How To Test The Starter Motor (GM 3.1L, 3.4L)

STARTER TEST 3: Voltage Drop Testing The Battery Cable

So far in your testing, you have verified that the battery is fully charged and in STARTER TEST 1 is not cranking the engine and in STARTER MOTOR TEST 2, you confirmed that the starter motor is being supplied with the Crank Signal (when your helper turned the key to crank the car).

The next step is to make sure that the battery positive (+) cable that attaches to the starter motor's starter motor is supplying full battery voltage and Amperage to the starter motor, because if it isn't (usually due to hidden corrosion somewhere on the cable), the starter motor won't be able to crank the engine. This verification is accomplished by doing a simple voltage drop test.

1. 1

   With your multimeter still in Volts DC mode from the previous test, attach the red multimeter test lead to the center of the bolt that attaches the positive (+) battery cable to the battery. The spot on this bolt, which the multimeter test lead will be touching, has to be clean and rust-free.
   
   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 bolt and someone else inside the vehicle (to crank it when everything is set up).

2. 2

   Now, at the starter motor solenoid, place the black multimeter test lead on the Stud that connects to the battery positive (+) cable. The orange arrow in the photo in the image viewer points to this stud. Maintain the black multimeter test lead in this position throughout the next step.

3. 3

   When everything is ready, have your helper, who should be inside the vehicle, crank the engine. Although the starter motor won't crank the engine, your helper has to turn the ignition switch to Start the engine for the voltage drop test to work.

4. 4

   OK, when your helper is trying to crank the engine, your job is to eye ball the multimeter. If everything is OK (no voltage drop), the multimeter will register 0.5 Volts or less (.5 V is really 0 Volts). If there's a problem in this circuit, your multimeter will register some voltage, usually 5 Volts or above.

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

CASE 1: Your multimeter indicated NO voltage drop (which is 0.5 Volts or less). This result indicates that the starter motor is receiving all of the battery voltage and amperage it needs to crank the vehicle.

This result also confirms that the starter motor is bad and needs to be replaced, and this is why:

1. STARTER TEST 1 confirmed that the starter motor does not crank the engine when you applied 12 Volts to the S terminal of the starter motor solenoid.
2. The result of STARTER TEST 2 confirmed that the ignition switch and the neutral safety switch are good and that the starter is getting the Crank Signal.
3. The test you just got done doing, confirms that the starter motor is getting all of the power it would need to crank the engine from the battery positive post.

So, all of the 3 test results confirm you have a bad starter on your hands.

Now, before you remove the starter motor, do two more things:

1.) Turn the engine manually (using a 1/2 ratchet and the appropriate socket). This will verify that the engine is not mechanically locked up.

2.) Bench test the starter motor. You can find the step-by-step instructions 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 a voltage drop is not a good thing to have in this circuit.

The solution is to simply disconnect the battery positive (+) cable from the battery and thoroughly clean it. Also, the end of this very same cable, that's attached to the starter motor solenoid, should be disconnected, inspected and cleaned.

Cleaning the end that attaches to the starter motor solenoid should be done with a small piece of sand paper. Once both ends of the battery positive cable are clean, reconnect everything and try cranking the car. If the voltage drop was the cause of the no-crank condition, your vehicle will now crank and start.

Symptoms Of A Bad Starter Motor

The most common symptom of a bad starter are:

1. 1

   Turn the key to crank the car (or pick up or SUV) and the engine does not turn over (crank).

2. 2

   A jump start does not help. The vehicle's engine still refuses to crank.

3. 3

   The battery has been charged and/or replaced and still the vehicle does not crank.

4. 4

   Turn the key to start the car (or pick up or SUV) and 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 the key is turned to crank the vehicle.

Related Test Articles

If you found this tutorial helpful, you might be interested in the other 3.1L and 3.4L GM test articles I've written. You can find them by checking out this link: GM 3.1L, 3.4L Index Of Articles.

At easyautodiagnostics.com, you'll find the following articles:

• How To Test The Ignition Coil Packs (at: easyautodiagnostics.com).
• How To Test The Ignition Control Module (at: easyautodiagnostics.com).
• How To Test The MAF Sensor (at: easyautodiagnostics.com).
• How To Clean The MAF Sensor (at: easyautodiagnostics.com).
• How To Test The Throttle Position Sensor (TPS) (at: easyautodiagnostics.com).