Alternator & ampmeter are fine...NOW WHAT?

Texas Tom

Texas Tom

Senior Member
Yesterday I posted a question about why my ampmeter would not 'excite' when I started the engine and revved it up. Several folks suggested my alternator is likely the problem.

This morning I tested the alternator with a voltmeter and found that when I started the engine, the voltage dropped. This usually indicates a faulty alternator. So I pulled a spare (new) alternator from my parts pile and installed it. Same result!

So I took both alternators to my NAPA store and asked them to test them. Test result: both alternators work just fine. Yikes!

So I'm back to square-one. There is nothing wrong with my alternator, and apparently nothing wrong with my ampmeter (I tried three ampmeters, which yielded identical results: very slight discharge).

So why is my system (12 volts) discharging when the engine is running? I have no clue. Everything worked fine before I took the car out yesterday to give it some exercise.

HELP!
 
Are you matching voltage and ground polarity of the alternator and battery?

Is the car wired as original, or have extra wires, fuses and switches been installed?
 
Alternator is one-wire GM alternator.

Key point: Before yesterday, everything was just fine with the electrical system. Don't know why things changed.

Could the coil have a role in this mystery?
 
I just worked on a friends A. 12v neg ground one wire alt. it quit charging. checked all wiring and connections. the pos alt terminal should run to the pos post of the battery with 10 ga wire, at a little over 1000 rpm it should excite. we took the alt to napa and it tested good. hooked it back up no charge but after three or four starts it started charging (used volt meter at alt output). then quit again when restarted. went back to napa bought a rebuilt one wire for $50 problem solved. if everything was working and it just quit I'd bet on bad alternator
 
armchair67 said:
Could it be something as simple as a bad battery?
Click to expand...

yes it could be and can also be a ground issue at the frame..or another connection

tex are you using an analog v meter.
whats it reading at the alt with a high idle

how do you have it wired from what to what
 
if the motor cranks without dragging I would suspect the battery and grounds are good, the alt uses the same ground. disconnect any wires at the back of the alt and run a wire from the alt post to the pos battery post, it should charge once you rev'er up. theres nothing else in the one wire circuit. there should be no spark at the alt terminal when hooking the battery wire to it.
 
Texas Tom said:
Yesterday I posted a question about why my ampmeter would not 'excite' when I started the engine and revved it up. Several folks suggested my alternator is likely the problem.

This morning I tested the alternator with a voltmeter and found that when I started the engine, the voltage dropped. This usually indicates a faulty alternator. So I pulled a spare (new) alternator from my parts pile and installed it. Same result!

So I took both alternators to my NAPA store and asked them to test them. Test result: both alternators work just fine. Yikes!

So I'm back to square-one. There is nothing wrong with my alternator, and apparently nothing wrong with my ampmeter (I tried three ampmeters, which yielded identical results: very slight discharge).




So why is my system (12 volts) discharging when the engine is running? I have no clue. Everything worked fine before I took the car out yesterday to give it some exercise.

HELP!
Click to expand...



Is the alternator output wired through the ammeter so it will register or is the
output wired to the starter/hot post bypassing the ammeter?
Bill
 
Have the battery checked. I once had one that acted exactly like yours and after 3 alternators I checked the battery and found a bad cell that wouldn't charge. The battery would start the car for a while but not charge. replaced the battery and all was well.
 
Batteries can do some UNEXPLAINED tricks, at times! Had several Lucas batteries that would reverse the polarity on 1 cell, under a HEAVY load test! That would cause the generator NOT to charge!
Bill W.
 
James, you may have solved the riddle. A bad battery is the only thing I can think of that would explain why the electrical system was just fine, and then it wasn't. I'll check it out. Thanks.
 
UPDATE

Well, I don't know whether my battery is the problem or not. I pulled it out today and was ready to test each cell with a hydrometer. Regrettably, the battery is 'maintenance free' and sealed. So there's no access to the cells.

I bought it at CarQuest 15 months ago, and it's the "Premium" line of batteries. I took it to CarQuest and explained my dilemma. They tested it on their battery tester and concluded that it was just fine.

Tomorrow I'll install another 12V battery that I have and see if it discharges when I start the engine (as happens with my CarQuest battery).

If the second battery performs as the CarQuest battery does, I'm back to square-one with zero ideas about where to look next....
 
Texas Tom said:
UPDATE

Well, I don't know whether my battery is the problem or not. I pulled it out today and was ready to test each cell with a hydrometer. Regrettably, the battery is 'maintenance free' and sealed. So there's no access to the cells.

I bought it at CarQuest 15 months ago, and it's the "Premium" line of batteries. I took it to CarQuest and explained my dilemma. They tested it on their battery tester and concluded that it was just fine.

Tomorrow I'll install another 12V battery that I have and see if it discharges when I start the engine (as happens with my CarQuest battery).

If the second battery performs as the CarQuest battery does, I'm back to square-one with zero ideas about where to look next....
Click to expand...


In post # 11 I asked how the ammeter is wired. The output from Bat. post on the alternator must pass through the ammeter and then to the battery to register. If it is not wired this way your ammeter will not show a charge even though the system is charging.
Hook a voltmeter to the battery, it should read 12 volts then start the engine and rev it up a little,if it shows an increase the system is charging but not showing on the ammeter. This would be the first test they would perform if you took it to a shop. Its called " Troubleshooting".
Bill
 
In answer to questions:

-All electrical components (horn, lights) work just fine.
-System is wired per diagrams for 12V systems found in various Model A publications.
-Battery shows 12.6 volts on analog voltmeter prior to starting engine.
-When engine is started, voltage on voltmeter drops to 10.5V or so.
-NAPA store says both my alternators (1-wire) are fine.
-CarQuest store says my 12V battery (which is sealed and is 15 months old) is fine.

Tomorrow I'll try an alternate battery and will report results.

Thanks to all for your assistance.
 
Go to Harbor Freight and pick up a LOAD TESTER for $20 to test the battery. A Hydrometer will tell you whether each cell has a charge, but a bad cell can still take a "Surface Charge" and fool a hydrometer.
A load tester will put a LOAD on the battery and you can watch the needle either hold good in the green, or go to red.
 
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