Please help me fix my car!

I imagine that trying to diagnose this over the Internet will be as futile as trying to do it over the phone with the guy at the auto supply store, but I’ll give it a shot anyway.

The free car I was given in July (1986 Chevy Cavalier sedan, 2.0 4-cylinder) waited until one week after I obtained my driver’s license to start going wack on me. (Sigh. No problems while practicing my driving from July to October.) There’s some kind of electrical problem going on here. So here’s what I’ve got so far:

• Brand new battery
• Alternator bench-tested: GOOD
• Purchased circuit tester, and tested every red wire I could find (or get to, at least): all good, near as I can tell.

Basically, the alternator is not charging the battery.

I’ve done the test where, with the engine running, I disconnect the positive battery cable. The engine dies when I do this. A couple guys who know more about cars than I do said this test would tell me if the alternator was bad; if the engine dies, the alternator is the problem. However, as I mentioned above, the alternator showed up as “good” on the bench test. Indeed, my circuit tester shows a current at the positive terminal on the alternator. So this leads me to believe that the alternator is pumping out the volts, but those volts aren’t going where they’re supposed to go. The result is that all of my electrical stuff is pulling from the battery instead of the alternator.

The next suggestion I got from my more knowledgeable friends was “check the voltage regulator”. So I ask you: could a bad voltage regulator be preventing the battery from charging? What does a voltage regulator do? Does it direct the flow of electricity? Could a bad one prevent the current from the alternator from going where it needs to go? And how do I identify the voltage regulator?

I realize that it would be best to just get my car into the shop, but my fundage is kind of low at the moment. If this is something I can fix myself in the next couple of days, I’d prefer to do so.


I had that problem on my Thunderbird and it was the main fuse. It’s a 200 amp fuse located on the side of the primary junction box (the fuse box under the hood). I blew it by accidentally grounding out the alternator wire. I don’t know where they are on Chevies, but you may want to search for one and check it.

Check all the connections again, wiggle every wire you can find, take out the dome light bulb (don’t ask, but do thank your personal dieties & lucky stars that you weren’t given a Chevy Vega – unless it works, of course – then cuss Chevy), have your battery tested (even though it’s new) and get another bench test from a different (reputable) shop.

It could be your alternator & a wonky test. If you went to a cheap chain auto parts store, start with the second bench test.

Good luck.

Aw man, you mean I gotta pull that thing again? :frowning: We’ve got three main auto supply places here: Schuck’s, NAPA, and the local place, Jerry’s Auto Supply. I bought the battery from Schuck’s, but had the alternator tested at Jerry’s. Jerry’s is very reputable. I’m confident about the battery; it worked for two days after I installed it, and works fine after I recharge it. It holds the charge fine overnight … until I start the car a few times. Now I think I should have had the old battery tested/charged before I bought a new one. But everybody I described the initial problem to said, confidently, “It’s your battery”, so I bought a new battery. Plus, the old battery was about seven years old, according to the previous owner. I figure it can’t hurt to have a new battery with winter fast approaching.

Basically, the car runs fine until the battery gets too low. Maybe I could persuade my soon-to-be-ex-brother-in-law, the mechanic, to come take a look.

Thanks for the suggestion. I guess I should go get one of those model-specific repair manuals so I can find this stuff. The owner’s manual is no help at all. I did find this forum here, so maybe somebody there has experience with my problem.

I didn’t mean to imply that all auto parts shops are automatically suspect. It’s just that no bench test is perfect and a certain chainstore is notorious for having poorly trained employees. Even competent testers can biff it once in a while, and the equipment isn’t always spot on. double checking it can’t hurt.

Reading your post out loud to a group of slightly drunk car freaks has resulted in suggestions (& moderately boring, very long stories – I won’t subject you to the details. You’re welcome :stuck_out_tongue: ) of also checking the starter (your shop guy can test it), disconnecting the seat belt buzzer (ignore that one – that’s a slow continuous drain on the battery), assorted evil fuses, miraculous wire wiggling, new battery cables, replacing the battery ground wire (hmmmm – there’s a thought! Try that! It’s cheap and easy.), dome lights and somedamnthing about new radios, speaker wires & a pot-smoking installation guy.

