Parasitic drain caused by alternator diodes

Bad diodes are highly unlikely. In the reverse direction, as they are when the alternator isn't operating, their job is to block current flow from discharging the battery. That reverse current will be in the order of microamps, not amps. The only way one of these diodes will pass any significant current in the reverse direction is if it's "leaky", and the excessive heat from that reverse current will cause it to short completely. In 99 percent of shorted diodes, they pass really high current when the alternator starts working, and that causes a terminal to burn open, similar to a fuse, then the short is out of the circuit.

Next, all alternators use a minimum of six diodes, and sometimes more, but those six are in two groups of three. You must have one diode shorted in each group to have a dead short, then the fuse link wire would burn open. One shorted diode isn't particularly common, and two at the same time is very rare.

Where you will find this kind of current is when the voltage regulator is built into the alternator, and it isn't turning off. If it keeps the field winding energized, that will draw very close to three amps in any brand of vehicle. That's enough to kill a good battery overnight. Chrysler has never used that design on their domestic vehicles. Their voltage regulators are built into the Engine Computer. Failures of those are rare but not unheard of, but I've never heard of one of them keeping the field circuit powered up. In fact, that field current of up to three amps comes through the automatic shutdown (ASD) relay which also powers the ignition coils, injectors, and fuel pump. If the ASD relay wasn't turning off, you'd hear the fuel pump running, and the battery would be dead in less than an hour.

You can unbolt the output wire at the alternator to verify that doesn't affect the symptoms, but keep in mind if the wrench touches that stud and anything metal on the vehicle at the same time, enough current can pass through the wrench to turn it red hot. Standard practice for safety is to disconnect the battery's negative cable, then you can work around the alternator without causing a problem. You will likely observe later that idle speed is too low. The engine may not start unless you hold the accelerator pedal down 1/4", and / or it is likely to stall at stop signs. The solution for that is the Engine Computer has to relearn "minimum throttle" before it will know when it must be in control of idle speed. To meet the conditions for that relearn to take place, drive at highway speed with the engine warmed up, then coast for at least seven seconds without touching the pedals.

Please clarify how you're measuring 2.25 amps. What kind of tool or tester are you using and where is it connected? Are you waiting long enough for all the computers to go to "sleep mode"? 2.25 amps is about right for computers that are still running.

You are using the amp meter the right way, but there is a problem you need to be aware of. Starting in the mid 1990's, most vehicles have at least one computer that needs up to twenty minutes to go to "sleep" mode. During that twenty minutes it can draw up to three amps. Logic would dictate you just have to wait for more than twenty minutes, then start the testing, but there is one more piece to the story. Anything you do to remove power to that computer, even for just a fraction of a second, wakes it up again, and then you have to wait another twenty minutes for it to go to sleep. Once the battery cable is removed, as soon as you connect the final meter lead, the computer will wake up and draw high current.

Most of the common, inexpensive volt/ohm/amp meters have an internal 2-amp fuse for the milliamps and 2-amp range, and they need to have the positive test lead moved to a special jack to use the 10-amp range. That 10-amp jack is rarely fused.

Most people will start the testing with the meter on the 10-amp range, then they can read that 3-amp drain until the computer goes to sleep. Once that happens, we need to switch to a lower range for more accuracy. That means removing the test lead from the 10-amp jack and moving it to the common volts/ohms/milliamps jack. Doing that breaks the circuit, and once reconnected, the computer will draw enough current to blow the meter's internal 2-amp fuse. "Frog fuzz!" Now the frustration begins.

If you are lucky enough to not blow that fuse, you will be on the 2-amp range, and once the computer goes to sleep, you will probably want to switch to a lower range, again, for more accuracy. All voltmeters with a rotary switch use a "break-before-make" switch which means as you slowly rotate the knob, the internal contacts break the connection to one range, then it makes the connection to the next range. That very minute gap is plenty to cause the computer to wake up again. Even if the fuse does not blow, you will get an over-range indication because the computer is drawing much more current than that range can handle.

