O wise Dopers, help me diagnose my car problem!

Factual Questions
1

Just as a preface, let me say that I generally consider myself a fairly bright individual. And although the inner workings of cars aren’t really my area of expertise, I’m generally comfortable dealing with brakes and oil changes and whatnot. But for some reason, I’ve always had a huge blind spot when it comes to electricity. Electrical physics was the only class I ever really had trouble with in college - it took me most of the semester to figure out what voltage is. So when I have an electrical problem with my car, I’m completely at the mercy of the mechanic. He could tell me I need a new $700 electrical vortex deseparator, and I’d have to shrug and pay it. So this time, before I take my car in, I’d like to have some idea what a reasonable explanation for the problem would be. So I turn to you, my fellow Dopers, to help me out!

The problem is, simply, that my car won’t start. When I open the door, everything’s fine - lights light up, beepers beep, all is well. Then I put the key in the ignition and turn it, and after a split second of everything electrical coming on as usual, there’s a soft thump, and it all goes completely dead. Luckily, a while back, I invested in a $50 portable jumpstarter, which has saved my bacon this weekend (this all started Saturday night). When I hook up the jumpstarter, the car starts up just fine, and once it’s started, it runs just fine. I don’t think the charge is draining slowly when the car’s off, because when I stopped for gas the other day, I had to jump it again, even though it’d only been off for about five minutes or less.

I’ve asked a few people I know what they think. One suggested a bad battery, but it’s less than a year old, so I hope it hasn’t died already. Another suggested the alternator, about which I only know that it’s really expensive to replace, but then someone else said they thought that if the alternator was bad, my car would tend to die while I was driving it.

So, please, help me out here! What’s going on? Any ideas? Oh, if it matters, the car is a '93 Ford Probe. Thanks in advance.

2

From your description --------- check and clean your battery terminals—if that doesn’t do it put a new battery in it —or---------if the battery’s that new exercise your battery warranty rights.

EZ

3

Bad starter? Selenoid?

Alternators aren’t terribly expensive. I’ve changed two in my life, and I believe both times, it was under $100.

4

I second cleaning and checking the battery terminals. I had the exact same problem you are describing in a t-bird I used to have. Cleaning the terminals and wire connectors cleared it right up.

5

I had a battery die on me after about a year.

6

I agree with Ezstrete – it sounds like it’s either a poor battery cable connection or a faulty battery. While a faulty alternator isn’t impossible, it’s unlikely that would explain what you’ve described. Its starting right up with the jumpstarter pretty well eliminates the starter as the problem.

7

Smeghead, would you mind if I added a problem that we’ve been having? It’s totally non-electrical, so only fits in with your thread as it’s a car problem and I don’t want to get hosed by a mechanic.

8

Start your own thread. Less confusing for you and for Smeghead.

9

The battery is supplying enough current to light the lights, but when you try and start the car it all goes dead, meaning that the battery does not have enough current capability to spin the starter. This means your battery is dead.

The real question though is what killed your battery. They don’t just die on their own. I’m guessing that the real problem here is your alternator. If the alternator doesn’t charge up the battery while the engine is running, there won’t be enough energy in the battery to restart the car later.

An alternator is easy enough to test. Measure the voltage of the battery with the car off. It should be a little over 12 volts. Now start the car. The voltage should be higher, maybe by a volt or two, and will probably go up a bit more if you rev the engine. If you are still reading the same as with the car shut down, the alternator is shot. Some auto parts stores will test your alternator and battery for you for free right there in their parking lot.

Lead acid batteries will destroy themselves if they get too far discharged. Even if the root cause is your alternator, the battery could be destroyed just from being discharged too much. Don’t be too surprised if you need to replace both the battery and the alternator at this point.

Either your battery is able to hold enough charge that you are able to drive around without the engine going completely dead, or else the alternator is either charging a little (but not enough, in which case you still have an alternator problem) or the alternator is charging fine and the battery is kaput. In the last case, we still don’t know why your battery got damaged in the first place. Did it possibly go dead at one point (because you left a light on or something) and was jumped and seemed ok after that, but then (maybe a week or so later) your starting problems started?

10

In addition to cleaning the connections at the battery, have a look at the other ends of the cables - the black wire will be bolted (typically) to the engine block or some part of the frame. As to the red, there’s often two or more fat red wires - follow them to their ends at the alternator, starter and power distribution block. These ends don’t tend to get corroded like the battery terminals do, bu they certainly can get cruddy or loose.