Seriously – good luck. Electric stuff & fuel filters are pure malicious freakin’ evil in my book.

For car parts on a budget, junk yards can be a good resource.

Chilton’s is pretty good, but Haynes (sp?) sort of, well, sucks. IME, of course.

Ha! I’ve actually been buckling my seatbelt every time I do a test start, so that freakin’ buzzer won’t be going off and being one more thing pulling on the battery :wink:

I’ll try your other suggestions. As best I can, anyway. Some of that wiring is pretty darned inaccessible.

When the car is running, what is the voltage measured at the battery terminals? What’s the voltage measured at the alternator’s main output terminal (usually, it’s under a red rubber boot) to the alternator’s body?

The two readings should both be close to 13.8 volts. If both are low, it’s most likely a bad alternator or voltage regulator - most cars in the past couple decades have this as an internal part of the alternator. If the alternator comes up at 13.8 volts and the battery is (say) 11, then power’s not getting from the alternator to the battery. This could be anything from a corroded connection to a blown fusible link. You’ll just need to follow the big red wire off the alternator on its path to the battery. On some cars, it’s a straight shot, right out in the open, and in others, the wire dives into a wiring harness and loops hither and yon.

First off, and may I suggest you write this down and commit it to memory.
Don’t ever disconnect a battery cable from a car with the engine running.
I am as serious as a heart attack about this. Anyone that tells you to do this does not know shit about cars that have been built in the last, oh say, 40 years. Back in the day when cars had DC generators you could do this and find out if the generator was working. The problem is if you do this with an alternator equipped car, you can damage the alternator (as in, it wasn’t bad before, but it is now) or you might just wipe out one or more electronic control units to the tune of several hundred dollars each.
OK now that we have that out of the way, if you have a digital multimeter we can probably find out where the problem is.
First off measure the voltage in the battery after the car has been sitting for at least 3 or 4 hours.
If the battery is fully charged you should have 12.6V or above
3/4 charge 12.4V
1/2 charge 12.2
1/4 charge 12.0
if the battery is below 12.4 recharge it.
If you have any doubts about the battery have it tested a an auto parts store.
Then start the car and measure the voltage at the battery cables (they are clean and tight aren’t they?) with everything turned off and the engine at a high idle (1500-2000)
You should see 13.5-14.5 (some cars might go a little higher but not much)
Below 13.5 suspect a voltage drop, or a bad alternator.
Much above 14.5 a bad voltage regulator.
If you see straight battery voltage you alternator is a dead puppy.
Now Turn on the headlights and the heater fan. measure the voltage again. Is it still between 13.5 and 14.5? Flip the meter to AC and measure voltage at the battery terminals. If there is more than 0.4V AC at the battery terminals, your alternator diodes are bad, and you need a new alternator.
if you have not found a problem by now, it is time to do a voltage drop test. Set the meter to DC, engine running, lights and heater fan on. Place the red lead on the big fat output lead of the alternator. Place the black lead on the + battery terminal The reading should be less than 0.2V. Absolutely no more than 0.4V. If it is above this figure you have excess resistance in the cable between the alternator and the battery. Clean, tighten, or replace as necessary to bring the value down. Voltage drops are like golf, low score wins.
Next place one lead on the alternator case, and the other on the battery negative terminal. Same numbers apply.
By now you should have found the problem if it is in the charging system. (we have covered about 90%+ of all charging system failures, there are a couple of oddballs, but they are outside the scope of a message board.)
If you have not found the problem you may have a draw. you will need a test light to test for this.
Hood open, key off and out, car locked. Remove the battery negative terminal. Place the test light between the battery and the cable. If the light lights, you have a draw, find it. Fix it.
If the test light does not light, you do not have a draw.
NOTE: You cannot use a voltmeter for this test.

