ASE Certification from my ASE/ BAR Specific Diagnostic Course: Electrical Resistance:
Electrical Resistance will cause very slow moving motors, dim headlights and other problems within an automotive vehicle. Understanding what resistance is and how it will affect a circuit is key in diagnosing a electrical problem.
When you watch my little video you can see in an electrical circuit, if the applied voltage is constant meaning it never changes, but the resistance increases the current will decrease.
A voltage drop is a very good way to find extra resistance before or after a load. Remember just checking for open circuit voltage does not find resistance, and resistance will cut down current in a circuit.
So let’s see the Resistance with a problem circuit:
You are diagnosing a fuel pump with low fuel pressure, and the customer is complaining of no power. You check fuel pressure and it’s below specifications.
Obviously most people will see low fuel pump pressure and change the fuel filter and fuel pump. I would, and don’t say you are special and you would check for other problems.
Now the new pump still has low pressure, what now change the fuel pump again it’s in the tank, drop the tank again. Well before you do that go grab your voltmeter and perform a voltage drop to check for high resistance.
That’s what I did on a problem car I had, the circuit had high resistance, remember the voltage is constant and the resistance went up, which means my current went down.
Slow moving pump, low fuel pressure found and fix resistance problem no charge on electrical repair.