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Many problems with modern vehicles involve checking electrical circuits. Basic testing is easy but a good deal of damage can be done by careless methods. These tips should make the process much easier.

basic tools for automotive electrical testing

Basic automotive electrical testing tools 

A digital volt ohm meter or DVOM is a tool every automotive enthusiast should own. The DVOM is versatile and we can use it to test many things. A DVOM need not be expensive and most of the simple models are adequate.  Another advantage to the less expensive models is they are often easiest to use. We can test switches, relays and lighting circuits with our digital volt ohmmeter. Even more involved testing is not that difficult, using simple procedures.

When buying a DVOM certain things make it easier to use. An audible signal on the ohm scale is a very handy feature to look for. This gives a beep, when we establish continuity between the leads. This audible signal is very handy when the digital readout is not readily visible.

A good set of probes and back-probe adaptors are also necessary when using the meter.  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. Some mechanics fashion these tips from wire but we find the commercially available models that are hardened work much better. A small set of alligator-clamp adaptors are also handy to have.

Beyond basic testing we will also need a wiring diagram for the circuit being checked. Often we can borrow these from a library.  Wiring diagrams are also available for purchase as repair manuals or even downloadable from the internet. A good wiring diagram will give color codes and starting and ending points for a circuit.


Using a volt meter to probe a circuit

Touching an electrical connector or wire with a test-lead is referred to as probing. Less is always best when using a probe. The terminals of modern vehicles are extremely delicate.  Forcing a probe into an electrical connector may easily cause permanent damage.

Never stick a probe into the terminals of a light socket

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 normally used for checking connections and the black lead is touched to a metal part of the vehicle chassis or ground.

For example we switch the meter to the volt scale in checking a light-socket for current flow. We touch the black lead to a body 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

Back probing an electrical connector

Often we need to check current flow without separating a connector. This is handy with voltage drop testing to measure resistance in a circuit under a load. We call the technique back-probing. We touch the probe to the electrical contact in the back of the connection.

Sometimes we may need to remove a back cover to access the connector. This is far superior to other test methods and does the least damage. Tiny probes are available for tight connection. The probe is only touched to the connector and not forced in. Excess force will damage the delicate pins attached at the wire ends.

damage from improper testing and proper back probing method

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.

corrosion in a circuit, caused by a damaged seal

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

An ohmmeter is very handy for testing a component off of the vehicle. When we switch the meter to ohms the scale will indicate OL for open lead.  This is similar to infinite resistance.  We may also get OL when checking a switch that is turned off. If we touch the leads together the reading drops to near zero. This means there is little or no resistance.  With an open lead or OL current does not flow.  With near zero resistance the current freely flows. An example is a switch that is on.

continuity is tested with an ohmmeter, as the switch is operated

Alligator clips are used to attach the probes and make testing easier in the photo above. 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 for power and ground at 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.

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