Mastering the Multimeter
Use All Functions to Your Advantage
All technicians are aware that the scan tool holds the No. 1 priority in our initial diagnostic strategies. We are also painfully aware of the costs of keeping our scan tools updated. In my personal experience, the No. 2 tool in diagnostics is the DVOM, known as the digital volt-ohm meter or multimeter. We also know that in the event of an electrical problem the DVOM is the essential tool. There are times when we suspect the scan tool is showing us some data that may be wrong and we need to look at the raw value with a DVOM. Or in cases where an electrical component is not working, we need to do some valuable electrical tests. This is essentially a tool that never needs updating and can be used instead of a host of other handheld testers once we learn the diagnostic value of the DVOM and all of its test functions.
The framework of this article is to convey to techs in the trenches the diagnostic value of the DVOM with its 10 to 12 diagnostic functions.
In this article we are going to use the Fluke 87 series DVOM and the Fluke 88 series DVOM as our examples. GM has endorsed the Fluke 87 series meter in their electrical training program while Ford and Toyota have endorsed the Fluke 88 series meter. There are many good DVOMs in the market — and some that are not so good.
When dealing with automotive electrical circuits the most common type of measurement is D/C voltage. When looking at D/C voltage measurements there are three types, one being what is called an Open Circuit voltage test. This term refers to a voltage test done when a component such as a fuel pump is unhooked. With the fuel pump physically unhooked we have taken the normal load out of the circuit. When doing an Open Circuit voltage test the DVOM with its 10 Mega ohm internal impedance will only require an amperage value in the micro amps range when measuring voltage. This means there is no loading effect on the circuit. While this may be an OK step, it may not be the best step.
The second type of voltage test and a better test would be the Dynamic Voltage test whereas we leave the fuel pump hooked up to create the normal load on the circuit as we back probe the feed wire with our positive meter lead and back probe the fuel pump ground wire with our negative meter lead.
Let’s say the Open Circuit voltage test indicated 12.4 volts with the fuel pump disconnected while the Dynamic Voltage test indicated 5.2 volts with the fuel pump connected. This would tell us that there has to be a voltage loss or voltage drop caused by unwanted resistance somewhere in the circuit, possibly in a bad connector or by chance in the fuel pump relay contacts or in the fuel pump ground circuit. The same applies to the standard test light when doing an Open Circuit voltage test. The standard incandescent test light only requires about 300 milliamps current flow when doing an open circuit voltage test.
The third type of voltage measurement is known as the Voltage Drop test. Voltage drops should only occur across the load or component in the circuit with an output device such as a fuel pump with the circuit activated.
We summed it up in Fig.1. The LED test lights draw a current flow value in the micro amps range when conducting an Open Circuit voltage test so there is no loading effect from an LED test light. The individual Voltage Drop test is summed up in Fig. 2. Electrically speaking, everything begins at the battery and everything ends at the battery. We are conducting an Open Circuit voltage test at the battery during key off conditions. Notice the 12.8 volts indicated by the meter in Fig. 3. A good 12 volt battery will generate 2.1 volts per cell, and with six cells will indicate 12.6 volts with a 100% charge. Our meter reading indicates a higher value due to a surface charge.
Now we are going to conduct a 10 second cranking period during a WOT (wide-open-throttle) clear flood mode. Notice the battery voltage dropped well below the minimum 9.6 volts indicating a poor battery in Fig. 4. Battery conductance testers are very popular and have been used in the industry for a long time. When conducting these battery tests using the DVOM it is more accurate with a Dynamic Voltage test when compared to the battery conductive testers.
I have literally seen battery conductive testers fail good batteries and pass bad batteries. When doing the 10 second WOT cranking test with the DVOM, the battery must have a full charge. When testing a battery with a conductive tester it is not necessary for the battery to have a full charge.
In the next test with the engine running at an off idle condition and a full electrical load, notice the good charging voltage of 13.95 volts in Fig. 5. Modern day engines now incorporate a smart charging strategy whereas the alternator will be turned off at idle with no electrical loads. This is why we electrically loaded the system at off idle conditions. A failed diode inside the alternator rectifier bridge can create unwanted A/C voltages and a laundry list of trouble codes along with engine performance issues. With the DVOM now A/C coupled and the positive DVOM lead on the output terminal of the alternator, we are blocking the D/C voltages from the alternator. The meter in Fig. 6 indicates the A/C voltages being emitted from the alternator. A/C voltages below 400 millivolts would be acceptable under full electrical loads and off idle conditions. High and low inductive current probes are essential tools to interface with the DVOM.
