Theory Of Operation
VOLTAGE DROP TESTING: A voltage drop test is an effective way to locate excessive resistance in a typical series circuit. A circuit must be complete and energized in order to perform a proper voltage drop test of the circuit, meaning that there must be a source voltage (usually 12 volts), some type of a load with resistance, and a path to ground. The acceptable amount of voltage drop across a circuit is dependent upon the resistance of the component in the circuit that is being tested. For example, a 12 volt supply circuit that has 2.0 Ohms of resistance will have a greater voltage drop measurement when connected to a component with 6.0 Ohms of resistance than it will if connected to a component that has 60.0 Ohms.
- A 12 volt supply circuit with 2.0 Ohms of resistance, connected to a component with 6.0 Ohms, would have a voltage drop of 3.0 volts.
- A 12 volt supply circuit with 2.0 Ohms of resistance, connected to a component with 60.0 Ohms would have a voltage drop of 0.3 volts.
Depending on access to the circuit being tested, there are different voltage drop testing techniques that can be performed to test a suspect circuit. The different voltage drop testing techniques are listed and described in the test steps below.
LOAD TESTING: The following information includes when and how to perform a load test and/or voltage drop technique to diagnose resistance in an electrical circuit.
- In a circuit connected to a component that carries a load (solenoid, motor, heater, bulb, etc.) conditions such as poor terminal contact, corrosion, faulty crimp or splice can limit the ability of the circuit to carry the load required for proper operation of the component. The affect that resistance in a circuit has on component operation is dependent on the total resistance of the component(s) in the circuit. A small amount of resistance will have a greater affect on components that have a lower resistance requiring a higher amperage draw. A small amount of resistance, such 2.0 Ohms, in these cases can have a negative affect on the component operation. For this reason, simply measuring the resistance of a circuit may not be an effective way of diagnosing the circuit. A load test or voltage drop test should be performed for any load carrying circuit that is suspect prior to replacement of a component or ECU.
- However, while load testing and voltage drop testing a circuit connected to a component that requires a load can be very effective, it is not a recommended method for checking sensor circuits. The signal circuits connected to a sensor are normally connected to a large pull up or pull down resistor inside the Electronic Control Unit (ECU). The resistors normally range between 5k and 15k Ohms. The pull up resistor, plus the sensor resistance, is designed to protect against small amounts of resistance in the sensor wiring having a large effect on the sensor's input signal voltage. Because of this, 2.0 Ohms of resistance in a sensor circuit will usually only cause a voltage change of approximately.003 volts in the signal voltage input to the ECU. The amount of resistance needed to cause the sensor signal voltage to change enough for a fault to set should be easily detected with a resistance check. Testing a sensor circuit using the load tool below or a voltage drop test could falsely fail the circuit.
If a circuit load test is determined to be necessary, isolating the circuit, connecting the load tool listed below, and checking for the bulb to illuminate brightly can be a quick way to determine if a circuit can carry the proper load to operate the component. To verify with certain there is no other resistance in the circuit, perform a simple voltage drop test across the bulb. To do so connect the leads of a DVOM to the alligator clips on the load tool with it connected in series to the circuit. Compare the voltage drop across the bulb to the voltage reading across the Battery terminals. The voltage dropped across the bulb should be equal to the voltage reading across the Battery terminals if there is no resistance in the circuit being tested. Resistance in the circuit would act as a second load and voltage will be used through that resistance. That would cause the voltage drop across the bulb to be less than Battery voltage.
The amount of voltage dropped through the resistance is dependent on the resistance of the bulb being used in the load tool. For example, the 3156 bulb in the load tool illustrated has approximately 6.0 Ohms when the bulb is powered on and draws approximately 2.0 amps. This means that 2.0 Ohms of resistance in the suspect circuit will cause the voltage measurement across the bulb to drop by 25% as compared to Battery voltage. The reason for this is that the 2.0 Ohms resistance in the suspect circuit makes up 25% of the total circuit resistance of 8.0 Ohms.
A 12-volt test light can be substituted for the load test tool, but only if the test light draws enough current to effectively load test the circuit. Many high impedance test lights draw very little amperage (less than 0.1 amps) and are not reliable to load test a circuit. To perform a proper load test of a circuit, the test light being used should draw more than approximately 0.75 amps.
The Load Test Tool shown below will require higher current flow to power the bulb; and this will indicate if the circuit being tested can carry the current required to operate a connected device. Here is an example of a simple tool you can build to test a circuits ability to carry a load. The different load testing techniques are listed and described in the test steps below.
To avoid possible serious or fatal injury, DO NOT load test any air bag/restraint system components or circuits using the procedures listed here. Refer to the Service Information for proper air bag/restraint system testing procedures.
Do not load test any circuits with components still connected to the circuit.
The following is a list of components required to build a load test tool:
- 3156 Bulb
- 3156 Bulb socket
- Negative alligator clip with covers
- Positive alligator clip with covers
- 2 Amp mini fuse
- Mini fuse holder
- 2' to 4' of Black 16ga wire
- 2' to 4' of Red 16ga wire
- Approved back probe tool
- Shrink tubing
- Solder
Build the tool as illustrated below:
Depending upon the location of the circuit in question, this test may require jumper wires.
Use the appropriate SYSTEM WIRING DIAGRAMS as a guide to trace the circuits and look for any in-line connectors where the circuit failure could occur intermittently. Look for any chafed, pierced, pinched, or partially broken wires. Look for broken, bent, pushed out or corroded terminals - clean/repair as necessary. Verify that there is good pin to terminal contact in the related wire harness connectors.