One more thing to check:

There are three wires connected to the alternator. One is a thick red/white wire connected the alternator output post. This goes to the battery to charge it. This should show battery voltage with the engine off. If it does not, there’s an open circuit that must be found and fixed. This wire should then show charging voltage (13.5-14.5) with the engine running. If it does not, the alternator is not charging (see below). If it does show charging voltage, the battery should show the same within a few tenths of a volt. If this is not the case, do the voltage drop test Rick described and find out why. And of course, make sure the connection here (nut screwed onto a post) is tight - BUT KEEP IN MIND THIS GOES TO THE BATTERY POSTIVE AND DO NOT LET ANYTHING METAL (like a wrench) TOUCH THIS AND ANY METAL ON THE ENGINE AT THE SAME TIME.

The other two wires are thin ones in a plug-in connector. The red/white one also connects to the battery, and is the trigger for alternator operation. This should show battery voltage. If it does not, there is an open circuit that must be found and fixed. If the thick red/white wire at the output post is tight and connected to the battery, and the thin red/white wire has battery voltage, then the alternator should charge. If there is not charging voltage at the alternator ouput post with the engine running, then the alternator is faulty.

The other wire in the plug-in connector is a brown wire to the alternator warning light on the dash. It should not be a concern here.

One of the strange things I found on some chevys is that if the alternator warning light doesn’t work the alternator won’t either. Turn the ignition on and see if the ALT or Battery idiot light comes on. Also check the stupid plastic plug that sticks into the side of the altenator. Since you have a test light you can probe from the back side of the altenator. Both wires in the plug should have voltage. As a quick diagnostic check you can turn your headlights on and note how bright they are. Then start the car and see if they get brighter. Rev the engine a little when you do this because sometimes the altenator doesn’t kick in until higher revs. Also you can hear the altenator whine when it is or putting out a lot of current.

Whoa! Thanks for that info. Everybody who told me to do that was over 50 years old (one born and raised in Mexico), so maybe they were remembering doing that with the old beaters they had in high school.

I’ll pick one up ASAP (they’re fairly cheap, right?)

What do you mean by “between the battery and the cable”? Between the negative terminal and the negative cable?

Additional info I just remembered: As I mentioned, none of these problems showed up until a week or so after I obtained my driver’s license. A couple days prior to taking my drive test, I replaced a couple light bulbs - one of my backup lights, and the rear-window brake light. That’s the only change I’ve made to the car prior to installing the new battery. Could I have mangled a wire somewhere while installing these lightbulbs? I just checked the backup light and didn’t see anything. I’ll check the brake light next. Would a pinched or partially-severed wire in one of these fixtures be enough to cause the problems I’m having?

Gary T, I’ll try that.

figure9, I have a light on the dash that says “VOLTS”. Same thing?

Yes. Volts, Batt, Alt are all labels used on the indicator light for the charging system.

A-ha! With the engine off, my test light says there is no current here …

… or here.

When the engine is running, there is a charge at the output post.

Mind you, I’m using a simple test light and not a voltimeter.

Well the fact is that you can usually get away with it even on modern cars. However, without the battery, there is little capacitance in the electrical system and it can be subject to voltage spikes which would otherwise be smoothed out. People usually beat the odds, but I wouldn’t try it.

There won’t be current there with the engine off. The alternator isn’t charging when it isn’t spinning. If there is current there with the engine running, then it’s charging.

Oh, and Gary T said voltage, not current.

Seriously, I doubt that you getting a driver’s license had much to do with it. :smiley:
I doubt that changing a light bulb is the source of the problem. Could be but I doubt it. Shit does break, ya know.
If the volts light on when the engine is running? If so look at the alternator.

Exactly. One end on the negitive terminal the other on the cable.

I dunno, the DOL could just be fuckin’ wit me … I mean, the timing! I only live a couple blocks from the DOL - somebody could have sneaked over to my house … [paranoid smiley goes here] :slight_smile:

Nah, it’s only on when I have the key turned on but the engine’s not running.

I’ll try that.