One of the quickest ways to get a quick health check on your electrical system is watch your ammeter! It will tell you all kinds of valuable information if you know how to read it! Most modern cars now use a voltmeter to provide limited information about your electrical system. Or even worse just a warning light to let you know your alternator has failed. Because voltmeters are now the norm the skill of interpreting the information the ammeter provides is becoming a lost art. Let’s walk through a driving sequence to understand what the ammeter will reveal about your electrical system.
Entering the car your the ammeter should be reading "0", straight up. You may see a quick defection to the minus side if your have an interior light that comes on with opening the door. It's at "0" because you are not using any or generating any current (engine is not running). When you turn on the ignition you will see the needle move slightly to the minus (discharge) side indicating a discharge of a couple amps. This means your ignition system is getting power. When you hit the starter the ammeter will deflect sharply to the left (minus 20-30 amps) as the starter spins. The energy for the starter is being drawn straight from the battery. As the engine fires the ammeter will quickly move to the plus side (charging) of the gauge in the 20-30 amp range. The energy that was drawn down from the battery while starting is quickly being replaced by charging current from the generator. As you start driving the voltage regulator will manage the amount of charge needed to go back into the battery. After around five minutes of driving typically the battery will start to approaches full charge and you will see a reduction of charge rate down to 1-3 amps on the plus side. At this point the battery has fully recovered from the starter discharge and now the generator is putting out only enough current to maintain the charge. The voltage regulator manages the on-going charge rate.
While your driving night time is coming and it is getting cooler. You turn on your headlights and start up the heater fan. Immediately you see the needle momentarily jump to the minus side, then come back to 1-3 amps on the charge side as the regulator manages the generator output to meet the increased demand. As you come to a stop sign and the engine speed drops, the ammeter will move sharply to the minus side, often 15-20 amps down. You notice the lights dim and the heater motor may slow. Right now your generator is not creating enough power to offset the increased load of the headlight and heater motor and is drawing backup power from the battery. This lack of sufficient power generation can fully discharge a battery if allowed to go for a long period. The short stop at the stoplight however, is not harmful. In fact, you can always bump the manual throttle to bring the idle up enough to stop the discharge. As soon as you accelerate from the stop the generator will again start generating sufficient current to replenish the energy pulled from the battery (expect a jump to 5-10 amps charge for a short period) before settling back to a trickle charge of a couple amps while driving.
So how can you use if for some basic troubleshooting? When you first get in and step on the break pedal, the ammeter should deflect slightly to discharge as the brake lamp lights. This lets you know the battery has some charge. No deflection? Battery is probably dead or disconnected. Also when you turn on the key if you don't see a slight discharge indication your ignition is probably not connected or functional. If when turning on the key and immediate your have a full discharge (minus 35 amps) you have a dead short that needs to be repaired. Immediately turn off the key and begin trouble shooting to find the electrical short. Otherwise you risk the very real danger of a wiring fire. Might start your troubleshooting at the headlight switch as they have historically been trouble spots due to corrosion resistance in the connectors. If you are running and suddenly see a continuous discharge usually this indicates a voltage regulator issue. Try tapping the regulator case with a screwdriver handle to see if a relay is sticking and it starts charging again. On teh other hand if you see a continuous rate of high charge (> 20 amps) that never goes down you may have a battery starting to fail (it's not taking or holding a charge) or a voltage regulator failing. Either way it's time to troubleshoot the generator and regulator charging circuit.
By watching the action of your ammeter your can easily tell if your electrical system is functioning correctly. It will tell you if you have a short, your battery is full charged, how fast it is charging and how much current your are consuming while driving. Compared to a voltmeter which simply gives system voltage, ammeters allow you active monitor your electrical system.
Share what on the road lessons have you learned by paying attention to your ammeter!