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When you check the electrical system to see why a vehicle doesn't start there are several tests to perform that can be done with a regular multimeter, or VOM, as some know them.

Unless you pull the battery clamps and give them a good cleaning the
first step is to test the connections between the battery and the cables. This needs to be done static, with no load on the system, and loaded.

Place the red VOM lead on the + battery CLAMP, and the black lead on the - battery CLAMP. Read the voltage with the key "OFF", "ON", and cranking. If the reading is close to 12.5 with the key off it does NOT mean the battery is definitely good. There are more tests to be done before that answer is known. Turning the key to "ON" should drop the reading no more than 1 volt, and less is better. A reading drop from key "OFF" to key "ON" of more than 1 volt at this point is noteworthy, and warrants checking the battery to clamp connections again. .

Next step is to disconnect the distributor or coil. You need to be able to crank the engine for a moment in order to check the system further. Cranking the engine after disabling the ignition should not produce a reading below 9-10 volts if the starter system and battery are healthy. If the voltage doesn't drop much at all, but doesn't crank, it's because your starter isn't working for one or more of several possible reasons, so it can't draw down the battery. It doesn't mean the starter needs replacing yet!

If it drops below about 8 volts, you could have a bad starter, or a bad battery.
If the reading drops to almost zero and it basically quits cranking, place the VOM leads on the battery POSTS, red on +, black on -, same as before. Crank again and watch the reading. If the reading stays in the 8v, OR ABOVE range, double check the connections between the battery and clamps, cable to starter connection, and battery cable grounds at the engine and chassis.
If the reading drops to almost zero volts after clearing any connection problems, your battery is defective.

To check the charging system you first need a base voltage reading. Again, a good battery fully charged should read close to 12.5 volts, key "OFF". A properly charged battery with the engine running at higher than an idle should read 13 to 13.5 volts. If the battery voltage is low before you start the car, the alternator very well may be trying to charge it, and under charging conditions 14-14.5 volts is common, maybe more. Remember that if you just started the car you drew some of the power from the battery so it will be trying to recharge that draw for a short time, longer if longer cranking drew out more power.
If the voltage does not increase after starting the car, and actually goes down it doesn't necessarily mean the alternator is bad. (By the way, if there is no charge coming into the battery with the engine running, it should have a lower running voltage than "OFF" voltage.) The wire between the battery and the alt. may be bad, maybe a fuse is bad, maybe you didn't see that the belt is laying 5 miles back on the road. And lots more possible reasons.

One common method to check the charging system came from the old mechanical regulator days, when there were no diodes in the systems to burn out. That method being pulling the positive battery cable off the battery while the cars running to see if it dies. If it dies that means the alternator isn't working. If you're a gambler, go ahead and give this a try. You'll be betting the voltage spike from a charging alternator doesn't blow out the working regulator in our modern systems. I recommend you do NOT try this. LOTS of people get away with it. My money is on the safer test route, thank you.
 

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great knowledge. particularly useful in my current situation. thanks, loren.

why hasn't anyone commented or added to this awesomeness thus far?
 

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Thanks man! I had to try the old pulling the positive battery cable thing off a couple months ago. Luckily it didn't screw anything up, but my car wouldn't start after sitting for a while and a mechanic told me that it was the alternator. I tried pulling the cable and the car didn't die. I replaced the battery and haven't had any problems since. I know where NOT to take my car to now. :angry:
 

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from battery experience pov, using a vom usually checks on just the full state of the battery and not the cells that make up the battery...an actual battery tester (from midtronics or equivalent) will give you the voltage and conductance (how well it takes the charge) of the battery...also, wiring could also affect the performance of the battery as it will act like a load...make sure that there is good connection on the battery post...

i don't think there is a regulator on the alternator and i think that is why it the charging voltage is too high which a lot of batteries don't like...if it mentioned that it was temperature based then i would've bought the explanation...but then again i haven't dissected an alternator...


cheers...
 
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