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5/12/13 2:35pm - Original Message: 'Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

Vette(s):
1969 Daytona Yellow. 350 / Automatic.

Joined: 10/3/2010
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68 Silver Toy said: Can someone help me on this. My gauge on the car is telling me there is a discharge. Could it be a ground problem or alternator? When I put anything on like the heat fan or headlights on it shows discharge more. Thanks

My '69 is doing the same thing.  Always points to the left.  Am I running off the battery all the time?

Jim
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5/12/13 6:59pm - Reply: 'Re: Ammeter is showing negative all the time.'
dyoes
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Zachary, LA - USA

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1974 L-48 Auto. Just your basic Vette. GM crate motor w/vortec heads and intake. Crane cam. Scorpion rockers, Edelbrock carb. Lots of parts still in boxes... :(

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Does the meter return to zero (straight up) when you turn off the car and turn everything off?

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5/12/13 10:52pm - Reply: 'Re: Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

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1969 Daytona Yellow. 350 / Automatic.

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dyoes said: Does the meter return to zero (straight up) when you turn off the car and turn everything off?
Yes, it always returns to zero when I shut her down.




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5/13/13 12:36am - Reply: 'Re: Ammeter is showing negative all the time.'
rraider1
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Woodland, WA - USA

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1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

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It seems to me it is indicating you are running on the battery which means your generator is not putting out (by it some wine)



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5/13/13 7:32am - Reply: 'Re: Ammeter is showing negative all the time.'
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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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An amp gauge that shows a discharge all the time(while running) indicates the alternator is not putting out enough juice to carry the load. This could be because the alternator is bad, or not rated high enough amp-wise. It could also mean a bad/loose connection. I would first determine how much voltage is at the battery terminals while running, and compare that to the voltage at the alternator while running. If both readings are low(at or below 12v), I would suspect the alternator.

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Joel Adams
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5/13/13 9:31pm - Reply: 'Re: Ammeter is showing negative all the time.'
dyoes
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Zachary, LA - USA

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1974 L-48 Auto. Just your basic Vette. GM crate motor w/vortec heads and intake. Crane cam. Scorpion rockers, Edelbrock carb. Lots of parts still in boxes... :(

Joined: 1/27/2013
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The ammeter is essentially a voltmeter... Compares voltage at the starter connection and at the horn relay.  Higher voltage at the starter shows (+) amps, and vice versa.

Adam is right on.  What you can also do is connect your voltmeter at the battery with the car shut off (ignition on), note the voltage, then start the car and note the voltage again.  If the voltage goes up, it's charging and the ammeter may be bad.  If it doesn't change or drops a bit, you're not charging. 

In the "old days" people would start the car and disconnect the battery.  If the car died, it wasn't charging.  I was told that with modern cars (not that C3s are modern, mind you!) pulling the battery cable while the car is running could cause voltage regulator failure.  Anyone know if that's true?


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5/13/13 10:30pm - Reply: 'Re: Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

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1969 Daytona Yellow. 350 / Automatic.

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Looks like its more electrical work for me.  Thanks to all for confirming.  I went out to disconnect the battery earlier and thought I'd start the car but we ran out of juice before it kicked over.  I'll check out loose wires as soon as it cools off out there.

If I check the voltage at the battery, won't I always get the voltage in the battery?

Jim


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5/13/13 10:36pm - Reply: 'Re: Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

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1969 Daytona Yellow. 350 / Automatic.

Joined: 10/3/2010
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Please ring in on this.

If I need to replace the alternator, is there any problem with kicking it up from 63 amps to say 100 amps???


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5/13/13 10:50pm - Reply: 'Re: Ammeter is showing negative all the time.'
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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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Checking the voltage AT the battery without it running will only tell you how much voltage the battery is storing at that time. What you are interested in is, is there  MORE than 12v there with it running? You should see at least 14v with the engine running, and no lights, or A/C, etc. on. Then check voltage AT the alternator, and see if there is MORE at the alt than there is at the battery. If there is, then you prolly have a bad connection somewhere. If the voltage is the same, your wiring is ok. Either way, if you do not see a minimum of 14v AT the alternator, engine running at around 1500 rpm, you have an alternator issue.

And...while it's true, we used to "test" alternators by  disconnecting the battery, it's not a good idea....it can definitely mess up a regulator, and/or the diode trio inside the alternator. Plus, that test doesn't really tell you how much the alternator is putting out....just that it is.
 It's best to use a volt meter. Thumbs Up


Oh...and no...if you want to up the ante with a higher amp alternator, there's no problem with that. 100 amp might be overkill, tho....


|UPDATED|5/13/2013 7:50:15 PM (AZT)|/UPDATED|


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Joel Adams
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5/16/13 7:06pm - Reply: 'Re: Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

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1969 Daytona Yellow. 350 / Automatic.

