Automatic Shutdown Relay

If it's chattering with just the ignition switch turned on, the battery voltage is too low. That relay turns on for one second after turning on the ignition switch, then again during engine rotation, (cranking or running). Pulses from the crankshaft and camshaft position sensors tells the Engine Computer to turn on the ASD relay which sends voltage to the ignition coil, injectors, fuel pump or pump relay, alternator field, and oxygen sensor heaters.

ASD relay and computer plays a role on the charging system-See below

Thanks for the kind words. Please do not remove the battery cable while the engine is running. The potential exists to damage a lot of computer modules from excessive voltage. That trick worked years ago with breaker point ignition systems. Your HVAC comment made me think you know more about electrical than the average do-it-yourselfer. If that's wrong, I can make it more understandable. Basically, once the engine is running, use an inexpensive digital voltmeter to measure battery voltage on the 20 volt DC scale. You should find between 13.75 to 14.75 volts. If it is low, around 12.0 to 12.6 volts, the alternator is not working. We'll cross that bridge when we get that far. A few simple tests will determine if it has the most common problem, worn brushes. Those can be replaced relatively easily. I never replaced them on a '96 or newer Caravan but I did that a lot on older donated ones to put "bugs" in for my students to troubleshoot. On those it can be done without removing the alternator from the engine. I can post pictures too if it comes to that.

Yup, you can measure right on the battery posts. If it is acceptable there will be no need to check further unless it is an intermittent problem. The output terminal on the alternator will have the same voltage unless a large value bolt-in fuse is blown in the under-hood fuse box. If the voltage is low, the circuit can be divided in half by measuring at the two small terminals or wires on the back of the alternator. One will have full battery voltage while the engine is running. The secret is the other one. It must have less voltage but not 0 volts. 0 volts means there is an open circuit between the brushes. The exact same voltage on both terminals means the voltage regulator circuit inside the Engine Computer is open or the wire to it is broken.

Here's the deal. The regulator doesn't actually have an output as such. Current flows from the ASD relay to one of the brushes on the back of the alternator, through the rotating field winding, out the other brush, to the regulator in the Engine Computer, then to ground and back to the battery. A magnetic field is formed around the field winding. Three things are needed to generate the output voltage. Those are a coil of wire, magnet, and movement between the two. The field winding is the electromagnet. The movement comes from spinning it with the belt. The only practical way to control the output voltage is to strengthen or weaken the electromagnet. THAT is the purpose of the voltage regulator. At a maximum there will be around three amps of current flow through the field. Think of the regulator as nothing more than a variable resistance in series with the field winding. Under most common situations the regulator reduces field current to around one amp. A wire ties battery voltage to the regulator so it can measure system voltage. The regulator's resistance varies continuously so the alternator's output voltage stays between 13.75 and 14.75 volts. That means battery voltage during charging, alternator output voltage, and system voltage are all the same. That's the same voltage that you will find at light sockets and motors.