TEST 2: Verifying The 12 Volt Start Signal
So far you have confirmed that the starter motor activates (to crank the engine) when you apply 12 Volts directly to its S terminal.
Now you'll check that the wire that connects to the S terminal is delivering a 12 Volts activation signal to the starter motor.
These 12 Volts should only be present in the wire when the ignition key is turned to the START position.
NOTE: You can perform this test with the S wire connected or disconnected to the S terminal.
These are the test steps:
- 1
Raise and place the front of the vehicle on jack stands (if it isn't already).
- 2
Place your multimeter in Volts DC mode.
- 3
Ground the black multimeter test lead directly on the negative (-) battery terminal using a jump start cable.
You can also Ground it on the engine, if you can find a clean, unpainted and rust-free spot of metal. - 4
Connect the red multimeter test lead to the S terminal wire using an appropriate tool.
Don't know which wire is the S terminal wire? It's the one that connects to the smaller of the three studs on the starter motor solenoid.
The S terminal wire (circuit) is the one that delivers the Start (Crank) signal from the ignition switch. - 5
Have your helper turn the key to crank the engine when the test is setup.
- 6
Your multimeter should read 10 to 12 Volts DC.
OK, let's take a look at your test result:
CASE 1: Your multimeter registered 10 to 12 Volts. This is the correct and expected test result, confirming that the starter motor is receiving its activation signal.
You can conclude that the starter motor is bad and needs replacement (or repair) if you have:
- Confirmed the starter motor activates when directly applying 12 Volts to the solenoid's S terminal (TEST 1).
- Confirmed that the wire that connects to the solenoid's S terminal is delivering 10 to 12 Volts when the key is turned to crank and start the engine (this test section).
CASE 2: Your multimeter DID NOT register 10 to 12 Volts. This test result confirms the starter motor isn't receiving an activation signal. Without it, the starter motor will not activate and crank the engine.
The missing starter motor activation signal is most likely caused by one of the following issues:
- A bad starter motor relay (if equipped).
- A bad park-neutral safety switch (automatic transmission).
- A bad clutch pedal switch (manual transmission).
- A bad ignition switch.
- An anti-theft system fault.
Although it's beyond the scope of this tutorial, your next step is to find the cause of the missing activation signal and resolve the issue.
TEST 3: Voltage Drop Testing The Battery (+) Cable
Your Chevy S10 or GMC Sonoma's battery is connected directly to the starter motor via a cable.
This cable channels the battery current the starter motor needs to crank the engine.
It's not unusual for the terminal ends of the cable to develop corrosion. Eventually, this corrosion will cause a big enough voltage drop that the starter motor will not able able to crank the engine.
We can easily check if hidden corrosion is causing a problem by performing a multimeter voltage drop test on the cable.
If your multimeter reports a voltage drop of 5 Volts or more, you can conclude that the cable's terminal ends have a corrosion problem or are loose.
Let's get the ball rolling:
- 1
Place your multimeter in Volts DC mode.
- 2
Attach the red multimeter test lead to the positive (+) battery post. The positive (+) battery post must be clean and corrosion-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). - 3
Place the black multimeter test lead on the starter motor solenoid stud that connects to the battery positive (+) cable (see illustration above).
The orange arrow with the plus (+) sign, in the illustration above, points to this stud.
Maintain the black multimeter test lead in this position throughout the next step. - 4
When everything is ready, have your helper turn the key to 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. - 5
The multimeter should register 0.5 Volts or less (0.5 V = 0 Volts).
If there's a problem in the wire, your multimeter will register some voltage, usually 5 Volts or more.
Let's interpret your test result:
CASE 1: Your multimeter indicated NO voltage drop (0.5 Volts or less). This is the correct test result, and it confirms the starter motor is receiving all of the battery voltage and amperage it needs to crank the engine.
You can conclude that the starter motor is bad and needs replacement if you have:
- Confirmed that the starter motor does not crank the engine when you directly apply 12 Volts to the starter's S terminal (TEST 1).
- Confirmed, in this test section, that the battery positive (+) cable does not have a voltage drop issue.
Now, before you remove the starter motor, do two more important things:
- Confirm the starter motor is receiving an activation signal. TEST 2 will help you with this.
- Check that the engine is NOT mechanically locked up by manually turning it via its crankshaft pulley bolt (using a 1/2" ratchet and the appropriate size socket).
If you'd like to bench-test the starter motor (after removing it). You can find the step-by-step instructions here:
CASE 2: Your multimeter registered 5 Volts or more. This result confirms that the starter's battery (+) cable has corrosion on one or both terminal ends.
The issue causing the voltage drop in the cable will prevent the battery's total amperage output from reaching the starter motor.
Your next step is thoroughly cleaning both ends of the battery positive (+) cable with a small piece of sandpaper.
Eliminating the cause of the voltage drop should get the starter motor to crank the engine.
If this info saved the day, buy me a beer!
