How To Test The Starter Motor (Jeep 4.0L)

STARTER TEST 2: Verifying The Start Signal

What makes the starter motor activate and crank the engine is the 12 Volt Start Signal it gets when you turn the key to start the vehicle.

So, in this test step, we're gonna' see if the starter motor solenoid is getting this 12 Volt Start signal on the S terminal wire.

You can use a multimeter or a 12 Volt automotive test light (although the following test steps assume you're using a multimeter).

OK, here's what you'll need to do:

1. 1

   Lift the vehicle and place it on jack stands (if it isn't already up in the air).

2. 2

   Now, while underneath it, connect the red multimeter test lead to the S terminal wire of the starter motor.
   
   You can either test for this signal with the S terminal wire connected to the starter motor or not, your choice.

3. 3

   Connect the black multimeter test lead to a clean and rust-free spot on the engine or on the vehicle frame.
   
   Here I'm going to recommend something to you: Use a battery jump start cable to Ground the black multimeter test lead to a clean Ground point on the engine. The reason why is that depending on how rusty and dirty the underneath of the vehicle, you may NOT be able to find a clean and rust-free spot to Ground the multimeter's black test lead.

4. 4

   Have your helper crank the engine.
   
   The engine won't turn over, but the idea is to verify that the starter motor solenoid is getting the 12 Volt start signal from the ignition switch.

5. 5

   Your multimeter should register 10 to 12 Volts DC.

OK, let's make sense of the readings that your multimeter recorded in the test:

CASE 1: Your multimeter registered a voltage between 10 to 12 Volts. This test result lets you know the starter solenoid is receiving the Start signal (crank signal).

This means that we can forget about the safety neutral switch and the ignition switch being bad. OK, now the next test is to do a very easy and simple voltage drop test. Go to: STARTER TEST 3: Voltage Drop Testing The Battery (+) Cable.

CASE 2: Your multimeter DID NOT register 10 to 12 Volts. This result exonerates the starter motor. Your starter motor is not bad.

Here's the reason why: Without this 10 to 12 Volt Crank Signal, the starter motor will not crank the engine. Now, although it's beyond the scope of this article to test the neutral safety switch or the ignition switch, you have eliminated the starter motor and this means saving money by not buying a part your vehicle does not need.

STARTER TEST 3: Voltage Drop Testing The Battery (+) Cable

Some of the engine does not crank complaints that I've diagnosed and repaired over the years, were due to hidden corrosion on the battery (+) cable or battery positive (+) post that was preventing full battery power from reaching the starter motor. The end result was that the starter motor wasn't able to crank the engine.

This also meant that the starter motor wasn't at fault!, even though the customer had already replaced the starter motor (or was about to). How did I find out? With a voltage drop test!

You may have already cleaned the battery cable terminals and the battery posts but there's still a good chance that hidden corrosion (on the battery positive cable) is blocking battery power from reaching the starter motor (this condition is known as a voltage drop).

The absolute best way to eliminate this possibility, is with a simple multimeter voltage drop test of the battery positive cable (that attaches to the large stud of the starter motor solenoid).

To further explain what a voltage drop is: A voltage drop is simply a condition in which unseen corrosion blocks a lot of the battery power from reaching the starter motor. When this happens, the starter motor will not be able to crank the engine in your 4.0L Jeep even though the battery is in a fully charged state.

IMPORTANT: Perform the voltage drop test only if your Jeep does not crank. If the engine cranks (and starts), do not perform this test.

OK, to get started, this is what you need to do:

1. 1

   Place your multimeter in Volts DC mode.

2. 2

   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 test lead make contact right in the center of the positive battery post.
   
   You may need two helpers for this test step, since someone will have to hold the red multimeter test lead onto the battery positive (+) terminal and someone else will need to crank the vehicle while you perform the next step.

3. 3

   Touch the center of the starter solenoid stud, to which the big battery cable attaches to, with the black multimeter test lead.
   
   You'll maintain the black multimeter test lead in this position throughout the next step.

4. 4

   Have a helper turn the key to crank the engine from inside the vehicle.
   
   This is important, since a voltage drop test has to be done while the component in question is working (or trying to work).

5. 5

   OK, if all is good (no voltage drop), your multimeter will register 0 Volts (0.5 Volts is still 0 Volts).
   
   If there's a voltage drop, your multimeter will register voltage (usually above 7 Volts DC.)

OK, now that the testing part is done, let's take a look at what your results mean:

CASE 1: Your multimeter registered 0 Volts (no voltage drop). This result indicates that the starter motor is receiving all of the battery voltage and amperage it needs to crank the vehicle.

This also means that the starter motor is bad, and here's why:

1. In STARTER TEST 1 you confirmed that the starter motor doesn't work when you apply power to the S terminal wire of the starter motor solenoid.
2. STARTER TEST 2 you confirmed that the starter motor is receiving the crank signal.
3. In this test step you have confirmed that no voltage drop exists on the battery positive cable.

These 3 test results, taken together, indicate that the starter motor is bad. Replacing the starter motor should solve your 'no-crank' condition.

I'm going to make two more recommendations to you:

1. Before removing the starter motor, manually turn the engine using a 1/2 ratchet and the appropriate socket on the crankshaft pulley. This is just to make sure the engine or the A/C Compressor have not locked up and causing the no-crank condition.
2.     -and-
3. Bench test the starter motor after removing it. This is a super easy test to do and you can find this article by clicking here: How To Bench Test A Starter Motor (Step by Step) (at: easyautodiagnostics.com).

CASE 2: Your multimeter registered 5 Volts or more. This result tells you that a voltage drop does exist and this is not a good result.

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 solenoid).

After cleaning, try cranking the engine. If it cranks and starts, no further testing is required.

More 4.0L Jeep Diagnostic Tutorials

You'll find a complete list of 4.0L Jeep diagnostic tutorials in the following index:

• Jeep 4.0L Index Of Articles.

Here's a small sample of the articles/tutorials you'll find in the index:

• How To Test Engine Compression (Jeep 4.0L).
• How To Test The MAP Sensor (Jeep 4.0L).
• How To Troubleshoot A Bad Fuel Injector (Jeep 4.0L).
• How To Test The Fuel Pump In 2 Tests (4.0L Jeep).