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Here's the scoop.  12 volts at the battery with engine off.  13 volts while running and 14 volts at the alternator while running.  But the battery meter shows -30 amps and goes lower if I gun the engine.

I guess I'm in the market for an ammeter, as soon as I figure how to remove the old one and how to test it for sure.

Yes, I'm troubleshooting two problems at the same time, horn and ammeter.  Don't think they're related.

Jim


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5/16/13 10:13pm - Reply: 'Re: Ammeter is showing negative all the time.'
dyoes
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Zachary, LA - USA

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1974 L-48 Auto. Just your basic Vette. GM crate motor w/vortec heads and intake. Crane cam. Scorpion rockers, Edelbrock carb. Lots of parts still in boxes... :(

Joined: 1/27/2013
Posts: 201

Yes, I'm troubleshooting two problems at the same time, horn and ammeter.  Don't think they're related.



Maybe they are...

The ammeter is connected to the horn relay and the starter lug (main harness).  (Black, and black/white wire if I remember.)  It measures a voltage between the two leads and infers a current from the voltage differential.  If you've got higher voltage at the alternator and battery when the car is running, it's charging.  So there's something in common.
 
Remove and clean the terminations to the horn relay, and check that the lugs are securely on the wire ends.

Adam, what do you think? 


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5/16/13 11:08pm - Reply: 'Re: Ammeter is showing negative all the time.'
Adams' Apple Lifetime Member
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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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We need to clarify the difference between an AMP meter, and VOLT meter. An AMP meter measures Current, while a voltmeter measures voltage...two different things, when discussing meters.
An amp meter runs In Line with the charge...meaning the meter is wired in series with the wiring from the alternator to the battery.
A voltmeter, on the other hand, simply measures voltage, so it is wired with a positive wire on one terminal, and a ground on the other terminal.
A bad Amp meter can certainly drop system voltage after the alternator...if the current going thru it is actually being used up by the gauge, it will lower the available voltage at the battery. An amp meter can also not allow voltage thru it if it goes open, in which case the meter(gauge) would not move at all.
I would certainly clean all of the connections, both at the horn relay, and the alternator. If no improvements are seen, then I would have to suspect a problem with the gauge itself....possibly the resistor/shunt bar on the back of the gauge.
In any case, it appears the charging system IS working, it's just not able to get the full output voltage back to the battery.


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Joel Adams
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5/17/13 2:28pm - Reply: 'Re: Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

Vette(s):
1969 Daytona Yellow. 350 / Automatic.

Joined: 10/3/2010
Posts: 361
Thanks everyone.  And Joel for the difference between the amp and volt meter.  I've ordered a new horn relay but will hold off on the ammeter guage to see if the connections or the horn relay itself were causing my ammeter problem.

There is excessive electrical tape on the wires at the terminals both at the relay and the alternator.  Hate to open all that up but, I'll do what I have to.


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5/17/13 3:47pm - Reply: 'Re: Ammeter is showing negative all the time.'
dskopp
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Oak Creek, WI - USA

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1981 Great White Shark. Red Interior, 350/190hp. PS, PB (SS), A/C CC, T-Tops, Going to remain as Stock as possible. Served three years in Active Duty Army, then Retired Air Force after 34 years! Badger State Vetts Car Club. 175,000 Original miles!!

Joined: 5/21/2008
Posts: 1956
rraider1 said:

It seems to me it is indicating you are running on the battery which means your generator is not putting out (by it some wine)




I had a similar problem with my 81.  Got new Alternator
, problem solved!!!!
 
"I never Drink...........Wine"
 
DanLOL


|UPDATED|5/17/2013 12:47:00 PM (AZT)|/UPDATED|


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5/17/13 4:37pm - Reply: 'Re: Ammeter is showing negative all the time.'
rraider1
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Woodland, WA - USA

Vette(s):
1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152
Something seams strange if the amp meter goes more negative as you race the engine but measured voltage goes up it seems the amp meter is hooked up backwards. when you go to put back in the new horn relay double ck the wiring hook up. and I think you should inspect what is under the tape you could have corrosion their also. With the car not running and you turn on the headlights does the meter go negative (discharge).

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Bob R
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5/17/13 9:44pm - Reply: 'Re: Ammeter is showing negative all the time.'
eldredjames Lifetime Member
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Temple City, CA - USA

Vette(s):
1969 Daytona Yellow. 350 / Automatic.

