Starter Voltage Drop Test: How To Simply Perform It

Starter Voltage Drop Test

A starter motor, also known as a starter, is an electric motor that initiates the rotation of the engine while the vehicle is in start mode. The starter motor is powered by the battery and turns the crankshaft to start the engine. The starter motor must have high torque to turn the engine over against the compression of the cylinders.

A starter voltage drop test is a quick and easy way to test the health of your car’s starter system. A voltage drop test can be performed with a multimeter and will tell you if there is an excessive voltage drop between the battery and the starter. If there is an excessive voltage drop, it means that there is a problem with the starter system and it needs to be repaired or replaced.

If you’re looking to know more about starter voltage drop testing, then this is the page for you. Keep reading to learn everything you need to know about this important test.

Starter Voltage Drop Test

Have you experienced that when you want to start the car, your engine cranks slowly but does not start? However, by testing the battery and starter, both are perfect.

The answer to this is due to resistance, the starter motor receives a little amount of voltage than what is required, and the voltage drops across the current path from the battery to the starter.

The voltage drop test is mostly overlooked while troubleshooting a starter motor circuit. It is the most reliable test. With the help of this test, you can diagnose the component or connection with high resistance without disassembling the parts.

As you know, every component is designed to operate on a certain amount of voltage, if the voltage is lower than the required voltage will affect the performance of the component.

With the passage of time, electrical connections, terminals, cables, and contacts become corroded or rusted resulting in a production of excessive resistance in the circuit.

Voltage Drop Explained

Car-Battery-Wire-Corrosion
Car-Battery-Wire-Corrosion

Voltage drop refers to the amount of voltage lost when it is flowing through a circuit. It is the difference between the voltage present in the battery and the voltage at the starter motor.

The voltage drop is caused by excessive resistance present in the wire due to undersized cables or poor connections. As a rule, every conductor has some level of resistance, which is thought-out normal.

Similarly, copper is the conductor that has the least resistance among all other conductors that’s why it is mostly used in the motor armature. When the current flows in a wire, it feels resistance, which causes to loss of voltage.

So, a little bit of voltage loss (Drop) is normal in a circuit; there is no universal specification for voltage drop in a circuit. Every car manufacturer has its own specification about voltage drop, but most manufacturers agree on a voltage drop of up to 0.5 volts is considered normal and standard.

Also, the voltage drop test is a great diagnostic test; it is a fast, reliable, and accurate way to deal with many electrical problems in a car especially, a Cranking-No-Start problem.

This is an Info
Every car manufacturer has its own specification about voltage drop, but most manufacturers agree on a voltage drop up to 0.5 volts is considered normal and standard.

What Causes Voltage Drop In Car

Voltage drop can be caused by many reasons.

  • Damaged or corroded cable increases the resistance of the circuit.
  • Wrong sized cable or smaller gauge cable
  • Loose or corroded ground straps
  • Damaged battery terminals
  • Corrosion inside the cable

Tool You Need

While doing the starter motor voltage drop test, the only tool you need is a digital voltmeter (DVM). Set your voltmeter scale on 20v DC volts.

Things To Remember Before Performing Voltage Drop Test With Multimeter

A Full-Charged Battery
A Full-Charged Battery

Before you start the voltage drop test, it is important to verify that the battery is fully charged. The starter motor draws a huge electric current in the car. It cannot run on a weaker battery.

Take the voltmeter and test the battery voltage. It should be 12.6 or higher volts. A 12.6 or higher volt battery is considered a fully charged battery. 12.4 volts, the battery is at 70% plus charged, and at 12.1 volts, the battery is 30% charged. So charge the battery before carrying out the test.

Remember this point, if you have charged the battery on a charger. It might have a surface charge, which is not a good thing and can rapidly discharge which might make you confused with a dead battery.

To remove the surface charge, turn ON the headlights for one minute, then turn it OFF and wait for five minutes to stabilize the voltage.

