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douglas.evans - Feb 16 2015 09:13 AM
Electrical Testing Techniques
February 1st, 2010
You don't use one testing method to diagnose drivability problems, do you? In fact, I bet you know more than one technique for testing nearly every system on the car. We learn these various techniques in order to accurately troubleshoot the problem at hand. What works well on one problem may not work at all on another. Troubleshooting electrical problems is no different. Here are a few techniques that I find especially helpful in tracking down electrical gremlins.
Testing Voltage Drop
If there is any one technique to learn and master, this is it. I've found and solved more electrical issues using this method than any other. Performing the test is the easy part. It's understanding what the resulting readings are trying to tell you that trips up most techs.
It's based on a simple electrical principle. Voltage is a measure of electromotive force, the force needed to overcome resistance in a closed electrical circuit. Ideally, the only significant source of resistance is the component doing the work in that circuit. All of the applied voltage should be used to overcome that component's resistance, with little left over on the other side. In other words, voltage "drops" across the source of the resistance.
Try this for yourself on a simple headlight circuit. Turn on the headlights and grab your Digital Multimeter (DMM). Connect the meter's negative lead to the battery negative terminal and use your positive meter lead to measure the voltage at the headlight, starting with the terminal supplying power to the bulb. You should be reading about the same as what's in the battery. Now measure the voltage at the ground side of the bulb. This reading should be nearly, but not exactly, 0.0 volt.
You just measured voltage drop.
Many electrical circuit problems are the result of poor grounds and loose or corroded connections. These all add resistance to the circuit. Each of these unwanted resistances will require voltage to overcome, no matter what side of the primary component they are on. Voltage will be split proportionally among them and will show up in one of the two readings you just practiced on. If an extra source of resistance is on the power side of the component, you'll measure less than system voltage on that side of the circuit. If the fault is on the ground side, you'll measure more on the ground side of the component.
Because we're looking for any potential thief, and that thief could be hiding anywhere in the circuit, testing the entire circuit path is critical. Be sure to always reference your DMM to the battery and begin your measurements as close to the circuit's primary component as you can. It also is critical that the circuit be turned on while testing or your readings will mean nothing.
Once you're comfortable with what the DMM is trying to tell you, you can add a variation to your technique to eliminate the effect varying voltage may have on your power side readings. Simply move your meter's negative lead to the battery's positive terminal when testing the power side of the circuit. Your meter measures the voltage potential between its leads, and any significant reading with the negative lead referenced to the positive terminal means someone is stealing potential away from the primary component.
You can see this for yourself, again using the headlight circuit. First, measure battery voltage and write it down. Next, with your DMM negative lead on the negative battery terminal, measure voltage at the power side of the bulb. It should be slightly less than what you measured at the battery. Write the amount of difference down. Last, move the negative meter lead to the positive battery terminal. This reading should be nearly identical to the difference you calculated earlier. The meter does the math for you!
Measuring Current Flow
Your boss hands you a ticket for a "no start" complaint. You quickly determine there's no fuel pressure. Next, you locate the fuel pump relay and test for power to the pump and then bypass the relay to wire the pump direct. Still no fuel pressure. Bad pump, right? Maybe, maybe not.
Many components are turned on and off by relays. These relays, in turn, typically are controlled by a control module or separate switch. So in any circuit with a relay there are two separate circuits to test: one in which the relay acts as the switch to operate the primary circuit component; and one in which the relay itself is the primary component.
The trick is to figure out quickly which of the two we need to focus our efforts on. Measuring current flow in the circuits can do this. Most DMMs are capable only of measuring up to 10 amps of current flow, so the initial start-up of the fuel pump may be too much for it. Instead invest in a low amp clamp you can plug into your meter.
Let's go back to the fuel pressure problem. Locate the fuse that supplies power to the coil side of the relay. Often, this is the same fuse that supplies power to the pump and if so, so much the better. It will save a step. Replace the fuse with a fused jumper wire you can clamp your probe around and turn on the key. Typically, the fuel pump will be energized for a few seconds without seeing the engine crank, then shut down. That is long enough to get diagnostic direction. You can use the Record function on your meter to catch the reading.
If a single fuse feeds the relay, you'll have your answer right away. No current flow means the relay was never turned on and you'll need to focus on that side of the circuit. Perhaps the fuse is bad or the control module is not seeing what it needs to see to close the circuit and energize the relay. Current flow of about 0.50 amp means the relay is energized, but there is no current flow to the primary component. Now your focus will be on the other half of the circuit.
Current flow over a few amps tells you that the relay circuit is intact, supplying power to the pump. For a fuel pump circuit, lower than normal current flow can indicate a voltage drop issue in the pump circuit, or fuel starvation from an empty tank or clogged inlet filter. Higher than normal current flow could be telling you the pump is mechanically binding.
If there is a separate fuse powering the pump, you'll need to move your amp clamp to test that portion of the circuit. This technique, with practice, can be used on any relay-controlled circuit to reduce your troubleshooting time. Just remember that the circuit being tested must be live.
I've seen lots of cases where EVAP purge and vent valves fail intermittently. The trick is in isolating those intermittent faults, and a technique learned from our own Jim Garrido has helped me do just that.
These valves are electrical solenoids. Usually, solenoids fail completely or only when heated. They can be controlled on the ground or power side by the ECM, so the first step in using this technique is to pull up the schematic. If it is a ground side control, power to the solenoid will be constant. If it is a power side control, the power supply will originate at the control module.
Two cautions here: only use this method on solenoids using a 12-volt power supply, and only energize them for brief periods or internal damage will result.
We're going to use our DMM as the control device in place of the ECM to operate the solenoid numerous times in succession while monitoring its electrical performance. If it is failing only when hot, this is one way to mimic the failure. Next, set your meter for amperage measurement using the 10 amp (or higher) scale. If you are operating a ground controlled solenoid, attach one meter lead to the battery negative post and then backprobe the solenoid ground connection with the positive lead.
With the key on, you should read amperage on your meter and the solenoid should be "on." Remember, you don't want to leave it on for more than a second or two, or you can damage the solenoid coil. Testing a power controlled solenoid only requires that you move your meter lead to the positive battery terminal and backprobe the power side of the solenoid with the remaining meter lead.
Each successive test should read about the same amperage. If the coil is failing when heated, the current flow will change as the resistance of the coil changes. A total open circuit will result in no current flow. You can use Ohm's Law to calculate the coil's resistance and compare that to any listed specification.
Electrical troubleshooting need not be a curse. Practice these techniques on known good components, then try them on your next "gremlin hunt."