Voltage Drop Diagnostics

Diagnosing an electrical problem should always start with the
basics, “Positives and Grounds” right? Can you get different readings
from one end to the other of a positive or negative connection? The
answer to both those questions is yes, and chances are that might be
because of a voltage drop.  So, what is a voltage drop? A voltage drop
is any loss of voltage from the source to the end component.
Meaning, if you start with a 12 volt positive (or negative) say, from
the lead at the battery (the source of the voltage) and follow that
individual wire to its component there should be 12 volts along its entire

Some of the signs of a voltage drop that your customer might
mention could be a slow reacting component, or a lower than normal
blower speed, intermittent components or erratic gauges, to power
windows that move at a snail’s pace. But, the big ticket components in
today’s cars are not immune to voltage drops either. Such as problems
with the charging and starting systems to engine and transmission
malfunctions. Loose connections, corroded terminals, frayed wiring, or
a high resistance in the wiring circuit can cause these voltage issues too.
With today’s intertwining circuits through different fuse boxes and control
modules, focusing on testing for a voltage drop is even more important than ever before.

One of the earliest forms of a voltage drop that I used to encounter back when the 70’s and 80’s models were common were the dim headlights or goofy warning lights on the instrument clusters caused by a loose or missing chassis ground.  All I needed to check for the presences of a voltage drop was a jumper wire attached to the negative battery terminal, then take the other end and attach it to the body of the car.  Usually you’d see a slight spark as the clip made contact, and of course the headlights got brighter again at the same time. No need in a meter or any other tool, a little voltage drop might dim the headlights or slow the alternator output, but you could get by for quite some time without any worries.  It was a simpler back then, but these days, it’s important to have every last bit of that energy being passed from the source to the component.

A more scientific explanation of voltage drop is Kirchoff’s Voltage Law. It states, “In any closed loop network, the total voltage around the loop is equal to the sum of all the voltage drops within the same loop which is also equal to zero.” In other words the sum of all voltages within the loop must be equal to zero. Kirchoff’s law is also known as the Conservation of Energy.

There’s no system immune to voltage drops.  Isolating where these drops are is the real challenge. Never assume that the voltage you see at the fuse box is the same amount of voltage getting to the other end of the circuit.  I tend to check both ends on most everything these days just for that reason. Sometimes, this voltage drop isn’t as obvious and sometimes the voltage drop is only present under certain conditions. 

When you’re talking about voltage drops it’s not just the positive side of the circuit that can cause it.  The negative side is just as prone to a loss of signal too, and sometimes with far worse consequences. I’ve seen PCM’s replaced by customers and other repair shops that didn’t need replaced because somebody didn’t take the time to trace out the ground leads completely.  Other times its gauge clusters or speedo’s that are replaced for nothing more than poor connections that made the gauges read erratically. 

Some of the worst offenders involving a voltage drop are probably the transmission related issues.  Poor diagnostics or overlooking a voltage drop could sometimes lead to a transmission change out that didn’t need to happen.  Typically you’ll get the same answers to your questions from the customer; i.e.… “Can’t shift, hard shift, etc…” and checking the wiring from end to end shows no signs of any problems.  But, what is overlooked is checking the amount of voltage coming from the source (the battery) through the ignition switch, through the fuse box, through the connectors, and finally to the transmission. (And don’t forget about the negative side either.)

These voltage drops can not only be from a 12 volt source but also come from the various modules that operate the systems in today’s car.  With so many 5 volt reference signals being passed back and forth from one end of the car to the other a small nick in a wire harness or a slowly corroding connector can reduce the 5V reference signal and quite possible set up a scenario of misleading codes to be stored.

Whatever the circuit, there’s always a chance to have a voltage drop somewhere in the system.  Checking for proper voltage and ground signals by whatever method you are comfortable with is going to insure a more complete diagnostics.  But, be sure to also check it under the conditions that your customer says the problem occurs.  Shifting wire harnesses, temperature, and vibration can also lead to an intermittent voltage drop.

It doesn’t take much of a drop to affect today’s complicated circuits, but a little extra effort in testing will go a long way to solving a voltage drop issue.

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