With some cars, excessive oil leaking onto the starter can dissolve the brushes and cause intermittent and / or slow cranking. All starter motors actually have two interconnected circuits, and if one fails, current flow will stay pretty close to normal, but the motor will have only half its normal power.
The place to start is by measuring battery voltage. First test it without cranking the engine. It should be 12.6 volts if it's fully-charged. If it's around 12.2 volts, it's good but discharged. We'll need to test the charging system.
Next, measure the battery voltage while a helper cranks the engine. We're looking for voltage readings when the starter is cranking, but too slowly. Put the meter probes right on the battery posts, not the cable clamps. If the voltage during cranking drops below 9.6 volts, suspect the battery.
Now move the meter probes to the cable clamps and take the reading again. It should be nearly the same as in the previous test. Move the negative probe to the engine block and take another reading during cranking, then move the positive probe to the terminal on the starter, and test again. At the starter, you want to be on the threaded stud, not the terminal bolted to it that's crimped to the end of the cable.
You're hoping to find a point at which the voltage drops a lot compared to the previous step. That indicates the point of a loose or corroded connection.
Thursday, January 15th, 2015 AT 2:48 PM