Car Alternator Problems

Most, I repeat, most cars will run if you remove the positive cable, if the alt. is good. I have a Chevy work truck that dies when I do this, but it's been modified, so I don't know if the mod affects this or it's particular to this vehicle.

Thanks for the info. We had discussion about this at work, the fellow who brought it said his died. He replaced the battery and it would still die after a few days. I tried to get him to try it.Maybe he will. Thanks again for your input,

Tony

I'll just add my two cents worth to the other posters who advised against removing the battery cable with the engine running, when voltage regulators were mechanical you could do that, the voltage spikes with solid state regulators not to mention the rest of the electronics are not a good thing.

One very simple way to get some idea if the alternator is working or not is to turn everything electrical on in the vehicle, with the engine idling headlights on highbeam and then bring the engine up to about 2000 rpm if the headlights don't get any brighter you have a problem. Not a very accurate test but sometimes it works.

The other solutions are much better, remove the alternator and have it tested, or get a meter and used the test procedures described in other post.

One other thing that could be tried is to remove the battery cable while the vehicle is not running. It is not uncommon to get some small amount of arc when you reconnect a battery cable on a modern vehicle, if you large spark you may have something draining power continuously. Seen that happen but it's not very likely, it sounds to me like a bad alternator.

Alternators are a lot cheaper than engine controllers. One other thing your friend should know is that standard automotive batteries are not meant to be deep cycled, that is drained until they're almost dead and then recharge, so besides being inconvenient to have to recharge his battery, he's also shortening its life.

If the battery voltage at a fast idle with no accessories on is not between 13.3 vdc (summertime) and 15.5 vdc (wintertime) it's probably a failed regulator. Continual current draw from the battery might be 50 mA to run keep-alive circuits.

I like to add if its below that (ie close to 12 V. its discharging from the battery, and under 12 V it shuts down.

One problem is check your connection.

You can always remove the alternator, run it down to the local auto parts store, and have them toss it on the test stand. Most Auto parts stores have the ability to test alternators and batteries. When testing the alternator ideally it should be tested under full load conditions, but a simple "how much voltage does it put out" can give a rough estimate if its working or not.

I've always heard that it's not good to do that if you have a computer.

Removing the cable with the engine running is a bad practice for several reasons. The simplest test is to measure the voltage of the battery with the engine off. If the battery has been sitting for a few minutes with the engine off, the voltage will be 12.6 or so. Then start the car and measure the voltage again. As you rev the engine to a comfortable speed above idle (i.e., the engine doesn't need to be screaming) the voltage will go to 14.5 or so, and level off. If so, the alternator is probably working fine. If the voltage stays at 12.6, the alternator is not functioning. If the voltage is 13.5 or so, then you'll need to check the specs for your car to see what the voltage should be under various conditions.

Good idea! Generally works for "quick check" but you should also note that after checking the Batt V (after starting) you should also "load" the system by turning on lights ,etc and comparing the V at the Batt to see if the altenator is "keeping up" with the load. This will generally tell you what your next step should be.

Donzi386

please do not remove cables from car while it is running. voltage spikes will very ofen damage diodes and the computer in modern cars. have it tested by someone who knows what they are doing. a volt meter will tell if the battery is getting charged. 99 percent of cars should read 14.3 volts with motor running and a little over 12 volts with the engine off. this is normal but there are other things that will show low voltage and you may still have a good alernator/ regulator. make friends with a good mechanic. how many miles are on the car? 100,000- 130,000 miles is a common amount for the alternator to die. also if the car has been jump-started many times or has been used to jump start other cars you may have hurt the internals of the charging system. check for loose belts and or soft tensioners on the belts. good luck.

I agree with all the posters that told you NOT to remove cables while the engine is running. In fact, if you are not 110% sure what you are doing, do not remove any when the engine is stopped either.....

A good digital voltmeter, if connected across the battery (assuming battery is good), will demonstrate if the alternator is good or not.

Observe battery voltage engine stopped.

Start your engine.

Battery voltage should slowly climb, increasing engine revs to around 2000 should speed this rate up.

Once battery has reached around 14.4 volts, you can say that the alternator is charging.

If the headlights are switched on and the 2000 rpm maintained, battery voltage should not drop.......if voltage starts dropping, probably a diode is defective in the alternator.....

If battery never reaches 14.4 volts, or only very slowly, then alternator is probably defective.....

Also:-

Check all cables carefully for corrosion and for hot connections when (or just after) starting the motor, this implies a bad connection if one gets hot - warning, you may burn yourself, wet finger and only a short tap on each connection to see if warm or not......

Best of luck.

One quick check I do for alternators, is to put a screwdriver blade near the bearing cap (undriven end) of the alternator. If magnetism is felt, then the field is on, no magnetism, then there is no field. I have found that most alternator problems have to do with the lack of a field generated. Usually, this is caused by a faulty regulator or bad brushes. Occasinally a diode will go, this usually shows up in a continually low charged battery. The screwdriver check is a quick and easy thing to do, especially when the alternator is buried.

Vic

Like it, I have to try that out sometime....

There was discussion of alternator problems in link below. In case I haven't attached it properly, it was started on 29 June 08, by batzee, in the Electrical Eng. section.

http://cr4.globalspec.com/thread/36784#newcomments

Cheers.........Codey

Seems I copied the wrong shortcut! Try this one

http://cr4.globalspec.com/thread/23394/Alternator-does-not-devolep-any-voltage

Codey

Many autoparts stores will check the alternator with it still in the car.

Careful with the "pull the lead" test. BOOM (PPE required) The test is correct but a good battery tester with a load meter will do the job. EVERY auto parts store will test it for you......it is on the car isn't it?