In this section, a general approach to DC Voltage testing will be described. Many variations are possible, and there is no single "correct" method. The electrical engineers reading along will probably recognize concepts from the formal circuit nodal analysis method.
The approach is probably best explained using an example. A somewhat
simplified GL500 horn circuit was selected for this purpose:
In this simple series circuit, current will flow through the horns when the ignition switch is on and the horn switch is depressed. The multimeter is set to measure DC Volts and the negative meter lead is connect to the negative battery terminal. Voltage measurement points, or nodes, have been assigned and arbitrarily numbered for discussion purposes.
With the negative meter lead always attached to the negative battery
terminal, and both switches closed, the positive meter lead is used to measure
the DC voltage at each of the various nodes. Assuming the circuit is
functioning properly, typical representative voltages measured at each point
are shown in the diagram below.
There are some minor voltage drops across the various items comprising the circuit, but the bulk of the voltage is dropped across the two horns. One horn has 12.163 - 0.027 = 12.136 Volts applied, and the other horn has 12.132 Volts applied. Assuming the horns themselves are not faulty, they should both be blaring!
Now, suppose there was a malfunction someplace and the horns no longer
worked. This time, the measurements taken might look like this:
As before, there are some minor voltage drops across some of the various items, but the vast majority of the voltage drop is across the wiring between points 7 and 8. And the horns have zero Volts across them so they are silent.
Wires are not supposed to have any significant voltage drop across them, so
the wiring from points 7 to 8 probably has a fault. In fact, this is a
real-life example of a failure that had occurred with the horn wiring on a
The wire feeding power to the horns had an internal break where shown. The fault location corresponds to point 8 on the simplified wiring diagram. This specific problem is actually fairly common with GL's. The horn wire harness flexes in the wind, and the point of highest mechanical stress is where it attaches to the horn. The failure could have been prevented if a wire tie had been used to secure the cable harness so it would not flex.
Let's return to the simplified wiring diagram.
You may be wondering why some items have a voltage drop across them and others don't. Basically, if there is no current flowing through an item with finite resisitance then there will be no voltage drop across it (Ohm's law.) Recall that this is a simplified wiring diagram and circuits unrelated to the horn are not shown. The nonzero voltage drops are due to currents associated with the other circuits (not shown) that just happen to be flowing through shared components (main fuse, ignition switch, etc.)
Measuring the voltage at every point in a circuit and then sitting down to analyze the situation is both inefficient and not necessary. A "divide and conquer" approach could lead to a diagnosis much faster. An initial measurement could be taken at some place near the middle, say point 5. The results would be the basis for where to take the next measurement. In this example, point 5 has 12.190 Volts so it would not make sense to measure point 3 next. On the other hand, point 9 would be a good candidate.
This simplified circuit is for the GL500 headlight wiring. Only the low beam
wiring is shown, and only a few voltage measurements are provided.
The voltage across the headlight is 12.088- 0.121 = 11.967 Volts. Unless the bulb is burned out, it should be glowing brightly.
Now consider the case where the headlight is not working and the following
measurements are obtained:
In this case, the majority of the voltage is being dropped across the dimmer switch and the logical and correct conclusion would be that it is defective. But note that the voltage drop across the headlight is not zero, so some small current must be flowing through it unless it is completely burned out (open). It follows that the dimmer switch has not failed in a completely open state, but in a high resistance state.
Anyone who has probed a headlight connector with a multimeter knows it can
be difficult to access. So why not simply unplug the connector and measure the
voltage on the connector to see if there is any power getting to the headlight?
With the fault condition unchanged, the result would probably resemble this:
With the headlight disconnected, there will be no current flowing through the dimmer switch. And since it has a finite resistance (even if very high), the voltage drop across it is essentially zero. The incorrect conclusion from this would be that the headlight must be bad - after all, it is getting voltage to it!
The major point is, measurements can be expected to change if the circuit changes. Care should be taken not to disturb or modify a circuit while trying to troubleshoot it or erroneous conclusions are likely.
Point 14 on the diagram represents where the main wiring harness is grounded to the frame. It should now be obvious why it is so important to have a good solid connection at this location.
The wiring diagrams in the factory service manuals are significantly more complicated than the simplified ones shown in these examples. Complicated diagrams can be very intimidating. I often find it helpful to sketch out a simplified diagram before jumping deep into a circuit - remember how the saying goes about not being able to see the forest for all the trees. However, the voltage node troubleshooting concept introduced here can be applied to even the most complex circuits.
Main Fuse and Grounding
DC Voltage Testing
Wiring Fabrication and Repairs
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