This 1991 Ford E150 van came in with the complaint that the battery will go dead overnight unless the customer disconnects the battery or leaves a battery charger hooked to the battery.
The first thing I had to do was to check for a battery drain. There are several ways to do this and I will show a couple. Start by loosening the battery negative cable, try not to break the circuit until you have your test device installed. Slide the cable up the terminal post while maintaining contact and connect one lead to the post and the other to the cable before you actually separate the two. The first picture shows the multimeter with one lead hooked to the battery post and the other lead to the battery cable. There is a 3.25 amp draw on the system. The second picture shows using a test light hooked up in the same manner as the multimeter and a strong illumination of the bulb. It is harder to tell if you have a very minor drain using the test light method because the bulb element may just barely glow. Also keep in mind that on later model vehicles with on board computers it may take an hour or more for all of the computer systems to go into sleep mode or time out. This van has only one computer and takes only seconds for it to time out.
With this large of drain and the type of vehicle that it is the first thing I wanted to check was the voltage regulator. It is the silver or grey colored box with rust on it behind the battery in the picture below. With this particular type of problem another simple test is to check to see if the regulator is warm to the touch on a cold engine with a charged battery. If it is warm,then the regulator is faulty. What happens to these regulators is that a circuit shorts out and sends a signal to the alternator to charge. With the engine not running and the alternator not turning it turns it into a drain condition rather than a charge condition. The alternator will also get very warm from this condition.
I disconnected the voltage regulator and the drain is now gone. The .01 amp drain is at an acceptable drain level and is most likely being caused by the memory circuit in the vehicle radio.
You can also see the element in the test light bulb is fully out.
I installed a new regulator and checked the charging system. More problems, although the voltage and amperage readings are within specifications the diode/stator light is on to the left of the gauges. This indicates that there is some ac voltage being sent out of the alternator rather than all dc voltage. The condition is usually caused by a shorted diode or stator assembly.
I like having this feature in my test equipment but if it is not available again a multimeter can be used to test for ac voltage. Simply set the meter to read ac voltage and connect it to the positive and negative cables. A reading of .154 VAC is too much. Anything over .05 VAC is too much. Time to change the alternator.
In case you were wondering, the customer’s problems started with a faulty regulator which was draining the battery. The customer would just barely be able to start the engine or would have to jump start the engine and then would just let the engine run to recharge the battery. This is a very big mistake. An alternator is design to keep a fully charged battery fully charged. It is not designed to recharge a discharged or dead battery. The alternator will be damaged under these conditions and it can take as little as 5-10 minutes. When a battery is discharged or dead it must be recharged with a battery charger and not the alternator.