and ground only a few tenths of a volt. A corroded connector or a poor ground point adds contact resistance. As long as no current flows, no voltage drops across it. That is why an unplugged connector shows the full 12.6 V to ground even though the contact is bad. Only with the load switched on does the fault become measurable as a voltage drop.
What the app shows
At the top a wiring diagram with battery, fuse, relay, positive wire, connector, load, ground wire, ground point and body. A multimeter with red and black probes measures voltage or, with the battery disconnected, resistance. At the bottom the voltage profile: the potential along the whole circuit, with the fault-free profile dashed. Steps above the guide value are highlighted, and the positive and ground side are summed separately. Four workshop jobs are ready: low beam, horn, rear window heater and blower.
Things to try
First unplug the connector and measure to ground: 12.6 V. Then switch the load on and measure directly at the load. Check the positive and ground side separately and narrow it down section by section until you find the step in the profile. Compare with a resistance measurement in the unpowered circuit. Name the cause in the Diagnosis panel and repair it. The slider changes the contact resistance, so you can see the voltage drop grow with the current.
Frequently asked questions
- How do you measure voltage drop in a car circuit?
- Switch the load on, then measure with the voltmeter across one section, for example from the battery positive terminal to the load positive (positive side) and from the load negative to the battery negative terminal (ground side). The reading is the voltage lost in that section.
- Why does an unplugged connector show 12 V although the wiring is faulty?
- With the connector unplugged the circuit is open and no current flows. By U = I · R, no voltage drops even across a high contact resistance. The voltage drop only appears under load.
- What voltage drop is acceptable?
- The vehicle manufacturer’s data are binding. As an order of magnitude from workshop practice, a few tenths of a volt per side apply; the app uses guide values of about 0.1 V per contact, fuse, relay or ground point, 0.3 V per wire and 0.5 V per side.
- How accurate is the app?
- The app calculates an ohmic series circuit with a 12.6 V battery and 10 mΩ internal resistance. Bulb, horn and motor are approximated as a fixed equivalent resistance at their operating point. In resistance mode the reading includes about 0.2 Ω of test leads. All values are examples.
Subject: Automotive engineering
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