When a device is turned on in a car with an ammeter what is the correct reading ? 12 volt system sbc with alt. with the key off, turning the lights on shows a discharge on the ammeter when the car first starts the ammeter shows charging on the + side for a bit then settles back to a slight + charge while running with a "ahem" larger amp draw device that comes on when driving the ammeter shifts to charging and stays charging while the device is on, once the device turns off it goes back to normal slight charge Is this correct? I ran a battery down today with a good power master alt and am wondering if i am wired correct
Sounds almost right...except the part about the ammeter showing a charge when a different load is placed on the system. My guess is the ammeter is not connected quite in the right place. It should be connected between the battery, and the rest of the system. It should not be between any load, and the alternator.
yea, when the fan kicks on the ammeter shows a charge to the battery that stays on unit he fan goes off
I would never use an ammeter in any of my cars-they are prone to starting a fire. Use a volt meter... R-
Ammeters were used safely for years (and if running a generator, actually more accurate as an indicator of charging system health), but once overall system continuous loads went above about 50 amps, they became an issue. Direct-read ammeters larger than that are a fire hazard as it's very difficult to build one. As a general rule, above that size is done with a shunt type, inductive type, or current-transformer type. The first type is very difficult to calibrate and not particularly reliable, inductive isn't very accurate, the current transformer type only works on AC.
connect it somewhere on the other side of the ammeter....that could be at the alternator, etc. Make sure the wire is big enough and properly fuse protected
No way I would use an ammeter again. Had one mounted in my 56 chevy once that had the wiring burn up and messed up my dash that had real nice pinstriping on it. Used voltmeters since. Gary
You may not be able to with an ammeter. What's the ammeter scale? 30-0-30? 60-0-60? What other electrical loads does the car have besides ignition and lights? How much current does the electric fan draw? Keep in mind that motors will instantaneously draw up to 1200% of their running current when starting (AKA as 'current inrush'), a 'safe' inrush number to use when sizing components is 300%. So if your fan draws 20 amps, you can see inrush current of 60 amps for the fan alone, equal to the maximum rating of the meter. Add in the other loads, and you're beyond the meter's rating... and a fire can result. Jim's advice above is correct, but doesn't take into consideration the rating of the meter or the effect that the additional load will have on the supply wire to the ammeter, any switches involved (like the ignition switch), any or all of which will probably be too small now. Note that low voltage to a motor will increase starting current, making the problem worse. I suspect that a fair amount of rewiring would be needed. Best advice in this case? Remove the ammeter and tie the wires connected to it together. Then replace it with a voltmeter, hooking one side to ground and the other side to the 'ignition' terminal on your ignition switch.
Ammeter is a SW 60-0-60 when the fan kicks it is at +30 about never more all I have are my lights, and electric fuel pump, wiper motor that is never used , and the fan there is a hot lead off my fuse panel that comes off the - side of the ammeter that is fused that I could use as the hot lead to the fan relay or use the winter to change my wiring...
If your load(s) are connected to a junction block on the Alternator side of the ammeter, as they should be, the ammeter does not read the current going to those loads - they are fed directly from the alternator to the load. The ammeter only reads the current going to or from the battery.
I've been using SW and original amp. gauges in my cars for 50+ years and no fire yet. I must be very, very lucky. Maybe the modern ones made out of plastic are the trouble makers.
With a roughly 30 amp running draw on the fan, that means you'll see spikes of 90 amps when the fan starts. Add in the ignition, lighting, and fuel pump loads and the ammeter will see 100 amp plus spikes if connected to read all loads. The 60% overload will kill the meter sooner or later, possibly with disastrous results. This is why ammeters disappeared; when continuous-use electric accessories on vehicles increased, ammeters weren't able to carry the loads involved. Replace it with a voltmeter...
Here's a very simple diagram of an automotive electrical system. Notice that all the loads (except the starter) come off the ammeter on the generator (alternator) side of the ammeter. When the engine is running, and the loads are switched in - none of the current to the load(s) flows through the ammeter. The generator (alternator) senses a drop in voltage as loads are switched in and increases output to compensate and maintain a constant voltage in the system. You could have 100 amps of current flowing to various loads in the system, and if the battery is charged, would only see a few amps of charging current flowing into the battery via the ammeter. Most wiring kits have a "switched hot" and an "always hot" junction block to connect loads to (there's not enough room to connect all the loads on the ammeter anymore). Take your fan hot lead off the starter post, and connect it to a "switched hot" post somewhere on the firewall. That way, when the key is turned off, the fan goes off too. If your battery drain problem still persists, use a 12V trouble lamp and the old process of elimination to figure out where the drain is (clock, stereo system with a memory, etc.).
the fan only works with the switch on as the wire from the fuse panel in the relay is switched, its the large wire into the replay that goes off the hot lead on the starter...
That doesn't matter... the main fan power isn't going through the ammeter, that's why the meter reads 'funny'....
That could be what's going on. With a 30 amp load from the fan, I think Steve has the right idea, don't use an ammeter on your car.
i'm a little slow, so if i change to a voltmeter, and I will will this change the way the fans draws power from the alt instead the battery?
If you use a voltmeter, the fan won't care...it will still draw power from the battery and alternator (which hopefully are connected together with a proper size wire). But you'll get to watch what the voltage does when you turn on the fan. Normally, the battery voltage will be around 12.5 to 13v when everything is turned off. When you start the engine, it will rise to between 13 and 15 volts as it charges, then settle to around 13.5 volts. When you turn on a load, such as the fan, it will probably drop, and if the alternator cannot provide enough current, it will drop below the "resting" voltage...ie below 12.5v, and the battery will drain as you drive.
btw I use mechanical fans on my hot rods. My wife's late model junk has factory electric fans, which work fine. But they are built a bit differently than my hot rods...
I am not sure about all after market ammeters, but the original early Ford unit in my 38 is based on inductive coil, the capacity of the cable is dependent on the size of the cable, and the meter is only reacting as a ratio of the 2 coils, as stated these ammeters may not necessarily be 100% accurate but to be fair do they need to be, all the operator needs to know is that the generator/alternator is charging the battery. The only way this can cause any fire is due to a fault (dead short) somewhere typically due to the cable going through the ammeter chaffing, the meter it self has no electrical connection as the meters reading is only induced by a ratio.
You could make an "electronically sensed" ammeter that uses a simple extremely low resistor in series with all loads and alternator (except starter). Then a simple current sensing IC can be used to operate a +/- meter. The sensing IC is connected across the sense resistor and uses the voltage drop across resistor for measurements.
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