How to Use a Multimeter

[Bill90Loyale]

Wish I really knew. Have worked on the Sub for years in near absolute ignorance of electrical troubleshooting. Tired of the ignorance. Just bought a Craftsman digital multimeter ($39). Paid another $10 for the cool-looking "simple and comprehensive guide" to its use. The booklet is written in a language I don't understand, and is not really helpful in a practical sense.

 

I've searched the threads to see if anyone ever published a practical tutorial. Didn't find one. Let me know if there's a good down-to-earth one on the WWW somewhere. Multiple suggestions to put the ohm meter on a part, but no instructions for the ignorant.

 

For example: "Test the EGR solonoid for resistance." Well, ok. Can you test it in place? With one or more wires removed? Have to remove the solonoid first?

 

It would help me alot (and I would guess a few others) if a veteran multimeter user could post a tutorial on what and how a multimeter can be used on our Subies to solve electrical problems. And what they CAN'T do.

 

Apologies for the long post, and thank you in advance to all.

[MorganM]

Electrical issues are the worst

 

I use mine for 2 things really. I dont know much about how to properly use all the fancy buttons and such but I get by lol.

 

The first setting I use a lot is just Volts testing. You set your meter to read within the range of volts you are testing. So in the case of a car, 12 Volt DC. Mine has a setting for upto 20 V DC. Now when I touch the red probe to a hot lead ont he car, and ground the black probe the meter will tell me how many volts (upto 20) is going to that wire.

 

The other setting I use is OHMs for resistance testing. Again I set the meter to the range I'll be working in. Put the red probe at a wire or device terminal (for example one spade end terminal on a Throttle Position Sensor) and ground the black probe. This will tell you how high the resistance is for that wire/device. The higher the resistance, the harder it is for electricity to flow.

 

Hope that helps somewhat I'm sure some electrical gurus will reply back with some more indepth descriptions on how to properly use a mutlimeter. Then hopefully I'll learn something also!

 

Peace.

[archemitis]

on the sensor testing, a water temp sensor works between 0 and 5 volts, for efi(say 0volts tells the computer it is 0 degrees, and 5 volts tells it 200degrees, not sure if the temps are on, or even close, but just as an example). so one wire goin in has 12, i think, and the other wire spits out the 0-5 volts for a signal. so if you test the wire goin back that should have 0-5 and it has something like 12 then you know that sensor is bad. alot of sensors can be tested like that, and if you have a factory service manual, not chiltons crap, i hear its all in there.

[WoodsWagon]

Resistance (ohms) is the horseshoe shaped icon on the meter. Volts DC is the Icon with a bunch of horizontal lines, some of them broken, or VDC. The black test probe is the negative one, and the red one is the positive one. If you hook them up backward, nothing bad will happen, the readings will be shown as negative. Since most cars are negative ground, the black probe should be contacting some good body metal, or the engine block. make sure the connections are good.

To test resistance, just remember that what the meter is testing is whats between the two probes. To test the EGR solenoid, disconnect the connector going to it, then touch the two probes to the two spade termanals inside the connector going to the solenoid. a reading of about 43 is good, a really high reading or OL means that there isn't a real connection there.

Hope this helps.

[MorganM]

Resistance (ohms) is the horseshoe shaped icon on the meter. Volts DC is the Icon with a bunch of horizontal lines, some of them broken, or VDC. The black test probe is the negative one, and the red one is the positive one. If you hook them up backward, nothing bad will happen, the readings will be shown as negative. Since most cars are negative ground, the black probe should be contacting some good body metal, or the engine block. make sure the connections are good.

To test resistance, just remember that what the meter is testing is whats between the two probes. To test the EGR solenoid, disconnect the connector going to it, then touch the two probes to the two spade termanals inside the connector going to the solenoid. a reading of about 43 is good, a really high reading or OL means that there isn't a real connection there.

Hope this helps.

Do you check EGR at full operating temp? Or when the engine is still cool?

 

Thx

[Bill90Loyale]

Electrical issues are the worst

 

The first setting I use a lot is just Volts testing. You set your meter to read within the range of volts you are testing. So in the case of a car, 12 Volt DC. Mine has a setting for upto 20 V DC. Now when I touch the red probe to a hot lead ont he car, and ground the black probe the meter will tell me how many volts (upto 20) is going to that wire.

 

The other setting I use is OHMs for resistance testing. Again I set the meter to the range I'll be working in. Put the red probe at a wire or device terminal (for example one spade end terminal on a Throttle Position Sensor) and ground the black probe. This will tell you how high the resistance is for that wire/device. The higher the resistance, the harder it is for electricity to flow.

 

Peace.

Thank you Morgan and Archemitus. Morgan, I take it that your first setting is to test the WIRE for continuity/power, then the second (ohms) is to test the individual device. Makes sense. How common is it for the wiring in these old cars to go bad?

[MorganM]

Thank you Morgan and Archemitus. Morgan, I take it that your first setting is to test the WIRE for continuity/power, then the second (ohms) is to test the individual device. Makes sense. How common is it for the wiring in these old cars to go bad?