In Fig. 7 we are looking at good charging voltage off idle from the DVOM on the right at 13.98 volts. As the alternator charges a good battery, the battery’s internal resistance increases as the alternator builds up the battery’s internal charge. Notice the DVOM on the left is coupled with a low inductive amp probe and clamped around the alternator feed wire and indicates 223 millivolts. The millivolt symbol represents 1 thousandths of a volt. We simply move a decimal point three places to the left which now represents 0.223 volts. The attenuation factor on this amp probe is that every 1 millivolt equals 100 milliamps. We simply multiply 0.223 X 100 to determine that this battery is pulling 22.3 amps. With no electrical load a good battery will require less than 10 amps after 10 minutes of running. Our 22.3 amps was captured before the 10 minute period expired. When using a low inductive amp probe the probe is inserted to the voltage and ground terminals of the DVOM. In Fig. 8 we are using a high inductive amp clamped around a battery cable while doing a WOT cranking amps test to the starter. Notice the good reading of 182.8 amps. Again when coupling an amp probe to the DVOM, clamp into the voltage and ground terminals of the meter and factor in the attenuation factor of the amp probe to convert millivolts to an amperage value.
Voltage Drop testing has always been an important function especially on ground circuits. In Fig. 9 we are conducting a Voltage Drop test on the battery post and cable connection during WOT cranking. Since this is a high current circuit, the maximum allowable voltage drop should not exceed 0.1 volt per connection. In Fig. 10 we are conducting a Voltage Drop test of the starter ground circuit. The positive meter lead is touching the engine block while the negative lead is at the negative battery post during a WOT cranking test. The meter indicates a value of 0.293 volts. There are three connections: the negative cable connection at the battery, the negative cable connection at the block, and where the starter gets its ground by being bolted to the block.
Fig 10. A Voltage Drop test of the starter ground circuit. The positive meter lead is touching the engine block while the negative lead is at the negative battery post during a WOT cranking test.
We previously covered the diagnostic value of using both the high and low inductive amp probes interfaced into the positive and negative terminals of the DVOM. Low inductive current probes are very useful in checking the current flow to fuel pumps and looking at current ramping on the primary side of the ignition coils. When using a current probe always remember to plug it into the female positive and ground voltage terminals of the DVOM. Also, when using the low inductive current probe it is always a good idea to use the Min/Max capture mode of the DVOM. The attenuation factor of the amp probe must be used when converting the DVOM’s millivolt reading to an amperage value.
Fig 11. A manual ammeter test is also available with the DVOM. In this test the circuit must be broken and the ammeter put in series with the circuit.
A manual ammeter test is also available with the DVOM. See Fig. 11. When doing this test the circuit must be broken and the ammeter put in series with the circuit. We now insert the meter leads into the amperage and ground terminals of the DVOM. Ammeter readings are not polarity sensitive, so if the meter shows a negative value simply reverse the leads. When using the manual ammeter test function make certain that the meter selection is made for amperage and not voltage. If the meter selection is set on voltage when using the manual ammeter selection the Fluke meters will emit an audible beep. If an audible beep is not heard then the fast blow fuses are blown. You may have noticed the lack of a fuel pressure test port on modern day systems. An amperage draw test of the fuel pump will not only verify fuel pressure but also alert us to an electrical problem with the fuel pump or the fuel pump circuit. On many applications we have a dedicated fuel pump fuse we can remove and by using the manual ammeter test function we can probe both female terminals from the removed fuel pump fuse. A value of 4 to 6 amps is common on most PFI systems. The early GM CSFI fuel pumps typically pull between 8 to 10 amps. Many applications will have a dedicated fuel pump relay that can easily be removed. In many applications the female terminals marked 30 and 87 are the power supply terminals to the fuel pump. We simply remove the fuel pump relay and jumper across the female terminals of the fuel pump relay by using the manual ammeter test function of the DVOM. On many Ford systems we can trip the inertia switch and jumper across the inertia switch terminals with our ammeter leads. On the Fluke series meters there are two scales. One being 0 to 400 milliamps