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Discussion Starter · #1 ·
Finally put my C900 on the road this weekend. We had changed out the engine from a turbo to an NA.

I installed a new battery late last fall/early winter and used it quite a bit testing lights, radio, top, etc. Always kept it on a charge.

I arrived home after about a 7 mile test run and went to restart and the battery was flat. Charged it and I tested at the alternator and I am putting out 13.7 volts there and at the battery. It seems to be weak turning over after running awhile.

Is 13.7 the right number? Could I just have a defective battery? Am trying to discern if possibly it is difficult to start after running awhile, but haven't gotten to that part. It seems to be running cool enough.

Thoughts??
 

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Anywhere between 13.5 and 14 volts is OK from the alternator. If you still have a flat battery after a run I would suggest the fault lies elsewhere. My first guess would be failing battery.
 

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13.7 is a bit low, 14-14.2 (some are running even higher). The newer battery chemistry that is used in todays low or no maintance batterys needs a slightly high voltage setting from the old type.
That being said, chargeing voltage alone doesn't tell you much. You need to know how much current it is, and is capable of, produceing.
Volt reading alone is next to the charge light in useless info. you really need to know volts and amps to have any idea on how things are doing.
 

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Discussion Starter · #4 ·
13.7 is a bit low, 14-14.2 (some are running even higher). The newer battery chemistry that is used in todays low or no maintance batterys needs a slightly high voltage setting from the old type.
That being said, chargeing voltage alone doesn't tell you much. You need to know how much current it is, and is capable of, produceing.
Volt reading alone is next to the charge light in useless info. you really need to know volts and amps to have any idea on how things are doing.
So how do I test for the current?
 

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Best way is with a chargeing system tester. With that you can load the system and see what it can and will put out. Barring that, you'll need to splice a ammeter (either direct reading or shunt type) into the charge wire to see how much current it is chargeing. A battery that sat on a charger all winter might be fried.
It is almost impossable to fully test a chargeing system on or off a car with the tools most have. The system need to be placed under a load to test the output current and voltage. Most do not even have a way to measure the current, let alone place a load on the system to test max output. For this reason I recomend takeing to someone who does have the tools and knowedge to test it correctly. With that info, you can either have them do the repair or do it yourself.
a while back Jim posted a suggestion for those without the tools. Turn everything on (lights, AC blower etc), and rasie the RPM to 2800-3000 and measure the voltage. It should be in the 13.8- to 14.4 range.
 

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So how do I test for the current?
Poor man's or field test method:
1) place a small wedge under the throttle stop to bring rpm's up to 3000.
2) measure voltage at the battery.
3) create load by turning on high beams, defrost, fan on high, a/c if U have it.
4) measure voltage at the batter.
5) compare voltage readings from step 2 & step 4.

They should be nearly the same if your charging systems in good shape.
If voltage drops off under load then you have a charging problem.

Check your alternators ground wire--by check I mean: remove, (NEVER while car is running!) clean, reinstall.
 

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Poor man's or field test method:
1) place a small wedge under the throttle stop to bring rpm's up to 3000.
2) measure voltage at the battery.
3) create load by turning on high beams, defrost, fan on high, a/c if U have it.
4) measure voltage at the batter.
5) compare voltage readings from step 2 & step 4.

They should be nearly the same if your charging systems in good shape.
If voltage drops off under load then you have a charging problem.

Check your alternators ground wire--by check I mean: remove, (NEVER while car is running!) clean, reinstall.
I endorse that as a rough & ready test, but it must be performed with a fully charged battery; then you need to load test the battery.
Your set of symptoms sound like a failed battery. Can you swap another one temporarily.
If you do a voltage drop test (search), you can identify any bad connections in the whole circuit; both positive and ground sides.
 

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Discussion Starter · #8 ·
Just took the battery to NAPA and they determined it had a bad cell. Gave me a new one - hopefully that's the end of the story.
 
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