I am fairly confident this is why you found 2.25 amps of current drain. You may have waited an hour, but you have to wait after making the final meter connection and selecting the range. There is an easier way though to solve this. Connect your meter like you did, and start on the 10-amp range. Wait a good twenty minutes or until you see the current drop significantly, then use a small jumper wire to connect the battery cable to the post. You can also jump between the two meter probes. Regardless how you do it, the meter is bypassed, or in effect, shorted out in the circuit. Voltage to the computer has never been lost and you are free to move the meter's probe to the milliamps jack. When you are ready, remove the jumper wire and read the meter.

If you see the current has dropped low enough to warrant switching to a lower range, put the jumper wire back on first, then switch the range. When you remove the jumper wire again, current will have no choice but to go through the meter to get measured.

Unless specified differently by the manufacturer, the industry standard is you're allowed up to 35 ma. (.035A), of current drain to keep all the computer memories alive. Chrysler says at that rate, a good, fully-charged battery will be able to crank an engine fast enough to start after sitting for three weeks. Cadillac is one brand I am aware of that allows up to 50 ma.

Getting back to my story about diodes, it is rare to see a shorted diode in an alternator that is still in the circuit. What I mean is, . . . a diode is a one-way valve for electrical current flow. When shorted, it acts like a piece of wire and lets current go both ways through it. That makes it get real hot real fast, then its lead burns open. At that point it's no longer electrically in the circuit. It is not uncommon to have an open diode. Those do not cause harm to anything else, but they do cause some common symptoms. All alternators put out three-phase output, then the diodes "rectify" those currents into DC that can be stored in a battery. Each phase uses two diodes at one time, then another two, then a different two, over and over. Because the diodes are shared between the phases, one defective diode causes a loss of 2/3 of the alternator's maximum current capacity. During a professional full-load output test, all you'd be able to get is 30 amps from the typical 90-amp alternator. That is not enough to meet the demands of the entire electrical system under all conditions. The battery has to make up the difference until it slowly runs down over days or weeks.

Three-phase output is used because it varies very little in voltage. Think of a one-cylinder water pump. You would get a big pulse of pressure, then nothing until the next big pulse comes along. With a three-piston water pump, even when two pistons are not doing anything, one will be developing pressure, so the pressure leaving the pump is very smooth and steady. In the alternator, when two phases aren't developing much voltage and current, one phase will be. The output voltage is very smooth and steady. Now, if there is one bad diode, you'll get a voltage and current pulse from one phase, then the next phase, then there will be a dropout when the dead phase does not develop anything. Voltage will drop real low for that instant. The difference between that low point and the normal voltage is called the "ripple" voltage. That is measured by most professional load testers. A few models actually show it as a voltage and include that on printouts, but most testers just show it on a relative bar chart and they do not offer a printout. The mechanic has to just write the numbers on the repair order.

When we see high ripple voltage and we can only get 20 - 40 amps on the load test, we can be pretty sure the alternator has a bad diode. It is usually not practical to replace a diode, both because of the cost and the high difficulty. You are better off with a professionally-rebuilt alternator with a warranty.

Consider looking at this article for more information:

https://www.2carpros.com/articles/car-battery-dead-overnight

In addition to Docs write up, a better way to test modern vehicles for voltage draws is to use a voltmeter on the millivolt scale and measure the voltage drop across the fuses. You will need to download the charts showing the voltage = amps from
https://www.powerprobe.com/fuse-voltage-drop-charts/
Then you do not need to disconnect anything to do parasitic draw testing. You pick a fuse, measure the voltage across the fuse and look on the chart to see the amp draw.
If you find a large draw then you simply look at the wiring diagram for the circuit that fuse powers. Just do not believe the names on the fuse box. Many times it will be marked for the primary circuit it feeds, but not all of the circuits on the fuse.