72' BB ammeter acts odd
Thread Starter
Racer
Joined: Feb 2013
Posts: 364
Likes: 39
From: Sebastian FL
72' BB ammeter acts odd
The ammeter on the '72 I am restoring acts in an odd way. On engine start the output of my alternator measured at the alternator is 17 amps at 14.6v and the dash ammeter reads 17 amps. When I turn the headlites on the dash ammeter shows a drop. Without the lites on or other loads the dash ammeter slowly drops to zero. It still shows a drop if the headlites are turned on again.
Voltage stays at 14.6v and amps at 17 as measured at the alternator all this time. Any idea what is going on?
Thanks, Phil
Voltage stays at 14.6v and amps at 17 as measured at the alternator all this time. Any idea what is going on?
Thanks, Phil
Race Director
Joined: Apr 2011
Posts: 11,183
Likes: 3,335
From: North of Toronto - Ontario
Is the dash amp meter wires into the original harness? I believe it's actually shunted and is measuring the voltage drop and displaying it as amps so if it's not tapped in correctly the readings will not follow the "real" results.
I think Noonie can step you through checking it.
I've got some papers from unknown origin that gave the test procedure for amp gage as the following
"...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 difference. 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.
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."
M
I think Noonie can step you through checking it.
I've got some papers from unknown origin that gave the test procedure for amp gage as the following
"...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 difference. 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.
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."
M
Thread Starter
Racer
Joined: Feb 2013
Posts: 364
Likes: 39
From: Sebastian FL
72' ammeter
Noonie, Here are second set of numbers: Battery before starting-12.3v, At engine start dash ammeter quickly goes from 17 to 15A and battery reads 19A at 14.6v and alternator reads 17A at 14.6v. After about 3 minutes the dash ammeter has gone to about 3a. Alternator doesn't change-stays at 17A, 14.6v and battery stays at 18-19A, 143.6v.
Forgot to mention-engine at idle about 750rpm. Racing engine has no effect on dash ammeter.
Phil
Forgot to mention-engine at idle about 750rpm. Racing engine has no effect on dash ammeter.
Phil
Thread Starter
Racer
Joined: Feb 2013
Posts: 364
Likes: 39
From: Sebastian FL
Is the dash amp meter wires into the original harness? I believe it's actually shunted and is measuring the voltage drop and displaying it as amps so if it's not tapped in correctly the readings will not follow the "real" results.
I think Noonie can step you through checking it.
I've got some papers from unknown origin that gave the test procedure for amp gage as the following
"...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 difference. 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.
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."
M
I think Noonie can step you through checking it.
I've got some papers from unknown origin that gave the test procedure for amp gage as the following
"...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 difference. 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.
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."
M
Thanks, Phil
