How To Test The Starter Motor (Jeep 4.0L)

Testing the starter motor, to see if it's the cause of the no-crank condition on your Jeep, is not that hard.

As a matter-o'-fact, testing the starter motor, to see if it's good or bad, is something that you can do at home. In this tutorial I'll show you how to do it in a step-by-step manner.

You can find this tutorial in Spanish here: Cómo Verificar El Motor De Arranque (4.0L Jeep) (at: autotecnico-online.com).

Important Safety Precautions

SUGGESTION 1: You'll notice that all of the photos I'm using show the starter motor off of the vehicle, well, this is only to make it easier to explain where you need to make your connections. You don't need to remove the starter motor to test it.

In case you do need to bench test it (test it off of the vehicle), the following tutorial will help you: How To Bench Test A Starter Motor (Step By Step) (at: easyautodiagnostics.com).

SUGGESTION 2: Before you start testing the starter motor, with the info in this tutorial, you need to make sure that the battery is fully charged. A discharged battery will cause you to reach the wrong diagnostic conclusion and have you wasting time and money! Also, the battery cable terminals and battery posts must be clean and corrosion free.

In many cases, dirty battery cables and/or battery posts are the culprit behind a no-crank condition and cleaning the battery cables and posts may actually solve your particular no-crank problem.

SUGGESTION 3: You'll need to raise your Jeep 4.0L equipped vehicle up in the air to access the starter motor. So use jack stands and don't trust the jack alone! Take all necessary safety precautions, like using jack stands to hold up the vehicle, wearing eye-protection (safety glasses), etc.

SUGGESTION 4: If your vehicle has a standard transmission, make sure that it's out of gear and in neutral, and the parking brake is activated/on.

Symptoms Of A Bad Starter Motor

When the starter motor goes bad on your Jeep vehicle, it will not crank the engine (this is called a no-crank condition in tech speak). This condition (a no-crank condition) is sometimes confused with a no-start condition.

In a no-start condition, the starter motor is cranking the engine, but the engine is not starting. The no-start condition is usually due to a lack of spark (think bad ignition coil or bad crank sensor), or a lack of fuel (think bad fuel pump). Knowing the difference between a no-crank and a no-start condition will save you some diagnostic time.

In a bonafide no-crank condition, you'll also see one or several of the following symptoms:

1. The engine doesn't turn over (crank) when you turn the key to start the engine.
2. A jump start doesn't help. The vehicle's engine still refuses to crank.
3. The battery has been charged and/or replaced and still your vehicle does not crank.
4. When you turn the key to crank the engine, all you hear is a small knock and nothing else.

Although the above list is a not a very complete list of symptoms, the theme that runs thru' them and any other related symptom, is that the engine will not turn over when you try to start it.

Tools Needed To Test The Starter Motor

You don't need expensive test equipment to test the starter motor on your Jeep vehicle but you do need a few things. These are:

1. Jack.
2. Jack stands.
3. Remote starter switch.
   • If you'd like to see what a remote starter switch looks like, you can follow this link: Actron CP7853 Remote Starter Switch For 6V And 12V Automotive Starting Systems
   • You can either buy this tool online or you can buy it at your local auto parts store (AutoZone, O'Reilly, Pepboys, etc.).
4. Multimeter or a 12 Volt automotive test light.
   • If you don't have a multimeter or need to upgrade yours, check out my recommendation here: Buying A Digital Multimeter For Automotive Diagnostic Testing (at: easyautodiagnostics.com).
5. A wire piercing probe.
   • This tool is not an ‘absolute must-have tool’ but I can tell you from experience that it makes it a whole lot easier to probe the S terminal wire for the Start Signal.
   • If you'd like to see what this tool looks like, you find out more about it here: Wire Piercing Probe Tool Review (Power Probe PWPPPPP01).
6. A helper.

As you can see, you don't need anything expensive. OK, let's turn the page and get started with the first starter motor test.

STARTER TEST 1: Applying 12 Volts To The Starter Motor S Terminal

The very first thing we'll do, to see if the starter motor is fried or not, is to manually apply 12 Volts to the S terminal of the starter motor solenoid.

The fastest, easiest, and safest way to do this is with a remote start switch attached to the starter motor solenoid's S terminal.

In case you're wondering, the purpose of this test is to eliminate or blame the starter motor as bad right off the bat, since this test will tell us right away if it's not activating due to it not receiving the 12 Volt Start signal from the ignition switch.

OK, to get this pot of water boiling, this is what you need to do:

1. 1

   Jack up your Jeep vehicle and place on it jack stands. Remember, the only way to gain access to the starter motor is from underneath the engine.

2. 2

   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.

3. 3

   Attach one of the remote starter switch's terminals to the S terminal of the starter motor solenoid.
   
   This is easier said than done, so take your time and make sure the connection is on the S terminal of the starter motor solenoid.
   
   Also, in case you're wondering, you can leave the starter motor solenoid's S terminal wire connected to the engine's wiring harness connector or not, the test will work either way.

4. 4

   Reconnect the battery negative (-) cable to the battery negative (-) post.

5. 5

   Connect the remaining remote starter switch's ‘alligator’ connector to the battery positive (+) terminal.

6. 6

   Now, apply 12 Volts to the S terminal wire of the starter motor starter solenoid with your remote starter switch.

7. 7

   You'll get one of two results:
   
   1.) The starter will activate and will turn over the engine.
   
   2.) The starter motor won't do a thing.

Let's take a look at what your test results mean:

CASE 1: The starter motor came to life and cranked the engine. This test result lets you know that the starter motor is OK and functioning. It also tells you that the probable cause, of it not working when you turn the key to start the engine, is a lack of the 12 Volt Start signal on the S terminal of the starter motor solenoid.

The next step is to go to TEST 2 and see if the starter motor is getting the 12 Volt Start signal on the S terminal wire (circuit). Go to: STARTER TEST 2: Verifying The Start Signal.

CASE 2: The starter motor DID NOT crank the engine. This usually means that your starter motor is bad and needs to be rebuilt or replaced.

I suggest 2 more tests and these are make sure that the starter motor is getting its 12 Volt Start signal and to test the battery cable (that attaches to the starter motor solenoid) for corrosion. This can be accomplished very easily with a voltage drop test.

• Go to: STARTER TEST 2: Verifying The Start Signal.
• Go to: STARTER TEST 3: Voltage Drop Testing The Battery Cable.