This is the negative circuit voltage loss. Voltmeter (–) test lead to the alternator ground stud or case.Ģ) Turn on and adjust the carbon pile to alternator-rated amperage output.ģ) Read the voltmeter and record voltage.Voltmeter (+) test lead to the battery (-) post.Immediately turn off carbon pile.Īlternator Circuit Voltage Drop Test (–) Sideġ) With the carbon pile tester still connected, connect the voltmeter to the negative battery cable. This is the positive circuit voltage loss.
Voltmeter (-) test lead to the battery (+) post.ģ) Turn on and adjust the carbon pile to alternator-rated amperage output.Ĥ) Read the voltmeter and record voltage.Voltmeter (+) test lead to the alternator output terminal.Connect negative lead to the ground at the alternator.Ģ) Connect the voltmeter (set on low scale) to look for excessive resistance on the (+) side.Connect the carbon pile tester positive lead to the B+ stud (output terminal) on the alternator.Part 2: Alternator Circuit with Carbon PileĪlternator Circuit Voltage Drop Test (+) SideĮnsure the vehicle’s batteries have been tested and that the terminals have been cleaned and tightened.ġ) Ensure the battery is charged to 12.4 volts minimum (AGM batteries 12. If not, excessive resistance could be present. Then turn off the devices and wait two minutes for the voltage toĤ) Once you have a reading that is between 12.4 and 12.6 volts you will then start the engine and bring it to normal operating temperature.ĥ) With the engine at 1,500 to 2,000 rpm, turn on all electrical loads (lights, blower fan, A/C, etc.). The charging voltage should be at least 0.5 volt above base voltage reading. (Your battery should have a minimum of 12.4 volts.) If it is less than 12.4 volts, charge the battery and repeat the test.Ģ) If the voltage is greater than 12.6 volts (AGM batteries 12.8 volts), surface charge must be removed.ģ) To remove surface charge, turn on the headlights and blower motor (high speed) for one minute. "One volt drop generally equals a loss of 30 engine cranking rpms."Ī voltage drop test will assess voltage losses at the battery, the alternator and the starter.ġ) With the voltmeter connected to battery, and key and engine off, record a "base" voltage reading. "A voltage drop test is a good ‘best practice’ during your scheduled preventative maintenance or before replacing the battery, starter or alternator," explains Clint Stohler, Director, Technical Service and Training. This is why the concept of voltage drop testing is the only sure method of determining if there is an adequate current delivery path for optimum starting, charging, battery performance and life. While a technician may regularly do a visual inspection of cables and connections it doesn’t really show what is going on inside the cables or inside the connections. High resistance is often caused by one of these four main issues: High or unwanted resistance is one contributing factor that leads to cranking and charging system problems. only to later discover that the root cause of the problem doesn’t reside with the alternator or starter.
It's not uncommon for an alternator or starter to be replaced because it’s believed to be faulty or run its normal life cycle. This is often manifested in short battery life, frequent jump-starts or perceived alternator and starter problems. If the street is too narrow to carry the current, it shows up in measurable "voltage drop" or loss. In turn, the alternator needs to replenish the battery and provide the power for the vehicle loads through the charging cables and connections. The battery needs to deliver its available power to the starter through the cables and connections for cranking the engine.
The electrical problems related to excessive voltage drops are more common than you may realize. Think of your starting and charging cables as a two-way street.