I've found it's rarely the wire that fails. Almost always a device or other part that fails. If the wire isnt cut, kinked, frayed or otherwise physicly damaged they are pretty much good to go. Points of failure tend to be connections in the wireing or the device its powering.

 

Peace !_!

[rallyruss]

ok I must be wierd because I like electrical and happen to almost understand it. only took me 10 years of study:rolleyes:

 

a water temp sensor works between 0 and 5 volts, for efi(say 0volts tells the computer it is 0 degrees, and 5 volts tells it 200degrees, not sure if the temps are on, or even close, but just as an example). so one wire goin in has 12, i think, and the other wire spits out the 0-5 volts for a signal.

sorry but I need to correct this. The sensor is fed a 5volt ref on one of the 2 wires. the temp sensor is a variable thermistor. so a cold motor will register 4. somthing volts depending on how stone cold it really is. as it warms up the voltage will drop to say .5 V . this voltage is read on the return signal wire to the ecm. the 5V supply should not change.

 

most SENSORS work on a 5 volt referance to make sure there is a stable voltage supply at all times. if it were to use 12 volts there would be too much cance of having a voltage supply drop during excessive loads on the charging system that fould effect the entire system.

 

voltage = amps x resistance (ohms law)

 

ohms(resistance) testing is verry easy but may give you a incorect diagnosis of a component. you also need to have what ever it is you are testing disconnected completely from the car.

 

voltage or voltage drop testing is the most valuable test method and the quickest once you understand the principle. ok I have attempted to explain this before and it was largly ignored so I will not try again. if you want to learn look it up under voltage drop testing on the net. There is plenty of info out there. (hint: the coolant sensor test is a voltage drop test of sorts.)

 

if you have any more questions feel free to ask. I could sit here typing all day in an attempt to explain the proper and entire operation of a multimeter.

 

try this link http://www.cnnw.net/~fourty/voltdroptest.htm

[Russell]

Just as a warning to you, don't try to measure resistance across something that you suspect will have a voltage. This can burn out the fuse in the meter. Measuring resistances is usually done with the device that you are measuring out of the circuit. This is in agreement with what was said above about measuring the egr solenoid. It should make no significant difference whether it is hot or not, since the egr is a solenoid it should have a finite resistance or infinite/short to indicate a problem. I just recently tested mine, the car had been sitting for perhaps 45 min. since I had it on the highway, i got a infinite reading on the egr solenoid. It had burn't out somehow and was open. Replaced it with another and the problem went away as verified in the diagnostic check mode on my spfi.

[Bill90Loyale]

I must be wierd because I like electrical and happen to almost understand it. only took me 10 years of study:rolleyes:

 

I could sit here typing all day in an attempt to explain the proper and entire operation of a multimeter.

 

try this link http://www.cnnw.net/~fourty/voltdroptest.htm

Thank you Rallyruss. Actually, I was sort of hoping that someone with your knowledge WOULD sit there typing all day and come up with a multimeter/subie guide on testing the solonoids and other electrical stuff that tends to cause mischief on our older vehicles. Could post the result in our "Repair Manual" section to encourage others to learn this stuff. I'll check out the volt drop link, and thanks again.

[Bill90Loyale]

i got a infinite reading on the egr solenoid. It had burn't out somehow and was open. Replaced it with another and the problem went away as verified in the diagnostic check mode on my spfi.

Thank you Russell. Any noticeable improvements in reducing surge, idle or any other performance improvements with the new EGR solonoid?

[Russell]

Thank you Russell. Any noticeable improvements in reducing surge, idle or any other performance improvements with the new EGR solonoid?

 

It is my girlfriend's car, but when I had a chance to drive it I felt there was an improvement in performance. She seemed to agree, difficult to tell though. Biggest thing for me was to get the check engine light to go off on the dash. I was pretty happy with the way the service manual laid out the flowchart for testing.

 

Measuring a voltage drop across a device is very similar to measuring the resistance of the device. The difference is the device is still in the circuit for a voltage drop test. Essentially if you were to measure the voltage drop across the egr solenoid you would do that with the solenoid in the circuit. If the voltage across the terminals to the solenoid was the same as that being supplied at the harness you could assume the solenoid to be an open circuit and needing replaced. I couldn't say what the voltage coming from the harness should be in this specific case, I would imagine though that it wouldn't just be some standard across all cars, since small effects like line resistance would change from one car to another. To measure the voltage drop on the solenoid you could:

 

1. disconnect solenoid from harness, measure voltage across the two terminals of the harness.

 

2. reconnect the solenoid to harness, measure the voltage across the two wires again.

 

as to what reading you would get with a working solenoid I couldn't say, but rallyruss seems to have an opinion on that. To me it would seem that you should still see the same voltage as in step one in both cases, that is assuming the source from the control unit is maintaining this voltage despite the draw of current by the solenoid. Maybe this is just making things more confusing, but I am trying to outline why I like the resistance test better. With a resistance test the device is removed from the circuit and you are performing a tried and true reading of the resistance characteristics of it. If they are not within some range of the specification then the device has failed. Of course, it is not always so easy to remove the device from the circuit... but I think for these instances the service manual would direct you to the right type of reading to take and what to suspect within a given range.

