The best way to test electrical circuits is voltage drop. Use your digital multi-meter (DMM), set to voltage and go find both ends of a single wire to test (Whatever is easiest). The B+ terminal on the alternator to the B+ on the battery. Start the engine. Voltage drop only works when current is flowing thru the circuit. At this point, turn on the headlights, blower fan to draw some amperage. The DMM does not care which meter led is used, if backwards, it will just show (-) in front of the reading. This test shows the "Loss" of voltage in the circuit, so the lower the voltage reading, the better. So a reading of 0.05 volts is great, but 0.40 volts is acceptable. If there is corrosion inside the wire, broken copper threads, or damaged / burn't fusible link, the problem will show up.
Major Advantage of Voltage Drop:
#1 You are not disturbing any wires, connectors or components than you have to.
#2 You have to have current flowing thru the circuit as intended to make the item work. So turn it on!
#3 With jumper wires, you can test relays for bad contacts across terminals 30 & 87, or 87a if equipped. Repeat test on relays multiple times. One high voltage drop is all you need.
Electrical Circuit Testing:
#1 A test for voltage only proves that it is present, not the amount of current it can carry. (Test light or DMM)
#2 A voltage drop test proves the condition of the wire & components in the circuit. Never forget the negative side.
#3 A scope will show the electrical activity inside the circuit.
DC brush type motors, all create a smooth consistent wave pattern. If jagged or missing hump, motor is bad or soon to fail. Fuel pumps are best tested this way, other than pressure and volume.
#4 A clamp-on current probe, will show the amount of amperage flowing in the circuit. If too high, the item is working too hard. Check for the reason why.
Say you check the blower fan with your DMM. The reading shows 12.4 volts. Great. It is a series circuit with only one path for current to flow. The fan is designed to consume all of the voltage in that circuit. However, each switch, connector, fuse on both positive and negative side, will have a very small amount of voltage drop across the component. If all where totaled up, it would equal the Battery available voltage...in theory.
But a voltage drop test is quick, easy and accurate. It never lyes and gives you additional information about the condition of the circuit and components. If the reading is above 1.0 volt, start going backward in the circuit until you find the resistance. Don't make it hard to gain access to a wire, unless you need to. This will isolate the problem every time.
As an ASE Master Tech since 1978, I grew up rebuilding alternators, carburetors, distributors, starters, etc. I hate the new "BOX" fixer's. If you don't perform best practices first by testing, the trips to the store will increase...with your box & receipt.
