Q: Occasionally will not start

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I have had the battery tested and everything checked out fine. I replaced the starter relay. Some days it will start others it will not. Sometimes if I jump start it it will start, sometimes it will not.

My car has 80000 miles.
My car has an automatic transmission.

I am assuming when you say it won’t start in this case, the motor will not turn. It’s an important distinction to make. Is it a "Crank no start", or is it a "No crank". With the crank no start, the motor cranks but will not run. With the no crank, the motor will not turn at all. In your case, based on your description, I am going to assume it is a no crank because you mentioned a jump start and you have replaced components that have only to do with cranking the motor.

With a condition such as this, it can usually be isolated to a bad connection somewhere in the primary or secondary circuits of the starting system. Bad connections are not always obvious. Electricity works on the atomic level and what we see with our eye is irrelevant to what electricity will do. For this reason we have to rely on tools such as a multi-meter, a test light, a carbon pile and an amp meter. In this case, the multi-meter and a test light are the tools of choice.

The test light is very easy to use, but not always the best tool with this kind of symptom. The biggest challenge is catching it when it won’t start. If you can consistently catch it in a no crank condition, have someone hold the key in the crank condition and check the power on the exciter wire at the starter. Check for power along the positive cable to the starter. I have seen many times where the bad connection is at the battery cable to battery post. You will be able to put the test light on the post and have the test light light up. Then put the test light on the cable end and have no power at all. This is the best example of a hard connection yet electricity won’t flow.

If this fails you, the next thing to do is a voltage drop. The voltage drop test is most often mis-understood. If you are so inclined to give it a try, Google it to find some detailed information on how to do it and give yourself due time to understand this test.

If this all sounds to daunting for you, I recommend booking a car is not starting inspection with YourMechanic.

Good luck! I hope I have helped you out.

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