The second thing you need to do is to disable the ignition system so that the engine will not start. For your kind information, we only want to crank the engine, not to start the engine. Remove the fuel pump relay or something not to start the engine.

And remember, a voltage drop test can only be performed on a live circuit. The current must be flowing in a circuit to test the voltage drop, and this can only be achieved when the driver is attempting to crank the engine.

In the voltage drop test, the voltmeter leads are placed in parallel with the circuit being tested. Below is the step-by-step procedure of the starter motor voltage drop test. You can use the same procedure for testing the voltage drop of the other components such as the alternator, fuel pump, etc.

In the starter motor voltage drop test, we will also test both sides of the starter motor, the power, and the ground side.

This is an Info
The voltage drop test with a multimeter can only be performed on a live circuit.

This is an Info
The battery might have a surface charge due to charging, turn ON the headlights for one minute, then turn it OFF to remove the surface charge.

Procedure Of Voltage Drop Test With Multimeter

Checking The Battery And Starter Motor Voltage
Checking The Battery And Starter Motor Voltage
  • First of all, record the “Base” voltage reading of the battery, which will help us in the future by comparing the voltage at the starter motor terminals.
  • Connect the voltmeter positive (red) lead to the battery positive post (remember to connect with the battery post, not to the clamp as it can be loosely attached and can increase the resistance).
  • Connect the voltmeter negative (Black) lead to the battery negative post.
  • Now crank the engine for 10 to 15 seconds and note the voltage reading at the voltmeter. This is your base voltage reading.
  • Now attach the voltmeter red lead with the starter motor feeding terminal stud (not the wire) and black to the starter motor casing.
  • Now crank the engine for 10 to 15 seconds and record the voltage on the voltmeter screen.
  • Now compare both voltage readings, the voltage reading at the starter motor and the base reading at the battery posts.
  • If both readings are within specification, I mean the same or a voltage difference is up to 0.5 volt. It means the connection is good, and there is no excessive voltage drop present on the starter motor circuit.
  • But, if in case, the voltage drop is higher than the normal 0.5 volts during the test, it means you have some unwanted resistance in the starter motor circuit. Now to find out the actual spot of resistance.
  • For this purpose, you have to perform the voltage drop test at both circuits of the starter motor, the positive side, and the ground side.

Testing The Voltage Drop Of The Positive Side Of The Starter Motor

Testing The Voltage Drop of The Positive Side of the Starter MotorTesting The Voltage Drop of The Positive Side of the Starter Motor
Testing The Voltage Drop of The Positive Side of the Starter Motor
  • Connect the voltmeter positive lead (red) to the battery positive post and the negative (black) lead to the starter motor feeding (input) terminal.
  • Now crank the engine for a few seconds and note the voltmeter reading.
  • If the meter reading is 0.5 volts or less than 0.5 volts, it is considered normal. The power side of the starter motor is working properly.
  • But, if the meter reading is more than 0.5 volts, it means the resistance is high.
  • You can pinpoint the spot where the resistance is high by moving the meter leads closer to each other along the power side of the starter motor circuit little by little until you reach the actual spot where the voltage drops to normal.
  • Remember the area where it drops the resistance after the actual spot of resistance, does have an actual voltage drop.

Testing The Voltage Drop Of The Ground Side Of The Starter Motor

Testing the Voltage Drop of the Ground side of the Starter Motor
Testing the Voltage Drop of the Ground side of the Starter Motor
  • Connect the voltmeter positive lead (red) to the battery negative post and the negative (black) lead to the starter motor case (a clean spot).
  • Now crank the engine for a few seconds and note the voltmeter reading.
  • If the meter reading is 0.5 volt or less than 0.5 volts, the engine ground strap is in good condition, and the ground side of the starter motor is working perfectly.
  • But, if the meter reading is higher than 0.5 volts, it indicates the resistance is high.
  • Then check for the starter motor mounting bolts. Mounting bolts should be tight and should have a good ground connection with the engine block, and also make sure there is no dirt or grease between the starter motor mounting bolts and engine block.

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