and the other being the 0 to 10 amp scale. If the measured values exceed these two values the fast blow fuses will blow inside the DVOM. We can also use a low inductive current probe by simply clamping the probe around the feed wire to the fuel pump. Two-wire magnetic crank sensors are popular on Toyota, Ford and many European systems. These sensors are sometimes referred to as variable reluctance sensors. A good crank with a no start condition will initially make us suspect a faulty magnetic crank sensor with no RPM signal on our scan tool. These sensors generate an A/C voltage signal so the meter selection must be set to A/C coupling. When the RPM is low during cranking for example, the amplitude of this signal is weakest. As the RPM increases the amplitude will increase. Looking at the schematic in Fig. 12, access the two wires at the easiest access point, either at the crank sensor or at the PCM. In Fig. 13 the DVOM on the right shows a good A/C signal during cranking after the sensor has been replaced. The DVOM on the left is now D/C coupled with the sensor disconnected during KOEO conditions indicating 0.98 D/C voltage. This voltage is from the A/D converter inside the PCM. PCMs cannot identify an A/C type voltage, so an A/D converter inside the PCM converts this A/C signal to a digital type signal.
Let’s look at a bench test of an oxygen heater element at room temp in Fig. 14. On a cold startup these sensors can pull over three amps. If the ohmmeter indicates an open circuit then the heating element is burnt open. There have been some cases where the resistance values of some aftermarket oxygen sensors are out of spec, causing the PCM to reset the oxygen heater code. An ohmmeter test of a coolant or air charge temperature is also a common test. These sensors are known as NTC (Negative Thermal Coefficient) thermistors, meaning as the sensor heats up its internal resistance decreases. This causes an increase in current flow which increases the voltage drop across the thermistor. When viewing the ohmmeter readings don’t miss the k symbol which means you must multiply the digital reading by 1,000. An example would be 1.5k ohms which represent 1,500 ohms. Another example could be where the ohmmeter reading indicated 2.3M ohm. You simply multiply 2.3 by 1,000,000 for 2,300,000 ohms. You can imagine how easy it would be to miss these symbols.
Another test using the ohmmeter may be a complaint from the car owner that the fuel level intermittently indicates empty even after refueling. The issue may be from a bad gauge inside the cluster, a bad sending unit, a circuit problem or a corrupted bus signal. A late model GM vehicle came in with a complaint where the fuel gauge would intermittently read empty even after a refuel. Before the car came into our shop a technician had replaced the cluster because while tapping on the cluster the gauge would read normal. However, the problem persisted after the cluster was replaced. When looking at the fuel sending unit schematic we would find that the fuel level sensor reports to the PCM, the PCM then busses this value to the IPC. When looking at the fuel level parameter from PCM data the fuel level also indicated empty. We now suspect a faulty fuel level sending unit. This is another good example of an ohmmeter test. After dropping the fuel tank and removing the fuel pump module let’s look at the schematic in Fig. 15. This is a perfect example of using the ohmmeter in the Min/Max peak detect mode while moving the float level up and down. The specs are also indicated.
Notice in Fig. 16 the diagnostic value of using the Min/Max peak detect mode of the Fluke DVOM. The Fluke 87 and 88 series meter will detect a glitch every 100 milliseconds. Touching the button under the Min/Max button on the Fluke 87 will put the DVOM in a 1 millisecond peak detect mode. When a record mode has been established, hitting the Min/Max button the first time will indicate the maximum values recorded. Pressing the Min/Max button the second time will display the minimum recorded values. A third press of the Min/Max button will display the average values recorded in the Min/Max record mode. While using the Min/Max peak detect mode of the Fluke meters an audible beep will be emitted every time the meter detects a change in a value. The Fluke DVOM’s Min/Max peak record mode works on all meter functions. We cannot say enough about this function especially when doing the popular wiggle and tap and tug test on connectors or components when detecting an intermittent poor connection. Without using the Min/Max mode of the DVOM and viewing the digital display on the DVOM the update is only four times per second. The bar graph display under the digital display will update 40 times per second. When using the Min/Max record mode it is not necessary to visually monitor the meter readings since they are being recorded.
