In this test step, I'm going to show you how to do a voltage drop test on the battery positive cable that connects between the starter motor and the battery on your Ford, or Mercury or Lincoln vehicle.
Don't worry... a voltage drop test is an easy, but super accurate test, that checks for hidden corrosion that could be keeping the starter motor from receiving full battery power (voltage and amperage). Why? Well, because if the starter motor doesn't get full battery power... it's not going to crank the engine and could lead you to think that the starter motor is BAD.
OK, on with the show... this is what you need to do:
You'll probably need two folks to help you with this test step.
With your multimeter still in Volts DC mode, probe the large stud on the starter motor solenoid (that has the large battery positive cable attached to it) with the RED multimeter test lead. Avoid probing the terminal itself... instead, probe the copper stud itself.
Now, have one of your assistants probe the very center of the battery positive post with the BLACK multimeter test lead. The very center of the positive post has to be clean, this is important.
Then, when everything is ready, have your second assistant crank the car (or pick up or mini-van) from inside the vehicle while you observe the multimeter.
like in STARTER TEST 1, the starter motor is not gonna' come alive, but the nature of the voltage drop test is that the test has to be done with the starter motor attempting to crank the engine (even if it doesn't).
If the starter motor is receiving full battery power across the battery positive cable (which means no voltage drop), your multimeter will register NO VOLTAGE. The display will register something like 0 Volts.
If the starter motor is not receiving full battery power, then the multimeter will register voltage. this voltage usually starts at 5 Volts and up. If this happens, well then... this is called a voltage drop, and this is not good.
OK, now that the testing part is done... let's take a look at what your results mean:
CASE 1: Your multimeter indicated NO voltage drop (which is .5 Volts or less): Well, I think you know what this means... the starter motor is BAD and needs to be replaced with a new one.
Now, just in case you want to be very thorough with your diagnostic (this is what I do at work when I'm testing a vehicle with a suspected BAD starter motor), I'm gonna' suggest two more tests:
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. After all, it could've thrown a rod and this is what is keeping the engine from cranking. Or the A/C compressor could be locked up too (I have seen this a lot!) and this will also make you think it's a BAD starter motor.
2.) Bench Test the starter motor. This is another easy test you can do once you have removed the starter motor from your Ford (or Mercury or Lincoln) car or pick up or mini-van. 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 the starter motor is not getting full battery power and this will keep the starter from cranking the engine.
Here's what you need to do to solve this voltage drop issue: Clean both ends of the battery positive cable. By both ends I mean the end that connects to the battery positive post and the end that attaches to the starter motor solenoid.
In the majority of the cases, this is all you have to do... since (once you done cleaning and re-attaching the ends of the cable to their respective places) the vehicle should now crank and start after this cleaning service.
To clean the end that connects to the starter motor solenoid, use a piece of sand paper to sand down both sides of the round terminal. You usually do not need to clean the stud on the starter motor solenoid.
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