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Top Ten Multimeter Tests - Part 2 of 2
(Continued from Part 1)
6 - More Resistance Tests
You can apply the same test to a number of other components that could be stopping your ride. For example, anything that uses a coil - ignition coils, solenoids, injectors and most sensors - should read only a low resistance between the power and ground terminals. A coil, after all, is just a spool with a lot of wire wound around it. If the wire has been broken, there is no continuity between the terminals and the coil cannot do its job.
Coil-on-Plug (COP) ignition coils can be checked individually. The best approach here is to test the coil resistance when everything is cold. Look for look for open circuits (infinite resistance) or an odd man out. If the car can be run while the low voltage coil wiring is reachable, run the car for a while and then stop. Quickly check the coils again and see if anything has changed significantly. This could help you diagnose an intermittent problem.
If you cannot easily get to the coil terminals with the COPs in the car, you could use an oven or a heat gun to heat them up to about 300 degrees F and check them - carefully - while hot. DO NOT microwave the components!
Another case where you would need to measure resistance would be during the install of a remote starter kit, in a GM vehicle that uses the Vehicle Anti-Theft Security (VATS) system. For the remote start feature to work, you'll have install a resistor in the wiring that matches the value of one that is embedded in the key. To figure out that value, touch the probes to either side of the pellet on the VATS key. With that knowledge, you can find and install the correct resistor.
7 - Engine Speed Test
OK, so you've got a tach inside the car but it's not simple to read when you've got your head under the hood. Hooking up the meter is easy, or a bit harder depending on whether you're dealing with a conventional or electronic ignition system. Some of the following details may vary by the brand of multimeter you're working with, but a little common sense will help get you through the details.
Before hooking anything up, you have to tell the meter about the engine you're checking, specifically the number of cylinders. Most meters have individual dial settings for RPM tests on 4, 6 and 8 cylinder engines. Some might have a separate switch for setting this.
Since most meters also only use a 3 or 3-1/2 digit display, you may be reading tens of RPMs and have to multiply the displayed reading by ten. So, if the meter is showing "215" then the engine is turning at 2150 RPM. Pretty obvious stuff, right?
For vehicles using a distributor, locate the ignition coil and connect the red probe to the negative terminal on the coil. Connect the black probe to a ground source. Start the engine and run it at the required speed. Read the speed from the meter display.
For distributorless ignition systems (DIS), locate the TACH signal wire going between the DIS module and the engine control unit (ECU). You will need to refer to the vehicle's service or technical manual for this information. Connect the meter's red probe to the TACH signal and the black probe to ground. Start the engine and run it at the required speed. Read the speed from the meter display.
8 - More Frequency Tests
In the engine speed test, what the meter was actually reading was the time between signal pulses. Each pulse corresponds to the coil firing. As the engine speeds up, the time between coil pulses gets shorter in proportion to the engine speed. There are other applications for frequency measurement.
Some kinds of MAF (mass air flow sensor) send a frequency signal to the ECU, according to how much airflow is detected. You'll need the service or technical manual for the vehicle to know what frequency range is involved. Not all multimeters have the capacity to handle high frequency signals.
The same can happen for Manifold Absolute Pressure (MAP) sensors, as well as Barometric Pressure sensors. Ford versions do use an output frequency, while those from GM, Chrysler, Honda and Toyota use a DC voltage.
Actually testing these sensors also requires a pressure and vacuum source, typically a hand pump designed for this kind of testing. As as result, we'll not get into the testing details. However, if you need to read any frequency signal, the general procedure is to set the meter to 4-cylinder RPM and test the component. Then, if need be, multiply the display reading by ten to get the RPM value, and finally divide that by 30 to get the frequency in Hertz, or cycles per second.
9 - Battery Drain Test
Cars equipped with onboard computers usually need a continuous source of power to retain information. This puts a small, but measurable drain on the vehicle battery. In normal circumstances, the draw is minimal, however, adding more electronics can increase the drain.
With older cars, about the only thing that draws from the battery, when everything else is shut off, is the clock. It is also possible that some other electric circuit could fail and start pulling power from the battery. The typical result is a no-start condition when you need to get going the next morning.
If you suspect that something in the vehicle is draining the battery, you can measure how much power is being taken from the battery when everything is off. You will need to disconnect the positive cable from the battery. If there is a hood light, take the bulb out, so that you're not measuring the draw from that. Close the trunk, doors and turn off any other electrics that might be running.
In this test, you'll be measuring current, as opposed to voltage. Current is measured in Amps (Amperes, actually) and the meter will have a fuse to limit the current flow through it. Too much current will blow the fuse, so for convenience always start at the highest level the meter is capable of... usually 10 amps. We're actually expecting a result less than 200 milliamps, but if you don't have a spare fuse, do it this way the first time.
Touch the black probe from the meter to the positive cable and the red probe to the positive terminal post on the battery. You shouldn't have any draw that the meter can measure in this range. If so, switch the meter down to the next range and try again. A current drain of a few or tens of milliamps (thousandths of an amp) can be expected on a modern car to keep the ECU memory alive.
If you read significantly more than this, you may have a problem somewhere in the car. You can try to isolate the problem area by leaving the meter connected and pulling fuses one-by-one from the fuse box. If the draw level drops when you pull a fuse out, check what equipment that circuit is for. The problem is likely to be in that area.