Another example of using the Min/Max record mode is indicated in Fig. 17 as we tapped into the oxygen sensor signal wire. Notice the recorded maximum value of 0.897 volts and the minimum recorded value of 0.123 volts while the average value is indicated as 0.488 volts. This tells us that we are in good fuel control. Many manufacturers have gone to an electronically controlled fuel pump with a fuel pump control module. Fig. 18 shows a GM electronic fuel pump control schematic. Notice that the fuel pump control module is powered by a fuel pump relay. When using a standard SAE electrical schematic the voltage source is shown at the top of the schematic while the ground is located at the bottom of the schematic. The fuel pump control module receives the voltage source from the fuel pump relay on the VT/GN wire at terminal No. 1. The fuel pump control module gets its ground at pin four. A good dynamic voltage test across these two terminals should indicate full battery voltage drop. The fuel pump control module supplies voltage to the fuel pump from pin five and supplies the ground at pin eight. The ultimate and final ground for the fuel pump and fuel pump module ground is at pin four. If we were to do a dynamic voltage test at the fuel pump positive and negative circuits with the engine running we would not see a full battery voltage value, since the fuel pump control module bangs the fuel pump 25,000 times a second with battery voltage and changes the on-time of the voltage applied to the fuel pump. The PCM will increase the duty cycle command on time to the fuel pump control module at pin three to increase the voltage on time to the fuel pump to increase fuel pressure as the engine load increases. If you elect to use an amp probe to current ramp these fuel pumps during idle no load conditions, a 3 to 5 amp value is normal.
On a conventional fuel pump circuit, a dynamic voltage check on the fuel pump feed wire and the fuel pump ground wire should indicate a full battery voltage drop or loss across the fuel pump. If, for example, a value of 5.2 volts was displayed, we have two possibilities. No. 1: We lost some voltage through a connector or the fuel pump relay. Or, option No.2: We have some resistance on the fuel pump ground circuit or likely the ground connection. I have had many tech calls after a fuel pump had been replaced with no improvement only to find a weak fuel pump ground. This is why when we do a dynamic voltage test to a component, we always use the component’s ground with our negative meter lead. We all know that the most reliable ground on any vehicle is the negative battery terminal. With our long jumper leads, if we used the negative battery ground and a full dynamic value of 12.4 volts was displayed, whereas the meter indicated 5.2 volts when using the fuel pump ground circuit, this would also verify that we had a weak fuel pump ground circuit or connection.
The DVOM also gives us the ability to check for a duty cycle control signal for example to a purge solenoid. Note Fig. 19 indicating a duty cycle command to a purge solenoid of 24.9%. If you are using the Fluke 88 series meter you would select the negative slope since most purge solenoids are ground side controlled by the PCM. However, late model Chrysler PCMs feed side control of these solenoids so the positive slope would need to be selected. This test verifies that the circuit is working electrically.
Many technicians opt to test the injector drive signal by using the popular noid lights. Keep in mind when using a noid light with the injector unplugged, the normal loading effect of the injector has been removed. The injector noid lights will only require an amperage value in the low microamps range while the normal injector load will require about 700 milliamps on most saturation type injectors. Using the Fluke 88 series meter with the injector plugged in and back probing the negative or switching side of the injector will not only verify a good drive signal but will also show the injector on time in milliseconds. See Fig. 20. The frequency counter function of the DVOM can also be an important function to use when looking for a glitch or signal drop out of a GM MAF sensor. This is a good example where a tap and tug wiggle test is used along with the Min/Max record mode as indicated in Fig. 21 from a good GM MAF sensor while doing a throttle snap test.
One popular OEM has issued several bulletins about terminal fretting causing an intermittent loss of conductivity in some of their connections. The aftermarket offers a contact enhancer known as stabilant 22A. This solution helps ensure conductivity on compromised connections. The BWD part number is CL85.
The objective of this article is to create an awareness of the importance of the DVOM and its many test functions. In these modern times of “buttonology,” patience and practice is the mother of talent. In other words when we intentionally use the DVOM more often we become more aware of its diagnostic value.
The industry is better because of your commitment.