It’s been over a month since I drove our Mustang. When I went to start the car this afternoon . . . well, no luck. Just a buzz from the engine compartment when I turned the ignition key.
Turns out that the battery was toast. It was just over seven years old, so I think we got our money’s worth from this particular AutoZone battery.
Diagnosing the No-Start Buzz
I’ve learned that whenever I have a no-start issue with the Mustang, the first thing to do ALWAYS is to check, clean, and tighten all battery and starter solenoid connections and grounds. So I did that with the main battery terminals and posts, and then cleaned and tightened the main engine ground. (A poor engine ground has caused me numerous problems over the years, so I always verify this first.)
Next, I cleaned and tightened the battery connection at the starter solenoid, which mounts right next to the battery, on the passenger fender. Again, in the past, I’ve had the Mustang not start — and just buzz at me — because this connection wasn’t quite tight enough.
With the relevant connections cleaned and tightened, and still no start-up, I moved on to the battery itself. Just the fact that it was seven-plus years old made me suspicious. When I attempted to turn on the headlamps, they came on, but were quite dim. So I grabbed my multimeter and tested the battery voltage. Set the dial to 20 volts DC or higher, as shown here:
When tested under no load, the battery showed between 12 and 13 volts, which is fine. However, when I monitored the voltage during attempted start-up, the voltage dropped to between 5 and 6 volts … when it ought to be at 9 volts or more. That’s bad, and indicative of an undercharged or failed battery.
Now, at this point, with battery voltage this low during a load, I’d typically try a 24-hour battery charge. But because I’d already had the battery charging overnight, I was pretty confident that the battery was shot. It just wasn’t putting out enough “oomph” to allow the starter to do its thing.
A quick trip to the local parts store got me a new battery (plus a new pair of anti-corrosion pads for the battery posts). I dropped the new battery into the Mustang minutes later. Sure enough, the engine started like a champ!
Multimeter Can Check Your Alternator, Too
In addition to testing a battery under load, a multimeter can help test your Mustang’s alternator when the engine’s running. Connect the red multimeter pin or clamp to the positive battery post, and the negative multimeter pin or clamp to the negative battery post. Turn on the engine. With the engine running, if the alternator is operating correctly, and charging the battery, then you should see more than 13 volts on your multimeter readout. (Source)