In the Car Talk Communities, there’s a question this week about a 2005 Mercedes-Benz C-Class that won’t start, despite the battery being “good.” Before you go buying new batteries, alternators or starters, there’s a sequence of diagnosis you should follow to figure out what’s going on. It could save you hundreds of dollars:
This diagram is a little basic and out of date, but it roughly explains the components of a typical 12-volt charging system:
The battery is a vulnerable component because it can be weakened by age, temperature and even vibration. But it’s not the only component that needs to be inspected and replaced, and as the years have gone by, it’s become one of the more expensive components in the charging system. There was a time when $75 would buy the best battery on the shelf. These days, you’re looking at $140 to $200.
You don’t want to spend that money unless the battery truly is the problem. We’re not only going to describe how to test these components, but the order in which you should be testing, which will uncover typical problem areas and save you from spending more than you have to.
You’re going to need a few diagnostic tools, but we’ll keep it to the absolute minimum.
The first thing to consider purchasing is a battery tester with a load tester. This kind of tester not only tests the state of charge at rest, but also the health of the battery while it’s subjected to the load of a start cycle.
There are fancier electronic testers, but this old-school analog tester has a couple of advantages:
- You can buy one for about $30
- It doesn’t require batteries to operate. This thing is going to be hanging in your garage — unused — for years. Something that requires a fresh set of AAs is going to fail when you need it to work most
If you don’t want to invest in a battery tester like this, you can always bring the car — or just the battery — to a good auto parts store. Most have a battery tester that can immediately tell you whether or not your battery is the problem.
The other thing to consider is a decent multimeter. You can spend a ton of money on one with features you’ll never use. Find one in the $29 to $45 range with a large digital readout and you should be fine.
This Klein Tools auto-ranging multimeter is about $50, and has all the features you’ll need. It’s available at big box home centers, so you won’t have to find one at a specialty electronics warehouse.
One note: a multimeter can test the voltage of a battery, but it won’t run a one-person start cycle on it the way a battery tester will, so we’d recommend getting both. You can have a second person start the car while you hold a multimeter’s probes against the battery posts, but it’s not as convenient.
The other thing every garage needs is a battery charger. Trickle chargers have gotten smaller, cheaper and more sophisticated over the years. You can pick up a fully transistorized Battery Tender that you simply plug in and not think about for about $29.
Testing in Order
If you want to make this as expensive as possible, just start replacing parts without knowing if they’re the problem.
If you want to get out of this as inexpensively as you can, you need to test a few components to find the root of the problem. We’ve laid these tests out in order of (a) ease, (b) expense and (c) likelihood of failure.
1. Test the Battery
Even if it’s new, you need to understand what’s going on with the battery. Attach the tester to the battery. If the needle’s not moving at all, then you’ve got a dead battery. What’s more important is its condition under load.
By pressing the “Load Test” button, you’re simulating a start cycle. The battery should be able to hold 8.5 volts for 15 seconds at 0 degrees F. If it doesn’t, then you know the battery is at least part of the problem.
If it does, then you need to start investigating elsewhere.
2. Test the Cables
Battery cables go bad all the time. Not only do the connections get crusted up with corrosion, but that corrosion can creep down inside the cables, rendering them all but useless.
A voltage drop test can tell you if your battery cable is the problem. Using the Voltage setting on the multimeter, first touch the probes to the battery terminals to determine the voltage of the battery. A fully charged battery should have about 12 volts ready to go.
To perform a voltage drop test of the cables and terminals, touch one probe to the battery post, and then the other to the terminal. It should read at or near zero. If it’s reading any lower (the numbers will be represented as negative decimals: -0.07, for example), then you’re losing voltage in your cables.
Watch the video for a more detailed explanation:
3. Check the Belt
If the battery is discharged and the cable connections are good, you’ll want to fully charge the battery and start looking at the condition of the charging system.
The second cheapest component in the entire charging system next to the cables is the belt, so let’s look at that first. For the last 30 years, cars have used serpentine belts with idler pulleys that maintain tension. If the belt it squeaking or showing signs of slippage, it might not be allowing the alternator to work the way it should If the belt hasn’t been changed, and it looks cracked, it’s time to replace it anyway.
It’s typically a $35 replacement, but you also want to replace the idler pulley at the same time. There’s a bearing in it that will go bad at the least convenient moment, and it will render the car dead at best, or dead with a blown head gasket at worst, since that same belt spins not only the alternator, but the water pump. They’re easy to replace and don’t cost much more than the belt.
4. Test the Alternator
Typically, if your alternator is no good, it’s going to throw a “CHARGE” light on the dash, or show you that you’re not running at 14.3 volts. But that idiot light or gauge might not be working the way it should.
Using your trusty multimeter, you can test exactly how much voltage the alternator is putting out. Simply attach the two probes to the positive and negative terminals of the battery, make sure the probe leads are out of the way of any moving parts in the engine bay, and fire the engine up.
The multimeter should show somewhere between 14.2 and 14.7 volts. If it’s showing less, the battery isn’t being charged as well as it should, and it will be especially apparent when you’re running accessories like lights, wipers, rear defroster and the radio. If it shows MORE than 14.7 volts, the alternator is overcharging the battery and will eventually cook the life out of it.
5. Test for Parasitic Drain
It’s hard to place this test using our hierarchy of ease, expense and likelihood of a problem. If you’ve left a light on, that should be easy and free to fix. If you’ve got a short in a hidden wire because a mouse chewed through it, that could be hard and expensive to diagnose and fix.
In general though, less than obvious parasitic drains aren’t as common as some of the other issues in our list. However, if your battery is discharging overnight — which you’re determining because you’ve charged the battery, then tested the voltage the next morning as described in step 1 — then you might actually have some kind of a constant drain that’s the problem.
To perform this test, remove the negative battery cable from the battery. With the multimeter set to the highest amp scale (read your multimeter’s instructions), touch one of your multimeter’s probes to the cable terminal, and one to the battery post.
The readout on your multimeter shouldn’t be higher than 50ma (milliamps). If it’s more than 50ma, you’ve got something that’s drawing power from the battery.
To determine what’s causing that draw, you’ll need to follow the instructions in this video. The process involves removing and replacing every single fuse in the car, one by one, until that voltage draw goes to fewer than 50ma.
5. Check the Starter
If the cables, belt, battery and alternator are ok, then, and only then, start looking at the starter. Testing it requires taking it out, meaning that you’re probably going to end up replacing it anyway, so unless you’re pretty ambitious, you’ll probably end up at a mechanic’s shop. But by this point, you’ll be able to provide your technician with a lot of information about the condition of your charging system.
We hope this information helps and saved you some inconvenience and a few bucks in the process.