Technical Ameter Reading Negative

Hi all, noticed when at higher speeds my ameter reads 'negative' and drains the battery. Same whether lights are on or the fans (car has no AC or other options) At idle it's zero. Changed the generator to an alternator and same reading. Volts seem ok at 13. It's a 12 V system. Battery is in the trunk with 2 GA wire. Not sure if that is the problem. At idle, when zero, I turn things on and yes it drops and comes up when the lights or fans are off. New battery btw. I would think at higher RPMs the alternator is driving even more power to the battery vs the engine using all the power up. Help requested.

 

I have heard amp meter is good for measuring the health of a charging system with a generator.

Now that all new cars have an alternator, we no longer see an amp gauge in the dash.

@squirrel can probably explain the difference in generator and alternator output that causes the ammeter to not work correctly.

 

Is it in a car that originally had a positive ground system? If so, did someone forget to reverse the connections when they converted it to negative ground? If the ammeter is wired wrong, it will read backwards.

 

Yes, car is a 56 Chevy so I believe that is a negative ground system. Ammeter reading is acting the same with the generator and now alternator. The new alternator is grounded directly to the firewall - maybe that is the issue? Im not sure what a negative ground is. I'll check the ammeter wiring as well.

 

56 car. So it’s an after market gauge. Recheck you volt readings both idle and at higher rpm’s. Sounds like gauge is bad or of wrong amp rating for charging system.

 

Negative ground means the negative terminal of the battery is connected to the frame / ch***is, I.E. ground. Most 12V American cars are negative ground and have been since the early 50's. Most 6 volt cars built before then used a positive ground where the positive post of the battery was hooked to the ch***is / ground.

 

It sounds like someone wired the ammeter incorrectly. By itself, an ammeter will not drain the battery unless it has a short and you will know that via the smoke. If the battery is draining, you need to find out why and fix that first, worry about the ammeter later. If you do not have any electronics on the car, a quick way to check for a drain is to connect a test light in series with either the negative or positive battery cable. Make sure everything in the car is turned off, including the dome light which will come on when the door is opened. Either close the door or depress the door switch. With everything turned off, the test lamp should be out. If it is turned on it means that something is on and will drain the battery. Start disconnecting circuits until the lamp goes out. The last one you disconnected is the problem circuit.

This test does not work well if you have electronic devices on the car. Many of them have "keep awake" circuits which draw a bit of current even when turned off.

 

Are any loads connected directly to the battery? When turned on they will read as a discharge. You want to connect everything past the ammeter. The ammeter should only read current in or out of the battery. If the alternator is keeping up with loads there should be no reading. The starter and ignition cranking byp*** are the the only things to connect directly to the battery.

 

Pick up one of these... S & G Tool Aid Corp 24600 | Alternator-Generator-Starter Cur (autotoolworld.com)
... and check the output of the alternator under various loads. If the gauge is bad this will show it.

You used to be able to get these at the parts stores, although I haven't seen them in a while. NAPA discontinued theirs.

 

BTW a 12 volt alternator should put out a little over 14 volts in good condition. What did you do with the voltage regulator for the generator when you switched them out?

 

have you tried just forgetting the ammeter and putting in a voltmeter for checking? If its charging 13-14 volts or so at all times, minus a heavy load like a/c, its probably fine. Check the alternator output first at the terminals. If you have a short to ground, it should show up.

 

All, problem solved. The gauge was wired backwards. + side to alternator.

 

Good to see that you figured it out and thanks for telling everyone you got it fixed. All too many forget that step.