Joined: 10/3/2010
Posts: 361
rraider1 said: Something seams strange if the amp meter goes more negative as you race the engine but measured voltage goes up it seems the amp meter is hooked up backwards. when you go to put back in the new horn relay double ck the wiring hook up. and I think you should inspect what is under the tape you could have corrosion their also. With the car not running and you turn on the headlights does the meter go negative (discharge).
My new horn relay is due on Monday.  In the meantime I'll check out the wiring.  I like to solder everything that's going to be covered by tape.  Oh Me Oh My Oh!  Son of a gun I'm having fun on the Vette-o!!




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5/17/13 10:51pm - Reply: 'Re: Ammeter is showing negative all the time.'
dyoes
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Zachary, LA - USA

Vette(s):
1974 L-48 Auto. Just your basic Vette. GM crate motor w/vortec heads and intake. Crane cam. Scorpion rockers, Edelbrock carb. Lots of parts still in boxes... :(

Joined: 1/27/2013
Posts: 201
Adams' Apple said: We need to clarify the difference between an AMP meter, and VOLT meter. An AMP meter measures Current, while a voltmeter measures voltage...two different things, when discussing meters.



True.  Except on our cars, the ammeter is not a true ammeter.  It's a DC shunt ammeter.  It carries only a tiny fraction of the total current going through the car's circuit, and approximates current based on differential voltage from two points.  The resistor between the two points causes a voltage drop which deflects the meter one way or another...



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5/18/13 1:18am - Reply: 'Re: Ammeter is showing negative all the time.'
rraider1
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Woodland, WA - USA

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1977 T-Top,350,Auto Black Ruby Pearl,Steeroids R&P conversion

Joined: 10/14/2003
Posts: 1152

Ammeter.  Refer to the below figure.


Troubleshooting the ammeter circuit can be difficult.  The following applies to cars equiped with alternators.  In the Chassis Service Manual there will be a simplified diagram of the charging system similar in part to that show above.   

 

Possibly the biggest mistake that people make associated with the ammeter circuit is not turning the ignition switch to either ACC or ON when trying to decide if the ammeter is showing proper discharge.  You can perform the following test and see if it makes a diference.  Start the engine and let it run for a few minutes until the ammeter reads close to zero.  Shut off the engine and with the ignition switch OFF, turn on the high beam lights.  Make a note of the ammeter reading. Turn off the headlights.  Turn the ignition switch to ACC and again turn on the high beams.  The amount of discharge shown by the ammeter should be about twice, or more, what it was before.

 

By referring to the above diagram, it can be seen that the ammeter reads the difference in voltage between the battery and the alternator.  The wire that goes directly from the battery to the alternator and appears to short out the ammeter is a meter shunt.  What the ammeter actually reads is the voltage drop across this wire.  The resistance of this wire is about 0.1 ohm.  There is no physical resistor in the circuit, the resistance is due to the length of the wire required to get from the starter solenoid to the horn relay around the engine bay.

 

The horn relay is not in the ammeter circuit but is a convenient connection point for the alternator output and the voltage regulator. 

 

When the output voltage of the alternator is greater than battery voltage, the ammeter shows a charge proportional to the difference in voltage.  When the battery comes up to full charge, the ammeter drops to near zero.  If the output of the alternator is less than the battery voltage, as under a heavy load, the ammeter will show a discharge proportional to the difference in voltage. 

 

The voltage regulator is set to have a voltage slightly higher than the fully charged voltage of the battery so that the alternator will carry most of the vehicle loads and maintain a slight trickle charge on the battery at all times the engine is running.  Under normal conditions, the ammeter will show a slight charge when the engine has been running for a few minutes and the system is working correctly.



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5/19/13 11:30am - Reply: 'Re: Ammeter is showing negative all the time.'
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Duncanville, TX - USA

Vette(s):
#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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I'm shocked....LOL
Just kiddin....this has turned into a very detailed discussion, and very informative, I'm sure. Remember, we're working on an early car here.
My point being that there is a loss of charge making it back to the battery somewhere...only a few things can cause that. Bad connections/wiring, or a problem with the gauge itself. The horn relay has nothing to do with the charging system, other than being a common junction box to connect the wiring to. Of course, if the terminals on the relay are corroded, it could create a problem.
The alternator in this case IS working, and the battery is being charged, although perhaps not completely, it seems. The issue is the gauge showing a discharge all the time....even tho there IS a charge getting to the battery.


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Joel Adams
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5/20/13 12:55am - Reply: 'Re: Ammeter is showing negative all the time.'
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I'd Like to thank everyone for their feedback. It helps me to solve my electrical problem. I had someone check the alternator before and they said it was fine. Another person said it might be the voltage regulator. Just like the person with the '69, my battery is draining. So I'm going to have to change the alternator.  Thumbs Up

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