Rick's Free Auto Repair Advice

Dash warning lights come on, but car won’t start

Diagnose Dash warning lights come on, but car won’t start

If you try to start your car and see the dash warning lights come on but car won’t start, you’ll most likely looking at a low battery condition. Lots of people say, “I’ve got a good battery because the dash lights come on.” But they’re wrong. It takes very little power to run the dash lights compared to how much power it takes to crank your engine.

Start by checking battery voltage

Using a digital voltage meter set to DC volts, touch one probe to the positive battery terminal and the other probe to the negative battery terminal. Battery state-of-charge is determined by the voltage on your meter, the type of battery in your car (standard flooded lead-acid, maintenance-free battery [no vent caps], or absorbed glass mat [AGM]}. Compare the voltage on your meter to the state-of-charge chart below. Measure battery temperature using a non-contact infra-red thermometer.

car battery testing
car battery state of charge chart

What the numbers mean

Generally, a battery voltage of at least 12.2 volts should crank your engine. However, if the voltage is less than than that, and certainly if the voltage is less than 9.6 volts, you may experience a dash lights come on but car won’t start condition.

What’s your next step?

Corroded battery terminals can’t always be diagnosed visually. If your battery terminals don’t have good contact with the battery posts, you won’t be able to start your engine.

One symptom of this is a dash lights come on, but car won’t start. Or, you turn the key and hear a single click or rapid clicking.

To diagnose a bad battery/terminal connection, conduct a voltage drop test using your digital meter.

How to conduct a voltage drop test on your car battery terminals and posts

Set your digital meter to the lowest setting on DC volts. Touch one probe to the battery terminal and the other probe to the battery post. The reading you get is the amount of voltage drop in the connection. A reading of less than 0.1 volt is considered a very good connection. The maximum amount of voltage drop is 0.4 volts. If your reading is at or above that 0.4-volt reading, the post-to-terminal connect is not good. You should remove the terminals and clean both the terminals and posts with a wire brush. Then retest.

car battery voltage drop

Test for voltage drop at the battery terminals and posts.

©, 2020 Rick Muscoplat

Posted on by Rick Muscoplat

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