TEST 1: Applying 12 Volts To The Starter Motor S Terminal
In this very first test section, we're gonna' bypass the ignition switch and apply 12 Volts directly to the starter motor's S terminal (to see if it activates and cranks the engine).
If the starter motor is OK, then it'll crank your Nissan Sentra or Altima's engine. If the starter motor is defective, then applying 12 Volts to it won't activate it.
The fastest, easiest, and the safest way to apply this voltage to the starter motor is with a remote start switch. You can find an example of this tool here: Sunpro Actron CP7853 Remote Starter Switch.
IMPORTANT: Remove the key from your Nissan Sentra or Altima's ignition switch to prevent the engine from accidentally starting.
IMPORTANT: Place your Nissan Sentra (Altima) on jack stands if you raise it to access the starter motor!
OK, these are the test steps:
Disconnect the battery negative (-) terminal.
You'll reconnect it back in one of the following steps, for now, it's a safety precaution as you set up the test.
Disconnect the wire that connects to the S terminal of the starter motor. This wire has a female terminal that connect to the male spade terminal on the starter motor's solenoid.
Attach one of the alligator-type terminals of the remote starter switch to the S terminal of the starter motor.
Reconnect the battery negative (-) cable to the battery negative post.
NOTE: Make sure that the battery cables and posts are clean.
Connect the remaining alligator-type terminal of the remote starter switch to the battery positive post.
IMPORTANT: If your Nissan Sentra has a standard transmission, make sure it's out of gear before you make this last connection.
Activate the starter motor with your remote starter switch. As you apply these 12 Volts (to the S terminal of the starter motor), you'll get one of two results:
1) The starter will activate and will turn over the engine -OR- 2) The starter motor won't do a thing.
Let's see what your test result means:
CASE 1: The starter motor cranked the engine. This is the correct and expected test result. You can conclude that the starter motor itself is not defective and thus not behind the problem causing the engine not to crank.
Since the engine isn't turning over (when you turn the key to crank and start it), we need to see if it's receiving the activation signal from the ignition switch. Go to: TEST 2: Verifying The Start Signal.
CASE 2: The starter motor DID NOT crank the engine. This usually means that your Nissan Sentra's starter motor is bad and needs to be rebuilt or replaced.
Before you replace it, I recommend that you do a voltage drop test on the battery cable that connects to the starter motor's solenoid. This is a very simple multimeter test and you can find it here: TEST 3: Voltage Drop Testing The Battery Cable.
I'm also gonna' suggest that you perform TEST 2 just to tie up any loose ends. Go to: TEST 2: Verifying The Start Signal.
If the above two tests confirm that the start signal IS present and there's no voltage drop on the battery cable (feeding battery power to the starter motor), then you can confidently conclude your Nissan Sentra's starter motor is bad and needs to be replaced.
TEST 2: Verifying The Start Signal
As you're probably already aware, the starter motor gets its start signal from the ignition switch.
This start signal reaches the starter motor thru' the wire that connects to the starter solenoid's S terminal.
In this test section, we're gonna' see if the start signal is actually present when we turn the key and try to crank and start the engine.
These are the test steps:
Disconnect the wire that connects to the S terminal of the starter motor solenoid.
Connect the red multimeter test lead to the female terminal of the wire you just disconnected from the S terminal of the starter motor.
Attach the black multimeter test lead to a clean and rust-free spot on the engine or on the vehicle frame.
I recommend that you use a battery jump start cable to Ground the black multimeter test lead directly to the battery negative (-) terminal.
Have your helper crank the engine from inside your Nissan Sentra (Altima).
The engine won't turn over, but the idea is to verify that the starter motor's internal solenoid is getting the 12 Volt start signal from the ignition switch (or not).
Your multimeter is going to register one of two results: Either 9 - 12 Volts DC or no voltage at all.
Let's examine your test result:
CASE 1: The multimeter registered 10 to 12 Volts. This is the correct and expected test result. You can conclude that your Nissan Sentra or Altima's starter motor is getting the start signal.
You can also conclude that the ignition switch and safety neutral switch are not defective.
The next test is to check the battery cable that connects to the starter motor. Go to: TEST 3: Voltage Drop Testing The Battery (+) Cable.
CASE 2: Your multimeter DID NOT register 10 to 12 Volts. This test result lets you know that the start signal is missing and that this is the reason that the starter motor is not cranking your Nissan Sentra or Altima's engine.
The most common cause of this missing start signal is usually a bad neutral safety switch or a bad ignition switch. Although it's beyond the scope of this tutorial to test these two, you have eliminated the starter motor as a cause of the problem.