I had a similar problem with my 81.  Got new Alternator

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


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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!!

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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"Let them that don't want none have memories of not gettin' any."
- Brother Dave Gardner

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.

I'm shocked....
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
C3VR Lifetime Member #56    
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

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. 

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Installed the new horn relay, cleaned up the wires a little (sanded the crud off), and viola! the ammeter works correctly now.

Haven't got the horn working yet but I need to put the horn button back together.  I wonder, rhetorically, why they couldn't send two wires, through the column connector, up the horn column to complete the ground with a simple button.

 

Two wires = twice the number of wires, twice the time to assemble, twice the $$$$.....now ya know! The same horn contact system has been in use for YEARS....it's fairly trouble-free, for the most part.

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Joel Adams
C3VR Lifetime Member #56    
My Link


(click for Texas-sized view!)             NCRS

"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"

My horn didn't work and it was the contacts in the horn relay which were the problem, you could hear the relay clicking but no sound, cleaned the contacts and horn worked BUT my ammeter doesn't work!

Hello all, new member here. When I bought my 82", the battery kept running down and in several cases, left me stranded when the car wouldn't start. I jumped it off and took it to my mechanic (old guy like me) assuming the battery or alternator was bad. The first thing he asked was "did you try reving up the engine?' I looked at him with a puzzled stare and he explained that these cars (read "old") need to have the engine revved to excite the alternator/diode. When I start my car the volt meter reads dead center or slightly below but when I goose the accelerator the meter moves into the charging area of the gauge and every since I've had no problems. Take this for what its worth!