10 - Temperature Measurement
Some multimeters come with temperature measurement capability. This is most often done using a thermocouple wire that plugs into the meter. A thermocouple is made from two wires of different metal. Those materials are chosen so that they actually produce a small electrical signal when in contact with each other. Even more handy is that the voltage produced changes with the temperature at the junction of the wires changes.
There are different types of thermocouple, depending on the metals used. If your meter came with a J-K type thermocouple (probably the most common for general use), you cannot use a different type. The specific performance of the thermocouple is programmed into the meter in order to give you a reading in degrees, rather than volts.
To measure the temperature of something, set the meter to read temperature in either farenheit or celcius degrees. Check if there are any range selections available and, if so, choose the appropriate one. Plug the thermocouple socket into the meter and touch the bare end to what you want to measure. Do take appropriate personal protective measures if working near high temperature components, so that you don't burn yourself.
You can hold the insulated wire (gently) with a pair of pliers without significantly affecting the reading. Thermocouples will often show an error range up to two or three degrees. You can check the accuracy by touching the tip to an ice cube, which should read about 32 degrees farenheit (0 deg Celcius). After, immerse it some boiling water. This should read about 212 degrees farenheit (100 deg Celcius). If you're not within a couple of degrees for each of these, check for damage of the wire, insulation and weld bead at the tip. Make sure that the tip is clean.
You probably got a three foot long thermocouple wire, which is handy for general work. Thermocouples are common industrial items and are available in a range of lengths. With a longer one, say 80 inches (a common size), you could measure intake air temperature on the road by putting the tip into the incoming air stream and running the wire back to the passenger compartment where the meter can be protected and seen easily.
There are many other possibilities, with the biggest challenge only being how to keep the thermocouple tip reliably in contact with the component of interest.
Dealing with Older Vehicles
I can tell you from experience that the most challenging electrical problems that you're likely to encounter in older vehicles result from either a lack of power or a lack of ground. Neither of these problems can be solved by just replacing components and hoping that the new one works.
Of the two problems, the lack of a good ground is more likely than a lack of power. Power problems usually come from a broken or cut wire. Ground problems most often result from corrosion. If you've ever seen another car's tail lights acting funny - for example, alternately blinking the signal and brake lamps - this is a sure sign of a ground problem.
You only need to do two things to diagnose most any electrical problem. First is an understanding of how the circuit of interest works. For example, a radio needs power, ground and a good signal from the antenna. If it isn't getting all of these, it won't work well, or at all.
The second thing is to take that understanding and break it out into a series of simple steps to check. Is the radio getting power? If not, why not? Is there continuity between the radio case and vehicle ground? If not, why not? In general, if there is no power, check the fuses first. If there is no ground, try running a separate ground wire from the radio case to a known good ground.
If there is power and ground, does the front panel light up? If not, the radio is likely dead. Is the antenna is good condition? Is the cable from the antenna plugged into the radio? Is there any evidence of damage to the cable? Does reception improve if you plug in another antenna? Answering these questions will bring you to a logical conclusion.
You may be looking to upgrade anyway, but its always good to know that you're not throwing good stuff out.
Dealing with Newer Vehicles
Modern vehicles are becoming more sophisticated in all aspects, including their electrics. Charging blindly into the wiring of your car or pulling components out for random checks isn't really the best way to approach things. Stick to the simple stuff unless you have the appropriate diagnosis and repair manuals. You will likely need much more sophisticated tools than the simple multimeter that we've been dealing with.
A better approach is to borrow or buy an OBD II code reader. Modern vehicles have On Board Diagnostics (OBD) and run diagnostic checks regularly. Problem issues are logged and may trigger a check engine light (CEL) or other warning indicator. The code reader can retrieve this information from the car's diagnostic system. What it tells you may give you an idea of how best to proceed.
For now, stick with the basics. If and as you get comfortable with how things work from the electrical side, you may want to take on greater challenges. Succeed at these and you'll be unique in the hobby. Even if you don't move beyond the basics, you're likely to save yourself a bunch of money and keep your ride on the road, instead of in the shop.
Copied from Another Revision:
Voltage drop is the reduction in voltage due to the load on the circuit. It's a better indicator of the circuit's condition than a resistance test, because you're powering up the circuit.
Let's use a headlight circuit as an example. Let's say that the wire at the terminal at the headlight has frayed, and only one or two strands are still attached. If you test it for resistance, the circuit will show to be good. However, when you turn on the light, it will be dim, because there isn't enough conductor to fully power the light. Connect one lead of the meter to the + battery post, the other to the terminal at the connector with the switch turned on. Set the meter to read 0~3 volts, or millivolts. You should seldom see more than .5v drop in a circuit.
You can also have a switch showing good continuity with no voltage applied (less than 1 ohm) but if the contacts have worn or corroded to the point of losing much of the contact area, the switch can't move the voltage necessary to power the component. To test a switch, place the meter leads on the input and output sides of the switch and turn the circuit on. Once again you'd like to see less than .5 volts, preferably around .050 v.
One of the best item to test for voltage drop is how well your starting system is grounded. Place one meter lead on the negative battery [B]post[B], and the other lead where the cable bolts to the motor. Have someone crank the engine and read the drop. If your cable isn't large enough, or if it's attached to a place with a lot of paint so the cable doesn't have good contact with the block, the starter will not crank, will crank slowly, and the cable will get hot.
Hope that helped.
Last edited by LevonH; Feb 19th, 13 at 9:51 PM.
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