Almost any problem with a modern vehicle involves checking electrical circuits. Basic testing is easy, but a good deal of damage can be done by careless methods. These tips will make the process much easier.
Basic automotive electrical testing tools
One tool every car-repair enthusiast should own is a digital volt ohm meter. This need not be expensive, the simplest models are adequate and often easiest to use. Testing of switches, relays and lighting circuits does not require a precision instrument.
A very handy feature to look for, is an audible signal on the ohm scale. This gives a beep, when continuity is established between the leads. When the digital readout is not readily visible, the audible sound is very handy.
Along with the meter, a good set of probes and back-probe adaptors are necessary. Probes are color coded, black and red, for identification. Pointed probe contacts are far more useful than those with blunt tips. Back-probe adaptors, slip over the probe ends and provide a tiny contact for testing circuits without damage. These can be fashioned from wire but the commercially available, hardened models work much better. A small set of alligator-clamp adaptors are also handy, but may be used less often.
Beyond basic testing, a wiring diagram, for the circuit being checked is also indispensable. Many times these can be borrowed from a library, purchased in a repair manual or even downloaded, from the internet. A good wiring diagram will give color codes as well as starting and ending points for a circuit.
Using a volt meter to probe a circuit
Touching a circuit, with a test-lead is referred to as probing. When using a probe, less is much more. The terminals of modern vehicles are extremely delicate and are permanently damaged, very easily.
A probe should never be wedged or pushed into a connector. Doing so will spread the contacts and ruin the connector. Instead very gently touch the metal portion of the connector with the lead. The red lead is used for checking voltage and the black lead it touched to a metal part of the vehicle chassis or ground.
For example, in checking a light-socket for current flow, the meter is switched to volt. The black lead is touched to a ground and the red lead to the metal contact to be checked. When the circuit is turned on, the volt meter should register, indicating current flow.
What is back-probing a circuit
There are many times when current needs to be measured while a connector is still attached. This technique is called back-probing. The tiny probe is pushed into the back of the connector to touch the wire connection.
Back-probing eliminates the need to puncture the insulation on the wire to test the circuit. Damage to insulation can create future problems with corrosion. Always back probe rather than puncture insulation. Any previous damage to insulation should be repaired by taping or sealing with liquid sealant.
Modern connectors are sealed to prevent moisture intrusion. When opening a connector be certain the seal is not damaged. The crankshaft position sensor above resulted in a vehicle dying problem, that was very intermittent. This was caused by an improper previous repair, that damaged the sensor seal. Such problems can be quite difficult to find and cause a great deal of distress.
Testing for continuity in a circuit
When testing a component off of the vehicle, an ohm meter is very handy. With the meter set to ohms, and the leads separated, the scale will indicate OL for open lead. This means current cannot flow, such as when a switch is turned off. When the leads are touched, the reading drops to near zero. This means there is little or no resistance, allowing current to flow. An example is a switch that is on.
In this photo, alligator clips are attached to the probes to make testing easier. When the switch is worked by hand, the buzzer on the ohmmeter signals continuity. When the switch is released the meter reads OL indicating the circuit is off. This switch is working as designed, and is not the cause of the brake light problem.
Basic electrical testing procedures can eliminate the need for needless part replacement and greatly increase the accuracy of diagnosis.
Electrical testing combined with other testing
Many fuel pumps are needlessly replaced, due to incomplete testing. For instance, the vehicle will not start, and the fuel pressure is checked. There is no fuel pressure, and the pump is assumed bad. Replacing the pump does not help and there is still no fuel pressure.
Testing the power and ground, going to the pump would have revealed a lack of current flow. With no current, the fuel pump does not run, so there is no fuel pressure. The actual problem may have been a tripped inertial switch, cutting power to the fuel pump.
Electrical testing is not difficult but does need to be properly done to avoid future problems. With a few simple tools and these tips, a great many electrical problems will be things of the past.
