Ammeter Question for you Electrical Gurus

[lookin-backtexas]

A little background info: I am running an alternator on my Model A and converted it to a 12-volt system several years ago. I have really liked the halogen headlights and the ability to drive at night and actually see where I'm going; the lights are as bright as any modern car - with my original 6-volt headlight bulbs I almost had to light a match to see if they were actually on.

In the past when I first started the car the ammeter would show +10 amps for a few seconds and then move to the "0" point and stay there while I was driving, and wouldn't even show a discharge if the lights were on or if I blew the horn.

While touring with my club yesterday I noticed that the ammeter showed +10 amps and would never return to the "0" point. I even tried turning on my headlights in the middle of the day to try and put some "drain" on the system to get it back to "0" and it just seemed stuck on the +10 level.

One of the other guys on the tour thought that perhaps a diode in the alternator might be about to go bad.

Thoughts or suggestions?

Thanks,
Fred

[BillLee/Chandler, TX]

Once had similar problem. Was a bad battery.

[lookin-backtexas]

Bill:
Don't think it is a bad battery - installed a new one November 19, 2012.
Fred

[MikeK]

The battery may have needed that charge rate, as it may have been 1/2 down. Since you can't readily open a modern 12V for a hydrometer test there is no easy way of knowing that, or if all cells are even.

Put the battery on a 10amp or larger charger and charge it several hours. Then start it up and see if you still show the same 10 charge rate on your ammeter. If the ammeter is now less, or normal, you need to look for the parasitic discharge reason. If not, the problem is in either the battery or alternator.

Parasitic Discharge:
With the engine off, and after several hours cool-down, disconnect the battery post ground clamp and put a meter set for milliamps in between the battery ground post and the disconnected lead. You are checking for parasitic draw. It should be 10ma or less if everything is healthy. If it is 50ma a typical 12V car battery will be 1/2 down in a few weeks. If more than that you have a problem. Model A dash lights are often left ON, not visible in the daylight. Check that first. Selectively disconnect branches of the wiring to find the drain source: Leads to lights, horn, ignition, alternator itself, etc.

Alternator:
Running at higher than idle speed, ~ 2000 rpm, measure the voltage between the alt. output stud and the alternator aluminum case. This should never, never, exceed 14.8V. Normal on a warm day should be 14.2V or less. Turning large electrical loads on/off should not make that reading move more than 0.1 or 0.2V measured at the alternator. (downstream in the wiring there are some losses)

Just a start, hope it helps.

[JBill]

You said it seemed "stuck" on +10. If it's still stuck there after the ignition is turned off then it's a bad ammeter!!

[steve s]

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Add to Mike's list of potential parasites an electric temperature gauge.

[lookin-backtexas]

Originally Posted by JBill

You said it seemed "stuck" on +10. If it's still stuck there after the ignition is turned off then it's a bad ammeter!!

The ammeter returns to "0" when the engine is turned off and "dances" like it is supposed to when you first hit the starter button so I don't suspect a bad ammeter.
Fred

[lookin-backtexas]

Originally Posted by steve s

Add to Mike's list of potential parasites an electric temperature gauge.

There is no electric temperature gauge installed.
Fred

[lookin-backtexas]

"Model A dash lights are often left ON, not visible in the daylight. Check that first."
MikeK: The dash light can be eliminated - I just checked it and it's definitely off; the ammeter still registers +8 while the engine is running at a fast idle as well as when I rev it up quite a bit. The trip yesterday was a little over 125 miles and the car was running at 45 mph the entire trip yet the ammeter never got back to "0". I just started the car and let it warm up and we're still +8 unless you retard the spark to a super slow idle - when it "Cadillacs" (ta-pucketa-pucketa-pucketa-pucketa) the ammeter bounces between -2 and +2 . . . give it some gas and we're back to +8.

As I mentioned above, the new 12-volt battery was bought and installed less than 90 days ago and my normal SOP is to pull the fuse anytime I park the car so I doubt there's anything that would be drawing down the battery while it's sitting unused in the garage.

I'll follow through with your other suggested diagnostic items tomorrow and give you an update on what I find.

Thanks for the suggestions.
Fred

[Tom Wesenberg]

As Mike said, check the voltage output at the alt. stud. It might have a bad regulator.

[P.S.]

Sure sounds like a bad cell in the new battery to me.

[hardtimes]

Hey Fred,
'90 day old 'new' battery'.....I bought TWO high end batteries, one for my truck and one for RV...both bad and found out to be bad within week. Moral to that is ...like other auto parts , bat can be bad ...new! Load test it if any doubt. Placeing HEAVY amp draw will reveal any flaw.
I second Toms sentiment...bad or going that way...regulator. Let us know

[steve s]

Originally Posted by MikeK

...



Alternator:
Running at higher than idle speed, ~ 2000 rpm, measure the voltage between the alt. output stud and the alternator aluminum case. This should never, never, exceed 14.8V. Normal on a warm day should be 14.2V or less. Turning large electrical loads on/off should not make that reading move more than 0.1 or 0.2V measured at the alternator. (downstream in the wiring there are some losses)

Just a start, hope it helps.

Mike,
Are those voltages the same no matter where the senser lug is connected to?

Thanks.
Steve

[Mike V. Florida]

Originally Posted by P.S.

Sure sounds like a bad cell in the new battery to me.

Me too

[MikeK]

Originally Posted by steve s

Mike,
Are those voltages the same no matter where the senser lug is connected to?

Thanks.
Steve

I have made the assumption () that this is a one-wire alternator, having the regulator sense input connected directly to the output stud. If it is a remote sense installation the voltage at the output lug will vary as the regulator adjusts the output to "see" the set point at the remote connection point. I personally would never use a one-wire, but that's a different issue not for this thread.

I will not offer any "I think it is the . . " guess. I like to go on hard data from tests. A battery load test is a good tool for an engine starting battery if you carefully look at the voltage breakdown peak numbers and slope as well as an age weighted actual output vs. rated ratio during the test. Just looking at the pretty red and green or good/bad zones for a particular size battery on the load tester gauge takes neither into account.

[takomaned]

Rather than guessing what may be the cause, I recommend you get some actual data so that we may make more accurate guesses. If you're able to drive it to a nearby shop, a volt / amp tester will give you the specifics. Since it is 12v. this should be easy. I presume you don't have a clamp-on amp probe