Vehicle Testing Meters
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About this ebook
This book was written for the professional auto/truck, as well as the interested “do it yourself” week-end, mechanic.
I have been a student, technician, and teacher. During my career I have wondered, questioned, assumed, used gut feelings, diagnosed, troubleshot, tested, scanned, and scoped.
The years of work I have performed could not have been as
successful had I not had a thorough basic understanding of the
vehicle testing meters that are used to diagnose and troubleshoot vehicle electrical and electronic problems. This book covers, in detail with 159+ photos and illustrations, how to select, when and how to use the volt, Ohm, and amp features of test meters, as well as the inductive pick-up, also called an amp clamp, current clamp, or current probe. During my years of teaching I have had many conversations with working automotive and truck technicians that have given me insight into testing techniques that they have used. These are shared in this book.
If you are not familiar with the features and limitations of today’s vehicle testing meters and inductive pickups, this book is for you. Thank you for your interest in learning.
Joe Glassford
The author, Joe Glassford’s experience: 29 years as a public school teacher (retired). Teaching assignments included: Auto Shop Automotive Electricity/Electronics 20 years teaching vehicle technicians. Teaching the classes listed for the following companies: Allen Test Products - Chicago, Indianapolis. Teaching Independent Auto Technicians. Computer Controlled Carburetion Throttle Body Fuel Injection Port Fuel Injection Triton College - River Grove, Illinois. GM ASEP School Teaching Automotive Technology students. Beginning Automotive Electricity & Electronics Advanced Automotive Electricity & Electronics Delta Corporate Services - Milford, Michigan Teaching: GM dealer technicians, Illinois, Michigan, California. GM Specialized Electronics Training GM Electronic Service Information Teaching: GM factory vehicle builders and experimental engineering personnel - Pontiac, MI Advanced Troubleshooting
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Vehicle Testing Meters - Joe Glassford
Shown here are the socket areas for three different meters.
Meter #1 has a 10A socket that is fused, and a 300mA socket that is fused.
Meter #2 has a 10A fused socket and a 400mA fused socket.
Meter #3 has a 200mA fused socket, and an UNFUSED socket with a 20A 15SEC MAX
statement. This statement and the owner’s manual statements let the purchaser know that if you hook this meter into a circuit that is drawing 20A, and leave it hooked up for more than 15 seconds, that will be the last time you use the 10A socket, and may be the last time you get to use the meter. I am not sure if the other sockets would continue to work, as I have not, as yet, smoked this meter.
The point is, be sure you select a meter that has all amperage sockets fused and buy some extra fuses, as you will sometime, as I have, overload the socket and blow the fuse.
About meter input impedance
10 Mega ohm (10,000,000Ω) is the standard today.
Input impedance is a measure of how much the meter’s circuitry will load (prevent high amounts of current from flowing) in the circuit being tested.
With this amount of impedance (internal meter resistance), the meter will not draw excessive current while testing any circuit, electrical or electronic.
Good meters have 10MΩ (10,000,000 Ω) of impedance.
Think of impedance like a parallel resistance added to the circuit under test.
Meters with low impedance draw excessive current, load the circuit being tested, and can skew your testing values.
If the meter impedance meets today’s standard, then the next thing to check is how accurate is the meter. Check the owners manual before you buy.
One definition of impedance:
The effective resistance of an electric circuit or component to alternating current, arising from the combined effects of resistance and reactance.
How Accurate Should Your Test Meter Be?
Accuracy of: Plus or minus 0.1% is Best Check the owner’s manual for your meter under DC accuracy.
Lets compare meter readings of Best (0.1% accuracy) to Poor (1% accuracy) If the actual charging voltage was 14.30V
If the actual battery open circuit voltage was 12.66
Less than .050V (50mV) is a common acceptable voltage drop on a sensor or computer ground.
If your meter is off by .126V (126mV), as illustrated in the example, you will never find the problem.
Good reason to invest in an accurate meter!
What You Need to be Aware of When Using an Auto Range
Meter
The photos above shows four range values of a meter. This is also known as scale values
. This meter was ranged from 4 volts up to 4000 volts by continually pushing the range
button. Some meters display the word AUTO
in the upper left of the window and start in the 4 Volt range, and some will display RANGE
in the upper left of the window and start in the 40 Volt range even though they can be ranged down to the 4 volt range. You need to always be aware of what range your meter is in in order to interpret correct results. Work with your meter. If it is an auto ranging
meter, turn it on and then see what range it defaults to. Most auto ranging meter allow you to chose the range you want to test in by pushing the range
button highlighted on the above meters. Some auto ranging meters do not have a range button. They range automatically. Know your meter.
To continually measure accurate values, always keep your range
as close to the value you think you are looking for. The further away the range is from the actual value, the more inaccurate your measurement will be.
When and Why You Should Change Your Meter’s Battery
Photo #1: To get this low battery light to appear on this meter, I had to drain the 9Volt battery down to 4.3Volts. As you can see in photo #2 even with this low of a battery, it was still giving a very accurate reading of voltage when placed on my power converter rated at 14V DC. It read 13.89V while the other two meters with fresh batteries read 13.91V and 13.90V.
In photos #2 and #3, the meter on the left with the low battery continued to toggle between 13.89V and 13.90V, while the meters with the fresh batteries locked on to the values shown and never did toggle. Continual toggling of a value
is a first indication that the battery is weak. After time, however, the meter on the left in photo #4 reads 15.00V while the true value of the converter never changed on the meters with the fresh batteries. They continued to register 13.91V and 13.90V. All three meters in the photos are hooked to the same power converter. A higher than true voltage reading on a meter is an indication that the battery in the meter is going dead.
Because each meter circuitry is different, and we do not always pay attention to the exact moment a low battery symbol shows on our meter, I suggest you always check to see if there is a low battery symbol, and as soon as you see it, put in a new battery. Make sure the meter is off before you open the case.
A variation of .010V (10mV) between different meters is due to each meter’s internal circuitry. No two meters contain exactly the same chemical make up in their solid state components. These three meters are rated at .1% accuracy. Differences are due to meter interpolation.
Does Your Meter Need to be Calibrated
?
This meter is in need of calibration. The meter is connected to read amps on a test board I used when teaching. The board has 3 incandescent light bulbs that each operate at about 80mA+/-.
In the top photo the positive probe is plugged into the 10A socket of the meter, the knob is selecting mA/A DC. The reading of 0248mA is accurate for the current that these three bulbs see in total.
In the bottom photo, the only change made to the meter hook up was to turn the meter off, remove the positive probe from the
10A socket and place it in the 400mA socket, then turn the meter back on. As you can