Voltage Drop
Ohm's law
(E=IxR) tells us that even very low resistance in the starter
circuit will cause the starter to turn slowly, because of low
voltage. For example: in a system drawing 200 amps, 0.01 ohms
resistance in the starter cable will cause a 2 volt drop in
voltage at the starter; 0.01 ohms is too little for all but the
most expensive and sophisticated ohmmeters to measure, but
measurements of voltage drop will indicate where there is
resistance.
In
automotive circuits even the smallest loss of voltage will cause
poor performance. Set your Fluke multimeter in the mV or VDC
setting and connect the meter + lead to the side of the device
nearer the battery + terminal and the - lead to the side nearer
the battery - terminal or ground and engage the Min/Max
function. Current must be flowing for the meter to register the
voltage drop found. This procedure is helpful on components and
connections (both on the + feed side and - ground side) except
solenoids, which read battery voltage if you measure across them
when the engine is being cranked.
Voltage drops should not exceed the following:
200 mV Wire or cable
300 mV Switch
100 mV Ground
0 mV to <50 mV Sensor Connections
0.0V Connections
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