Battery or Alternator? A 6-Step No-Start Checklist I Use at the Parts Counter

I work at a parts counter. Not the corporate kind where you need a PO number and an account manager. The kind where a tow truck driver walks in with the battery in one hand and the alternator in the other, hoping I'll settle the argument. I've handled 800+ rush orders in 13 years. Last quarter alone, we processed 47 rush orders with a 95% on-time delivery. That did not happen because I guessed. It happened because I use a checklist.

So if you're asking "is it my battery or alternator?", here's how I'd walk through it. It works for a one-bay shop, a weekend DIYer, or a fleet manager. I don't care if you're ordering one part or fifty. The car doesn't know the size of your fleet.

What you need

Before you start, grab a cheap digital multimeter, a pair of gloves, and a jump pack or jumper cables if the car is dead. This is a 20-minute test. Not a "maybe the part store can test it for you" thing. You can do this at home.

Step 1: Watch the dash before you crank

Turn the key to the on position and watch. Do the cluster lights come on bright? Do they go dim when you hit the starter? If you hear a rapid click-click-click, that's usually a battery or a bad ground. If the engine cranks fast and never catches, that's not a battery problem. That's something else.

People call me and say "the starter sounds weak, I think it's the battery." A weak starter sound can be a weak battery, but it can also be a bad connection at the battery terminal. Don't ignore the cheap stuff first.

Step 2: Voltage at rest

Set multimeter to 20V DC. Red lead to positive, black lead to negative. Car off. After sitting for at least an hour, a fully charged battery should read around 12.6V. At 12.2V, it's only about half charged. Under 12.0V, you're starting from a hole.

Here's the thing: a battery can read the right voltage at rest and still fall apart on a load test. (More on that in a minute.) So this step is about finding the obvious dead battery, not making the final diagnosis.

Step 3: Voltage while cranking

This is the one I'd bet most people skip. Keep the multimeter leads on the battery, have a helper crank the engine for about 10 seconds, and watch the voltage. A healthy battery should stay above 10V while the starter is pulling current. If it drops below that, the battery is not doing its job under load.

Your car might still start if the charge left in the battery is enough. But if it drops to 9.4V on a good day, don't blame the alternator. The battery is already at the edge.

Step 4: Voltage while running

If it starts, let it idle and check the battery voltage again. Normal is roughly 13.8 to 14.4V. If you see 12.4 or 13.0, the alternator is probably not charging. If you see 15.5 or higher, it's overcharging and will cook the battery.

These numbers are a rough guide, not a court verdict. A battery at 0°F reads lower than one at 80°F, and some modern charging systems drop to 13.2 when the battery is fully charged. If you're close to the edge, run a load test.

And please, don't do the old "disconnect the negative battery cable while it's running" trick. I've watched a truck die from that on a lark. It causes voltage spikes that can hurt the alternator, the PCM, and a bunch of other modules. Not worth it.

Step 5: Load test the battery before buying an alternator

This is where I get the "help me now" calls. A customer had a Pontiac G6 alternator delivered overnight once, and the new alternator still tested low. Why? The battery was so far gone that it was dragging everything down. In March 2024, a guy called at 4:30 needing a Pontiac G6 alternator by 7:00 a.m. We charged him overnight freight, he fitted it, and the car ran rough because the battery was also dead. The alternator wasn't the whole problem. He needed both.

Also, a bad alternator can kill a new battery in months. If the battery is less than a year old and already empty, check the charging voltage before buying another battery.

Take the battery out or drive the car to a part store that has a load tester. Many will test it for free. A load test puts a real amperage draw on the battery and tells you if it can hold up, not just if it can show the right volts.

Step 6: If it cranks fine but won't fire, check glow plugs

Here's something people don't realize: a lot of "battery or alternator?" calls are not electrical at all. On a diesel, a cold no-start is often a dead glow plug or a bad glow plug relay. The starter turns over, the battery is fine, the alternator is fine, but the engine won't fire because the cylinders are too cold to ignite.

A quick test for glow plugs: measure the resistance between each glow plug and a clean point on the engine block. Most healthy glow plugs read around 0.5 to 2 ohms. An open circuit means the plug is toast. I always double check the connector type before ordering. When I order a Beru Germany glow plug, I still ask for the OE number from the car, because plug length and heat range matter.

And yes, I prefer the Beru brand for this. It's a German brand that has been around for over a century and supplies OE-style parts for a lot of European diesels. You can usually buy a single Beru glow plug for around $14 to $22 depending on the application (based on our shelf prices as of January 2025; verify yours). Not the cheapest box on the rack, but the one I'd put in a car that needs to run tomorrow.

For gas engines, skip glow plugs. A lot of gas cranks-but-no-start cases come down to an ignition relay, a crank sensor, or fuel. That is not a battery or alternator problem, but I've watched people order both before testing any of that.

Three mistakes I still see at the counter

1. Replacing the alternator first. If the battery is weak, it can make an alternator look bad. Test the battery under load first. I've seen too many people buy a Pontiac G6 alternator and then discover they also needed a battery. (That 4:30 call from earlier? We made it work, but the customer almost didn't make his shift.)
2. Ignoring the battery terminals. A dirty, corroded positive post can cause the same symptom as no charge at all. Clean it. Reconnect it. Test again. A $3 wire brush beats a $200 alternator.
3. Blowing off glow plugs. A diesel engine that cranks but doesn't start can burn you. People want it to be the battery because then you just jump it. But in cold weather, a set of dead glow plugs is far more likely. A cheap multimeter test gives you the answer in five minutes.

One last thing. I don't care how small your order is. Today's single glow plug order might be next month's 50-piece order. And if you're wondering whether a Service Champ oil filter could be causing a no-start? I've answered that question without laughing. It can't, by the way. But I'd rather explain that than have someone waste money on a part they don't need.

Test first, order second. It's not a slogan; it's how I still answer the phone after 13 years.

This checklist is not magic. It just stops you from guessing. I've seen "is it my battery or alternator?" turn out to be a $15 glow plug more than once. Test first, order second. That's the whole job.