TEST 3: Voltage Drop Testing The Battery (+) Cable
Our last test is a voltage drop test of the battery cable that connects the battery to the starter motor.
This battery cable is the one that delivers the current (from the battery) that the starter needs to turn the engine over.
In some cases, this battery cable will have hidden corrosion that keeps a lot of the current from reaching the starter motor. The end result is a starter motor that won't crank the engine and that gives the impression of being defective.
IMPORTANT: Don't disconnect the battery cable from the starter motor's solenoid. In the illustration the battery cable is disconnected from the starter motor just to make it easier to show the multimeter test connections.
These are the test steps:
Place your multimeter in Volts DC mode.
Attach the red multimeter test lead to the center of the positive (+) battery terminal.
If the positive battery post isn't clean, clean a spot right on the top of it. It's important that the multimeter lead make contact right in the center of the positive battery post.
With the black multimeter test lead, touch the center of the starter S terminal stud to which the big battery cable attaches to. You'll maintain the black multimeter test lead in this position throughout the next step.
Have your helper crank the engine even though the starter motor isn't cranking the engine.
This is important, since a voltage drop test has to be done while the component in question is working (or trying to work).
If all is good (no voltage drop), your multimeter will register 0 Volts (.5 Volts is still 0 Volts).
If there's a voltage drop (which is bad), your multimeter will register voltage (usually above 7 Volts DC.)
Let's interpret your voltage drop test result:
CASE 1: Your multimeter registered 0 Volts (no voltage drop). This is the correct test result. It also tells you that the starter motor is receiving all the available battery current it needs to crank the your Nissan Sentra's engine.
You can conclude your Nissan Sentra's starter motor is defective if you have:
- Confirmed that the starter motor doesn't work when you apply power to the S terminal wire of the starter motor (TEST 1).
- Confirmed that the starter motor is receiving the crank signal (TEST 2).
- In this test step you have confirmed that no voltage drop exists on the battery positive cable.
Replacing the starter motor should solve your no crank condition.
I'm going to make two more recommendations to you:
- Before removing the starter motor, manually turn the engine using a 1/2 ratchet and the appropriate socket. This is to make sure that the engine or the A/C compressor have not locked up and causing the no crank condition.
- Bench test the starter motor after removing it. This is a super easy test to do and this tutorial will help: Bench Testing The Starter Motor.
CASE 2: Your multimeter registered 5 Volts or more. This result tells you that a voltage drop does exist and that the battery's full power is not reaching the starter motor.
The good news is that this can easily be corrected, since a voltage drop is always caused by some sort of corrosion issue on the battery positive cable or terminals or the battery positive post.
The solution is to thoroughly clean the battery positive post and the battery positive terminal (both the end that attaches to the battery positive post and the end the connects to the starter motor's battery (+) cable stud.
After cleaning, try cranking the engine. If it cranks and starts, no further testing is required.
More 2.5L Nissan Tutorials
To see all of the 2.5L Nissan specific articles, go to: Nissan 2.5L Index Of Articles.
Here's a sample of the articles you'll find in the Index of Articles:
- Coil-On-Plug (COP) Coil Test 2.5L Nissan Altima, Sentra (2002-2006).
- How To Test Engine Compression (2.5L Nissan Altima, Sentra).
- Testing A BAD Alternator: Symptoms And Diagnosis.
- How To Bench Test The Starter Motor.
- How To Troubleshoot a Blown Head Gasket (2.5L) (at: troubleshootmyvehicle.com).
If this info really saved the day, buy me a beer!