[usaweapon777]

Hey guys, I have a friend who needs some help with his car and would like some secondary opinions. He called me over telling me that the battery was dead and that it needed an alternator. He had replaced the battery recently so we knew it wasn't the battery. While the car was running I read 12 volts off of the battery. I thought that if the car is running I should see slightly more than 12 volts. Do you need to read it straight off of the alternator? I pulled the alternator and took it to advance, They tested it and said the alternator was fine. So we put it back in the car and jumped it and it seemed to run fine on its own.

Since the alternator tested good and seems to be working I started looking elsewhere for something that could be draining the battery. However, I think other issues are causing his problem. He failed to tell me that it takes him on average at least 20-30 seconds of cranking for the car to start. So I'm figuring that he's loosing his prime to the carb from probably cracked fuel lines or something else?

Also when we went to shut off the car after jumping it, it wouldn't shut off. The car went from a smooth idle to coughing and sputtering after turning back the key but it wouldn't die until i choked it out. What would allow power to the ignition even with the key off?

Thanks for any advice.

[oldbogie]

Originally Posted by usaweapon777

Hey guys, I have a friend who needs some help with his car and would like some secondary opinions. He called me over telling me that the battery was dead and that it needed an alternator. He had replaced the battery recently so we knew it wasn't the battery. While the car was running I read 12 volts off of the battery. I thought that if the car is running I should see slightly more than 12 volts. Do you need to read it straight off of the alternator? I pulled the alternator and took it to advance, They tested it and said the alternator was fine. So we put it back in the car and jumped it and it seemed to run fine on its own.

Since the alternator tested good and seems to be working I started looking elsewhere for something that could be draining the battery. However, I think other issues are causing his problem. He failed to tell me that it takes him on average at least 20-30 seconds of cranking for the car to start. So I'm figuring that he's loosing his prime to the carb from probably cracked fuel lines or something else?

Also when we went to shut off the car after jumping it, it wouldn't shut off. The car went from a smooth idle to coughing and sputtering after turning back the key but it wouldn't die until i choked it out. What would allow power to the ignition even with the key off?

Thanks for any advice.

With the engine running propped up to 1000-1200 RPMs to be sure it's spinning fast enough to produce a charge. The line voltage must be above 12 in order to charge the battery. Typical would be 13 to 15 volts, less than 13 the battery won't stay charged and more than 15 sustained for a long time will overheat the battery resulting in boiling off the electrolyte to warping plates, an excursion to 16/17 is OK as long as it tapers off within a minute or two.

The ignition getting power with the key off is most likely the key switch is worn and not breaking contact with the run position when switched to off, assuming somebody hasn't messed with the wiring. But your description sounds like the engine is simply running on a common ailment of carbureted emission engines, there being something in one or more cylinders that's glowing hot and keeps firing any fresh mixture that comes its way. You'll have to figure this out as to which of these two symptoms is the one that keeps the engine going.

A short large enough to siphon off 3 or 4 volts when it's running would be a large leak. Typical of this is a battery going bad where the alternator can work till goes up in flame and never recharge the battery. Being new is not a guaranty that it's any good. The best test for a battery is a high load test, this times how long under a large discharge the battery can hold its nominal voltage. then how long does it take and how far does it fall.

Leakage can be tested with everything off by disconnecting the ground lead of the battery then placing a voltmeter between the ground terminal which is now the positive lead to the meter and the other lead to the ground cable. A voltage tells you something is on, modern cars will show some volts here because there are housekeeping functions in the computers that don't fully shut down.

Another test is back EMF leakage through the alternator's diodes, put the meter between the positive terminal of the battery with the cable disconnected and the large red lead from the alternator. Positive lead to terminal, negative lead to #10 wire from alternator. A voltage here tells you the alternator is leaking backward through the diodes to ground.

Bogie

[usaweapon777]

Thanks for the information.
Also your description of it firing because it is a heavy emissions engine seems probable since the key turned off in the ignition with no problem and the engine went from running fine to sputtering and shaking but still firing enough to keep it running.

[oldbogie]

Originally Posted by usaweapon777

Thanks for the information. Where is the proper area to test for voltage while the vehicle is running? After I put the alternator back and started the car I had him raise the idle and i still measured 12 volts after I was given back the alternator being told it was good.

Also your description of it firing because it is a heavy emissions engine seems probable since the key turned off in the ignition with no problem and the engine went from running fine to sputtering and shaking but still firing enough to keep it running.

Where ever you choose to test, the line voltage should show nearly the same from the battery's positive terminal to ground to the positive side of a tail light that's switched on. That's with the engine at a high idle so at that point the alternator should be carrying the entire electrical load. With all the lights, on, heater fan running, stereo on, the line voltage shouldn't be less than 13. If it drops around 12 to 12.4, that would show the battery is supporting everything and no voltage is left over to charge it.

The best place to test is at the battery from it's positive terminal to ground as this eliminates the vagrancy's of what can happen in the car's wiring harness.

Also, check the engine grounds, this completes the circuit for the starter, ignition, and alternator. A weak ground (missing/broken wires or corrosion of and in terminals) can mimic a weak alternator or battery. We usually think of electrical problems as being with the positive side of the system but grounds are needed to complete the circuit so they bear careful inspection as well.

Bogie

[T-bucket23]

Make sure to start with that the positive battery cable is connected to the back of the alternator. Next unplug the 2 prong plug and turn the key on. On the plug you should Have voltage on both terminals. The #1 may be less than battery voltage, probably 8-10 volts.
If that check out, check to be sure the engine is grounded.

[usaweapon777]

Figured I'd post the resolution to this thread. I went back to my buddy's and went over everything again while it was still light out. The battery was dead since he thought my trickle charger would somehow charge a dead battery in a few hours and unplugged it . So I jumped the car with my truck and rechecked the voltages the alternator started out good at 14 volts but after the car had warmed up, voltage dropped down to the 11-12 range. i had him load the alternator up and voltage dropped down to 8-9. I checked the plug connections and it was receiving power there so I said to myself that the Advance Auto Parts guy who tested it was simply wrong and the alternator was at fault. 15 minutes to drive to the parts store(NAPA mind you). 65 bucks for the alternator, and a half an hour later and the car was running like a champ. I let it idle for 15 mins after replacing the alternator and jumping it and she started on her own with no problem. I put the trickle charger on to fill up the battery good overnight. Thanks for the help and advice guys.