 

whew... that was probably more than anybody wanted to hear. By the way, electronics is where I have personally been trained. In addition to physics, so I would like to think that I know something... Somebody could always come and prove me wrong though :-\ .

 

Let me know what you come up with on removing the O2 sensor if you do, the "special" tool from the dealer for this task is $130!!!!! I've been trying to think of something else to use or even build to get at that darn thing.

[TomRhere]

I know it's a bit late, but I just ran across this.

 

Go to autospeed.com and click on the tech section. There's a 3-part series on purchase and use of a multimeter there. I haven't read thru it myself, but it should be some good info for those needing a multimeter.

[rallyruss]

well I have not been clear enough I guess.

1. disconnect solenoid from harness, measure voltage across the two terminals of the harness.

 

2. reconnect the solenoid to harness, measure the voltage across the two wires again.

there is no need to disconnect any thing when voltage drop testing.

 

I couldn't say what the voltage coming from the harness should be in this specific case, I would imagine though that it wouldn't just be some standard across all cars, since small effects like line resistance would change from one car to another.

ALL actuators/solinoids ect. work off battery voltage. yes that may varry a bit but its real easy to check what B+ is. as for "line resistance" a ruel of thumb is that you are allowed .25V drop accross connectors and at most .5v at a switch. the sum of all voltage drops will allways = battery voltage

 

resistance testing is easy to understand and removes the thinking part of troubleshooting. it also will help diagnose a bad component. this is all very true. let me give just a few examples of when I would say it is not usefull.

 

checking battery cables and connections. a BAD battery cable may show little to no resistance when checking ohms because a meter does not push enough amps to test the circit as it would actually be used. a tiny strand of wire may have verry little resistance but will not start you car.

 

I have seen plenty of bad solinoids and coils with GOOD resistance readings to know not to trust resistance alone.

 

just recently I was trying to find a bad ground in a prelude dash system. I started out checking resistance and it all looked good. less than a couple of ohms on the ground side. I tore into everything. finally I decided to check voltage drops on the ground circit. when the power was on it droped 5v across a bracket to the body that supplyed the ground to the cluster.

 

I have been bit too many times by resistance testing. It just does not simulate a real load on a circit properly to give you accurate readings.

 

Let me know what you come up with on removing the O2 sensor if you do, the "special" tool from the dealer for this task is $130!!!!! I've been trying to think of something else to use or even build to get at that darn thing.

use a 7/8 wrench or 22mm. if its really difficult to get to us a blue point tool# YA8875 or generic versions avl. at many parts stores.

[Russell]

yea, now that you say it I can see my post is definitely wrong. Instead of saying to measure the voltage drop -- do 1 and 2. it should say to read the resistance -- do 1 and 2. Sorry about that, and thanks for the correction rallyruss.

 

As for the rest, my point is mainly to make it simpler for somebody new to multimeters to make a measurement of a bad component. Though perhaps, as you say, this isn't the best advice. It is however the advice given by the service manual. At least in 89.

 

In my opinion a circuit like the egr solenoid which comes directly from the ignition to the ecu, should give a reading of 12 volts as if you were measuring the source, regardless of the condition of the solenoid, if it were open you should still read the same voltage across the terminals of the solenoid in the circuit. For this reason I can't see how the voltage test is any better at finding a fault.

 

Am i missing something else?

[rallyruss]

yes same stuff different smell.

 

you are correct a voltage drop test alone on a solinoid would not be too helpfull. I should have stated the need for a vacume gauge to verrify that the solinoid is switching.

 

I prefer the V drop test because it tests the ENTIRE circit in a verry short time in conparison to following a flow chart full of resistance tests.

 

I get paid to diagnose and do it quickly. my labor costs the customer over 100$ an hour. people ask questions when you are into a job over an hour without a diagnosis. common sense and a good undrerstanding of the circit in question will get me a result faster than a factory manual flow chart.

 

I call truce. we each have our own favorite for various reasons. The last time I tried to bring a new method of troubleshooting to the masses I was chased with burning torches and called a voltage droping freak. looks like I will have to hide my methods for fear of persicution again.

 

this was what I was afraid of in the begining.

hopefully this shined some light on BOTH testing methods.

[Russell]

yes same stuff different smell.

 

Yeah, hey no hard feelings rallyruss. I was only trying to defend my position and not attacking yours. I really have no strong feelings about it at all. The flowchart in the service manual doesn't seem to start you on the most obvious problem first, rather I see it as some complete guide to follow if you haven't touched it before. Which is why I personally use it. For although I have experience with electronics, I haven't been working on cars for that long.

 

Speaking of your above advice, I guess I need to remove the heat sheild on the cat to get at the 02 sensor with a normal wrench. It doesn't look like its going to come off easy, you know what I mean. Did you recommend another tool I could find at a local store? What was it?

[rallyruss]

Did you recommend another tool I could find at a local store? What was it?

most parts stores and sears type places sell em. short littler guy that you use a 3/8 ratchet with.