Voltage drop testing will catch automotive electrical thieves - Vehicle electrical system problems can be solved with voltage drop testing. - Motor Age - Automotive training, certification & parts
Problems in modern electrical systems can cause all sorts of complaints, from a taillight that doesn't work to an engine that
runs poorly. All of us, whether in school or on the job, have learned basic electrical testing techniques; measuring voltage,
resistance and maybe even current. Yet, still I meet techs of all skill levels that miss problems that could be discovered
had they understood and used one of the most effective techniques there is — voltage drop measurement.
It all starts with the infamous Ohm's Law and the interrelationship between voltage, current and circuit resistance. If you
don't remember, that relationship is expressed as:
Voltage = Current x Resistance
While this may be an extreme example, even the connection at the battery itself can rob the load of the voltage it needs to
work properly.
While plugging in the numbers has some use in diagnosis, the key points related to Ohm's Law that I want you to remember are
these:
For a constant voltage, an increase in resistance will cause a decrease in current.
For a constant resistance, a decrease in voltage will cause a decrease in current.
A circuit is not complete until it gets back to the battery. Often, loose or corroded ground points from the harness are the
cause of unwanted resistance.
Current is what makes an electrical component work. If the current received is less than what it should be, the component
won't work as designed, if it works at all. Source voltage, usually supplied by the battery, is the "push" needed to overcome
any resistance in the circuit, including that of the component itself. Resistance is just that — the restriction to current
flow in a circuit or component.
QUICK STUDY
For the majority of electrical circuits, the only significant resistance in a circuit is the resistance of the component doing
the work, which I'll refer to as the load. Some loads are probably obvious to you, like light bulbs, ignition coils, and fuel
pumps. But ANY component that performs work is a load, and this includes some that may not be as obvious, like relays, Hall
effect sensors and control modules. Many components play more than one role, depending on their function in a given circuit.
For more, check out the electrical features in the January and February 2009 editions of Motor Age.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
